 Chapter 8 of History of Astronomy This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. History of Astronomy by George Forbes Chapter 8 Newton's Successors Holly, Euler, Lagrange, Laplace, etc. Edmund Holly succeeded Flamsteed as Second Astronomer Royal in 1721 although he did not contribute directly to the mathematical proofs of Newton's theory, yet his name is closely associated with some of its greatest successes. He was the first to detect the acceleration of the moon's mean motion. Hipparchus, having compared his own observations with those of more ancient astronomers, supplied an accurate value of the moon's mean motion in his time. Holly similarly deduced a value for modern times and found it sensibly greater. He announced this since 1693, but it was not until 1749 that Dunthorne used modern lunar tables to compute a lunar eclipse observed in Babylon 721 BC, another at Alexandria 201 BC, a solar eclipse observed by Teon 360 AD, two latter ones up to the 10th century. He found that to explain these eclipses, Holly's suggestion must be adopted, the acceleration being 10 seconds in one century. In 1759, Lalanda again fixed it at 10 seconds. The Paris Academy in 1770 offered their prize for an investigation to see if this could be explained by the theory of gravitation. Euler won the prize, but failed to explain the effect, and said it appears to be established by indisputable evidence that the secular inequality of the moon's mean motion cannot be produced by the forces of gravitation. The same subject was again proposed for a prize, which was shared by Lagrangian Euler, neither finding a solution, while the latter asserted the existence of a resisting medium in space. Again in 1774, the Academy submitted the same subject a third time for the prize, and again Lagrang failed to detect a cause in gravitation. Laplace now took the matter in hand. He tried the effect of a non-instantaneous action of gravity to no purpose, but in 1787 he gave the true explanation. The principal effect of the sun on the moon's orbit is to diminish the Earth's influence, thus lengthening the period to a new value generally taken as constant. But Laplace's calculations showed the new value to depend upon the eccentricity of the Earth's orbit, which, according to theory, is a periodical variation of enormous period, and has been continually diminishing for thousands of years. Thus the solar influence has been diminishing, and the moon's mean motion increased. Laplace computed the amount at 10 seconds in one century, agreeing with observation. Later on, Adams showed that Laplace's calculation was wrong, and that the value he found was too large, so part of the acceleration is now attributed by some astronomers to a lengthening of the day by tidal friction. Another contribution by Holly, to the verification of Newton's law, was made when he went to St Helena to catalog the southern stars. He measured the change in length of the seconds, pendulum, and different latitudes due to the changes in gravity foretold by Newton. Furthermore, he discovered the long inequality of Jupiter and Saturn, whose period is 929 years. For an investigation of this, also the Academy of Sciences offered their prize. This led Euler to write a valuable essay, disclosing a new method of computing perturbations called the instantaneous ellipse with variable elements. The method was much developed by Lagrange. But again, it was Laplace who solved the problems of the inequalities of Jupiter and Saturn. By the theory of gravitation, reducing the errors of the tables from 20 minutes down to 12 seconds, thus abolishing the use of empirical corrections to the planetary tables and providing another glorious triumph for the law of gravitation. As Laplace justly said, these inequalities appeared formally to be inexplicable by the law of gravitation. They now form one of its most striking proofs. Let us take one more discovery of Holly, furnishing directly a new triumph for the theory. He noticed that Newton described parabolic orbits to the comets which he studied, so that they come from infinity, sweep around the sun, and go off to infinity forever, after having been visible a few weeks or months. He collected all the reliable observations of comets he could find, to the number of 24, and computed their parabolic orbits by the rules laid down by Newton. His object was to find out if any of them really traveled in elongated ellipses, practically undistinguishable in the visible part of their paths from parabolae, in which case they would be seen more than once. He found two old comets whose orbits, in shape and position, resembled the orbit of a comet observed by himself in 1682. Apeen observed one in 1531, Kepler the other in 1607. The intervals between these appearances, 75 or 76 years, he then examined and found old records of similar appearance in 1456, 1380 and 1305. It is true, he noticed, that the intervals varied by a year and a half, and the inclination of the orbit to the ecliptic diminished with successive apparitions. But he knew from previous calculations that this might easily be due to planetary perturbations. Finally he arrived at the conclusion that all of these comets were identical, traveling in an ellipse, so elongated that the part where the comet was seen seemed to be part of a parabolic orbit. He then predicted its return at the end of 1758 or the beginning of 1759, when he should be dead. But as he said, if it should return, according to our prediction, about the year 1758, impartial posterity will not refuse to acknowledge that this was first discovered by an Englishman. 1. Synopsis Astronomy, Commitaceae, 1749 Once again, Holly's suggestion became an inspiration for the mathematical astronomer. Clairot, assisted by Lalanda, found that Saturn would retard the comet 100 days, Jupiter 518 days, and predicted its return to Perihelion on April 13, 1759. In his communication to the French Academy, he said that a comet traveling into such distant regions might be exposed to the influence of forces totally unknown, and even of some planet too far removed from the sun to be ever perceived. The excitement of astronomers towards the end of 1758 became intense, and the honor of first catching sight of the traveler fell to an amateur in Saxony. George Palach, on Christmas Day, 1758, reached Perihelion on March 13, 1759. This fact was a startling confirmation of the Newtonian theory, because it was a new kind of calculation of perturbations, and also it added a new member to the solar system, and gave a prospect of adding many more. When Holly's comet reappeared in 1835, Ponticolon's computations for the date of Perihelion passage were very exact, and afterwards, he showed that with more exact values of the masses of Jupiter and Saturn, his prediction was correct within two days, after an invisible voyage of 75 years. Hind afterwards searched out many old appearances of this comet going back to 11 BC, and most of these have been identified as being, really, Holly's comet, by the calculations of Cowell and Chromellen, of Greenwich Observatory, who have also predicted its next Perihelion passage for April 8-16, 1910, and have traced back its history still farther to 240 BC. Already in November 1907, the astronomer Royal was trying to catch it by the aid of photography. End of Chapter 8 Chapter 9 of History of Astronomy This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. History of Astronomy by George Forbes. Chapter 9 Discovery of New Planets Herschel, Piazzi, Adams, and Leveria. It would be very interesting, but quite impossible in these pages to discuss all the exquisite researches of the mathematical astronomers, and to inspire a reverence for the names connected with these researches, which for 200 years have been establishing the universality of Newton's Law. The Lunar and Planetary Theories, the beautiful theory of Jupiter's satellites, the figure of the Earth, and the tides, were mathematically treated by Maclaurin, D'Alembert, Legendre, Clairo, Euler, Lagrange, Laplace, Almsley, Bailey, Llanda, D'Lombra, Mayer, Hansen, Burchard, Benet, D'Amousseau, Plana, Poiseau, Gauss, Bessel, Beauvard, Aerie, Ivory, Dillonne, Leveria, Adams, and others of later date. By passing over these important developments, it is possible to trace some of the steps in the crowning triumph of the Newtonian theory, by which the planet Neptune was added to the known members of the solar system, by the independent researchers of Professor J.C. Adams and of Meijer Leveriaire in 1846. It will be best to introduce this subject by relating how the 18th century increased the number of known planets, which was then only six, including the Earth. On March 13th, 1781, Sir William Herschel was, as usual, engaged on examining some small stars, and noticing that one of them appeared to be larger than the fixed stars, suspected that it might be a comet. To test this, he increased his magnifying power from 227 to 460, and 932. Finding that, unlike the fixed stars near it, its definition was impaired, and its size increased. This convinced him that the object was a comet, and he was not surprised to find on succeeding nights that the position was changed, the motion being in the ecliptic. He gave the observations of five weeks to the Royal Society, without a suspicion that the object was a new planet. For a long time people could not compute a satisfactory orbit for the supposed comet, because it seemed to be near the perihelion, and no comet had ever been observed with a perihelion distance from the Sun greater than four times the Earth's distance. Lexel was the first to suspect that this was a new planet, eighteen times as far from the Sun as the Earth is. In January 1783, Laplace published the elliptic elements. The discoverer of a planet has a right to name it, so Herschel called it Georgium Cetus after the King, but Lalanda argued the adoption of the name Herschel. Bode suggested Uranus, and this was adopted. The new planet was found to rank in size next to Jupiter and Saturn, being four point three times the diameter of the Earth. In 1787 Herschel discovered two satellites, both revolving in nearly the same plane, inclined eighty degrees to the ecliptic, and the motion of both was retrograde. In 1772, before Herschel's discovery, Bode had discovered a curious arbitrary law of planetary distances. Opposite each planet's name, write the figure four, and in succession, add the numbers zero, three, six, twelve, twenty-four, forty-eight, ninety-six, etc., two to four, always doubling the last numbers. You then get the planetary distances. Mercury, with a distance of four, four plus zero equals four. Venus, seven, four plus three equals seven. Earth, ten, four plus six equals ten. Mars, fifteen, four plus twelve equals sixteen. A blank, four plus twenty-four equals twenty-eight. Jupiter, a distance of fifty-two, four plus forty-eight equals fifty-two. Saturn, ninety-five. Four plus ninety-six equals one hundred. Uranus, the distance of one ninety-two, four plus one ninety-two equals one ninety-six. Another blank, four plus three eighty-four equals three eighty-eight. All the five planets and the earth fitted this rule, except there was a blank between Mars and Jupiter. When Uranus was discovered also fitting the rule, the conclusion was irresistible that there was probably a planet between Mars and Jupiter. An association of twenty-four astronomers was now formed in Germany to search for the planet. Almost immediately afterwards, the planet was discovered, not by any members of the association, but by Piazzi, who was engaged upon his great catalogue of stars. On January 1st, 1801, he observed a star which had changed its place the next night. Its motion was retrograde till January 11th, direct after the thirteenth. Piazzi fell ill before he had enough observations for computing the orbit with certainty, and the planet disappeared in the sun's rays. Goss published an approximate ephemeris of probable positions when the planet should emerge from the sun's light. There was an exciting hunt, and on December 31st, the day before its birthday, Desac captured the truant, and Piazzi christened it series. The mean distance from the sun was found to be 2.767, agreeing with the 2.8 given by Bode's law. Its orbit was found to be inclined over ten degrees to the elliptic, and its diameter was only 161 miles. On March 28th, 1802, Olbers discovered a new seventh magnitude star, which turned out to be a planet resembling series. It was called Palos. Goss found its orbit to be inclined 35 degrees to the elliptic, and to cut the orbit of Ceres. Wenz Olbers considered that these might be fragments of a broken-up planet. He then commenced a search for other fragments. In 1804 Harding discovered Juno, and in 1807 Olbers found Vesta. The next one was not discovered until 1845, from which date, asteroids or minor planets, as the small planets are called, have been found almost every year. They now number about 700. It is impossible to give any idea of the interest with which the first editions, since prehistoric times, to the planetary system, were received. All of those who showered congratulations upon the discoverers regarded these discoveries in the light of rewards for patient and continuous labors, the very highest rewards that could be desired. And yet they remained still the most brilliant triumph of all, the addition of another planet like Uranus, before it had ever been seen. When the analysis of Adams and Laverrier gave a final proof of the powers of Newton's great law to explain any planetary irregularity. After Sir William Herschel discovered Uranus in 1781, it was found that astronomers had observed it on many previous occasions, mistaking it for a fixed star of the sixth or seventh magnitude. Altogether, 19 observations of Uranus' position from the time of Flamsteed in 1690 had been recorded. In 1790, D'Lombra, using all these observations, prepared tables for computing its position. These worked well enough for a time, but at last the difference between the calculated and observed longitudes of the planets became serious. In 1821, Brevard undertook a revision of the tables, but found it impossible to reconcile all the observations of 130 years. The period of revolution of Uranus is 84 years. So he deliberately rejected the old ones, expressing the opinion that the discrepancies might depend upon some foreign and unperceived cause which may have been acting upon the planet. In a few years, the errors, even of these tables, became intolerable. In 1835, the error of longitude was 30 minutes. In 1838, 50 minutes. In 1841, 70 minutes. And by comparing the errors derived from observations made before and after opposition, a serious error of the distance, the radius vector, became apparent. In 1843, John Couch Adams came out Sr. Wrangler at Cambridge and was free to undertake the research which, as an undergraduate, he had set himself. To see whether the disturbances of Uranus could be explained by assuming a certain orbit and position in that orbit of a hypothetical planet, even more distant than Uranus. Such an explanation had been suggested, but until 1843 no one had the boldness to attack the problem. Vessel had intended to try, but a fatal illness overtook him. Adams first recalculated all known causes of disturbance using the latest determinations of the planetary masses. Still, the errors were nearly as great as ever. He could now, however, use these errors as being actually due to the perturbations produced by the unknown planet. In 1844, assuming a circular orbit and a mean distance agreeing with Bode's law, he obtained a first approximation to the position of the supposed planet. He then asked Professor Chalice of Cambridge to procure the latest observations of Uranus from Greenwich, which Erie immediately supplied. Then the whole work was recalculated from the beginning, with more exactness and assuming a smaller mean distance. In September 1845 he handed to Chalice the elements of a hypothetical planet, its mass, and its apparent position for September 30, 1845. On September 22, Chalice wrote to Erie explaining the matter and declaring his belief in Adams' capabilities. When Adams called on him, Erie was away from home, but at the end of October 1845 he called again and left a paper with full particulars of his results, which had, for the most part, reduced the discrepancies to about one minute. As a matter of fact, it has since been found that the heliocentric place for the new planet, then given, was correct within about two degrees. Erie wrote to expressing his interest, and asked for particulars about the radius factor. Adams did not then reply, as the answer to this question could be seen to be satisfactory by looking at the data already supplied. He was a most unassuming man, and would not push himself forward. He may have felt, after all the work he had done, that Erie's very natural inquiry showed no proportionate desire to search for the planet. Anyway, the matter lay in embryo for nine months. Meanwhile, one of the ableist French astronomers of Erie experienced in commuting perturbations was independently at work, knowing nothing about Adams. He applied to his calculations every possible refinement, and considering the novelty of the problem, his calculation was one of the most brilliant in the records of astronomy. In criticism, it has been said that these were exhibitions of skill, rather than helps to a solution of the particular problem, and that in claiming to find the elements of the orbit within certain limits, he was claiming what was, under the circumstances, impossible, as the result proved. In June 1846, Laverrier announced, in the Comte Rendue de la Académie des Sciences, that the longitude of the disturbing planet, for January 1st, 1847, was 325, and that the probable error did not exceed 10 degrees. The result agreed so well with Adams, within one degree, that Erie urged Chalice to apply the splendid Northumberland Equatorial at Cambridge to the search. Chalice, however, had already prepared an exhaustive plan of attack, which must in time settle the point. His first work was to observe and make a catalog, or chart, of all stars near Adams' position. On August 31st, 1846, Laverrier published the concluding part of his labours. On September 18th, 1846, Laverrier communicated his results to the astronomers at Berlin, and asked them to assist in searching for the planet. By good luck, Dr. Bremerker had just completed a star chart of the very part of the heavens, including Laverrier's position, thus eliminating all of Chalice's preliminary work. The letter was received in Berlin on September 23rd, and the same evening, Gaul found the new planet of the eighth magnitude, the size of its disc agreeing with Laverrier's prediction, and the heliocentric longitude agreeing within 57 seconds. By this time Chalice had recorded, without reduction, the observations of 3,150 stars, as a commencement for his search. On reducing these he found a star, observed on August 12th, which was not in the same place on July 30th. This was the planet, and he had also observed it on August 4th. The feeling of wonder, admiration, and enthusiasm aroused by this intellectual triumph was overwhelming. In the world of astronomy, reminders are met every day of the terrible limitations of human reasoning powers, and every success that enables the mind's eye to see a little more clearly the meaning of things, has always been heartily welcomed by those who have themselves been engaged in like-researches. But since the publication of the Principia in 1687 there is probably no analytical success which has raised among astronomers such a feeling of admiration and gratitude as when Adams and Laverrier showed the inequalities in Uranus's motion to mean that an unknown planet was in a certain place in the heavens, where it was found. At the time there was an unpleasant display of international jealousy. The British people thought that the earlier date of Adams' work and of the observation by Chalice entitled him to at least an equal share of credit with Laverrier. The French on the other hand, who on the announcement of the discovery by Gaul, glowed with pride in the new proof of the great powers of their astronomer Laverrier, whose life had a long record of successes and calculation, were incredulous on being told that it had all been already done by a young man whom they never heard of. These displays of jealousy have long since passed away, and there is now universally an entente cordial, that to each of these great men belongs equally the merit of having so thoroughly calculated this inverse problem of perturbations as to lead to the immediate discovery of the unknown planet, since called Neptune. It was found that the planet had been observed and its position recorded as a fixed star by Lalande on May 8th and 10th, 1795. Mr. La Sel in the same year, 1846, with his two feet reflector, discovered a satellite with retrograde motion, which gave the mass of the planet about a twentieth of that of Jupiter. End of Chapter 9 Chapter 10 of History of Astronomy This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. History of Astronomy by George Forbes Chapter 10 Instruments of Precision State of the Solar System Having now traced the progress of physical astronomy up to the time, when very striking proofs of the universality of the law of gravitation convinced the most skeptical. It will still be borne in mind that while gravitation is certainly the principal force governing the motions of the heavenly bodies, there may it be a resisting medium in space, and there may be electric and magnetic forces to deal with. There may further be cases where the effects of luminous radiative repulsion become apparent, and also Crook's vacuum effects, described as radiant matter. Nor is it quite certain that Laplace's proof of the instantaneous propagation of gravity are final. And in the future, as in the past, Tycho Brahe's dictum must be maintained, that all theories should be preceded by accurate observations. It is the pride of astronomers that their science stands above all others in the accuracy of the facts observed, as well as in the rigid logic of the mathematics used for interpreting these facts. It is interesting to trace historically the invention of those instruments of precision which have led to this result, and without entering on the details required in a practical handbook, to note the guiding principles of construction in different ages. It is very probable that the Chaldeans may have made spheres like the Armillary sphere, for representing the poles of the heavens, and with rings to show the ecliptic and zodiac, as well as the equinoxial and solstitial coliores. But we have no record. We only know that the Tower of Beluce on an eminence was their observatory. We have, however, distinct records of two such spheres used by the Chinese about 2500 B.C. Gnomans, or some kind of sundial, were used by the Egyptians and others. And many of the ancient nations measured the obliquity of the ecliptic by the shadows of a vertical column in summer and winter. The natural horizon was the only instrument of precision used by those who determined star positions by the directions of their risings and settings, while in those days the calypsidra or water clock was the best instrument for comparing their times of rising and setting. About 300 B.C., an observatory fitted with circular instruments for star positions, was set up at Alexandria, the then center of civilization. We know almost nothing about the instruments used by Hipparchus in preparing his star catalogs and his lunar and solar tables. But the invention of the astrolabe is attributed to him. In modern times Nuremberg became a center of astronomical culture. Waltherus of that town made really accurate observations of star altitudes and of the distances between stars. And in 1484 A.D. he used a kind of a clock. Tycho Brahe tried these but discarded them as being inaccurate. Tycho Brahe, 1546-1601 A.D., made great improvements in armillary spheres, quadrants, sextants, and large celestial globes. With these he measured the positions of stars, or the distance of a comet from several known stars. He has left us full descriptions of them, illustrated by excellent engravings. Previous to his time such instruments were made of wood, Tycho always used metal. He paid the greatest attention to the stability of mounting, to the orientation of his instruments, to the graduation of the arcs by the then new method of transversals, and to the Aperture site used upon his pointer. There were no telescopes in his day, and no pendulum clocks. He recognized the fact that there must be instrumental errors. He made these as small as possible, measured their amount, and corrected his observations. His table of refractions enabled him to abolish the error due to our atmosphere, so as it could affect naked eye observations. The azimuth circle of Tycho's largest quadrant had a diameter of nine feet, and the quadrant radius of six feet. He introduced the mural quadrant for meridian observations. The French Jesuits at Peking in the 17th century helped the Chinese in their astronomy. In 1875, the writer saw and photographed on that part of the wall of Peking used by the Mandarin's as an observatory, the six instruments handsomely designed by Father of Her Beast, copied from the instruments of Tycho Brahe, and embellished with Chinese dragons and emblems cast on the supports. He also saw there two old instruments, which he was told were Arubaic, of date 1279 by Kosho King, astronomer to Kobay Khan, the grandson of Genghis Khan. One of these last is nearly identical with the Armalay of Tycho, and the other with his Armalay Equatory Maxime, with which he observed the comet of 1585, besides fixed stars and planets. The discovery of Galileo, of the isochronism of the pendulum, followed by Huygens adaptation of that principle to clocks, has been one of the greatest aids to accurate observation. About the same time, an equally beneficial step was the employment of the telescope as a pointer, not the Galilean with concave eyepiece, but with a magnifying glass, to examine the focal image, at which also a fixed mark could be placed. Kepler was the first to suggest this. Gas coin was the first to use it. Huygens used a metal strip. A variable width in the focus is a micrometer to cover a planetary disk, and so to measure the width covered by the planet. The Marquis Malvasia in 1662, described the network of fine silver threads at right angles, which he used in the focus, much as we do now. In the hands of such a skillful man as Tycho Brahe, the old open sights, even without clocks, served their purpose sufficiently well to enable Kepler to discover the true theory of the solar system, but telescopic sights and clocks were required for approving some of Newton's theories of planetary perturbations. Picard's observations of Paris from 1667 onwards seemed to embody the first use of the telescope as a pointer. It was also the first to introduce the use of Huygens' clocks for observing the right ascension of stars. Alois Romer was born in Copenhagen in 1644, in 1675, by careful study of the times of eclipses of Jupiter's satellites, he discovered that light took time to traverse space. Its velocity is 186,000 miles per second. In 1681 he took up his duties as astronomer at Copenhagen and built the first transit circle on a window sill of his house. The iron axis was five feet long and one and a half inches thick and the telescope was fixed near one end with a counter-poise. The telescope tube was a double cone to prevent flexure. Three horizontal and three vertical wires were used in the focus. These were illuminated by a speculum near the object class, reflecting the light from a lantern placed over the axis, the upper part of the telescope tube being partly cut away to admit the light. A divided circle with pointer and reading microscope was provided for reading the declination. He realized the superiority of a circle with graduations over a much larger quadrant. The collimation error was found by reversing the instrument and using a terrestrial mark, the azimuth error by star observations. The time was expressed in fractions of a second. He also constructed a telescope with equatorial mounting to follow a star by one axial motion. In 1728 his instruments and observation records were destroyed by fire. Helvelius had introduced the vernier and tangent screw in his measurement of ARC graduations. His observatory and records were burnt to the ground in 1679, though an old man he started afresh and left behind him a catalogue of 1500 stars. Flamsteed began his duties at Greenwich Observatory as first astronomer royal in 1676 with the very poor instruments. In 1683 he put up a mural ARC of 140 degrees and in 1689 a better one, 79 inches radius. He conducted his measurements with great skill and introduced new methods to attain accuracy using certain stars for determining the errors of his instruments and he always reduced his observations to a form in which they could be readily used. He introduced new methods for determining the position of the equinox and the right ascension of a fundamental star. He produced a catalogue of 2935 stars. He supplied Sir Isaac Newton with results of observation required in his theoretical calculations. He died in 1719. Halley succeeded Flamsteed to find that the whole place had been gutted by the latter's executors. In 1721 he got a transit instrument and in 1726 a mural quadrant by Graham. His successor in 1742 Bradley replaced this by a fine brass quadrant eight feet radius by bird and Bradley's zenith sector was purchased for the observatory. An instrument like this, specifically designed for zenith stars, is capable of greater rigidity than a more universal instrument and there is no trouble with refraction in the zenith. For these reasons Bradley has set up his instrument at cue to attempt the proof of the earth's motion by observing the annual parallax of stars. He certainly found an annual variation of zenith distance, but not at the times of year required by the parallax. This led him to the discovery of the aberration of light and of nutiation. Bradley has been described as the founder of the modern system of accurate observation. He died in 1762, leaving behind him 13 folio volumes of valuable but unreduced observations. Those relating to the stars were reduced by Bessel and published in 1818 at Königsberg in his well-known standard work Fundamenta Astronomy. In it are results showing the laws of refraction with tables of its amount, the maximum value of aberration and other constants. Bradley was succeeded by Bliss and he by Masculine, 1765, who carried on excellent work and lead the foundations of the nautical Almanac, 1767. Just before his death he induced the government to replace birds quadrant by a fine new mural circle, six feet in diameter by Troton. The divisions being read off by microscopes fixed on piers opposite to the divided circle. In this instrument the micrometer screw with a divided circle for turning it was applied for bringing the micrometer wire actually in line with a division on the circle, a plan which is still always adopted. Pond succeeded Masculine in 1811 and was the first to use this instrument. From now onwards the places of stars were referred to the pole, not to the zenith, the zero being obtained from measures on circumpolar stars. Standard stars were used for giving the clock error. In 1816 a new transit instrument by Troughton was added and from this date the Greenwich Star places have maintained the very highest accuracy. George Bedell Airy, Seventh Astronomer Royal commenced his Greenwich labors in 1835. His first and greatest reformation in the work of the observatory was one he had already established at Cambridge and is now universally adopted. He held that an observation is not completed until it has been reduced to a useful form. And in the case of the sun, moon and planets these results were in every case compared with the tables and the tabular error printed. Airy was firmly impressed with the object for which Charles the Second had wisely founded the observatory in connection with navigation and for observations of the moon. Whenever a meridian transit of the moon could be observed this was done. But even so there are periods in the month when the moon is too near the sun for a transit to be well observed. Also weather interferes with many meridian observations. To render the lunar observations more continuous, Airy employed Troughton's successor, James Sims, in conjunction with the engineer's ransom and may to construct an altismith with three foot circles and a five foot telescope in 1847. The result was that the number of lunar observations was immediately increased threefold, many of them being in a part of the moon's orbit which had previously been bearer of observations. From that date the Greenwich Lunar Observations have been a model and a standard for the whole world. Airy also undertook to superintend the reduction of all Greenwich Lunar Observations from 1750 to 1830. The value of this laborious work, which was completed in 1848, cannot be overestimated. The demands of astronomy, especially in regard to small minor planets, required a transit instrument and a mural circle with a more powerful telescope. Airy combined the functions of both and employed the same constructors as before to make a transit circle with a telescope of 11 and a half feet focus and a circle of six feet diameter, the object class being eight inches in diameter. Airy, like Bradley, was impressed with the advantage of employing stars in the zenith for determining the fundamental constants of astronomy. He devised a reflex zenith tube in which the zenith point was determined by reflection from a surface of mercury. The design was so simple and seemed so perfect that great expectations were entertained, but unaccountable variations comparable with those of the transit circle appeared, and the instrument was put out of use until 1903 when the present astronomer Royal noticed that the irregularities could be allowed for, being due to that remarkable variation in the position of the Earth's axis included in circles of about six yards diameter at the north and south poles, discovered at the end of the 19th century. The instrument is now being used for investigating these variations, and in the year 1907 as many as 1,545 observations of stars were made with the reflex zenith tube. In connection with zenith telescopes, it must be stated that Raspeagi at the Capitol Observatory at Rome made use of a deep well with a level mercury surface at the bottom and a telescope at the top pointing downwards, which the writer saw in 1871. The reflection of the micrometer wires and of a star very near the zenith, but not quite in the zenith, can be observed together. His mercury trough was a circular plain surface with a shallow edge to retain the mercury. The surface quickly came to rest after disturbance by street traffic. Sir W. M. H. Christie, eighth astronomer Royal, took up his duties in that capacity in 1881. Besides the larger altizimith that he created in 1898, he has widened the field of operations at Greenwich by the extensive use of photography and the establishment of large equatorials. From the point of view of instruments of precision, one of the most important new features is the astrographic equatorial set up in 1892 and used for the Greenwich section of the great astrographic chart just completed. Photography has come to be of use not only for depicting the sun and moon, comets and nebulae, but also to obtain accurate relative positions of neighboring stars, to pick up objects that are invisible in any telescope, and most of all, perhaps, in fixing the positions of faint satellites. Thus Saturn's distant satellite, Phoebe, and the sixth and seventh satellites of Jupiter have been followed regularly in their courses at Greenwich ever since their discovery with the 30-inch reflector erected in 1897, and while doing so, Mr. Malote made in 1908 the splendid discovery on some of the photographic plates of an eighth satellite of Jupiter at an enormous distance from the planet. From observations in the early part of 1908, over a limited arc of its orbit, before Jupiter approached the sun, Mr. Cowell computed a retrograde orbit and calculated the future positions of this satellite, which enabled Mr. Malote to find it again in the autumn, a great triumph both of calculation and of photographic observation. The satellite had never been seen and has been photographed, only at Greenwich, Heidelberg, and the Lick Observatory. Greenwich Observatory has been here selected for tracing the progress of accurate measurement, but there is one instrument of great value, the heliometer, which is not used at Greenwich. This serves the purpose of a double-image micrometer and is made by dividing the object glass of a telescope along a diameter. Each half is mounted so as to slide a distance of several inches each way on an arc, whose center is the focus. The amount of the movement can be accurately read, thus two fields of view overlap, and the adjustment is made to bring an image of one star over that of another star, and then to do the same by a displacement in the opposite direction. The total movement of the half object glass is double the distance between the star image in the focal plane. Such an instrument has long been established at Oxford, and German astronomers have made great use of it, but in the hands of Sir David Gill, late His Majesty's astronomer at the Cape of Good Hope, and especially in his great researches on solar and on stellar parallax, it has been recognized as an instrument of the very highest accuracy, measuring the distance between stars correctly to less than a tenth of a second of arc. The superiority of the heliometer over all other devices, except photography, for measuring small angles, has been specially brought into prominence by Sir David Gill's researches on the distance of the Sun, that is, the scale of the solar system. A measurement of the distance of any planet fixes the scale, and as Venus approaches the Earth, most nearly of all the planets, it used to be supposed that a transit of Venus offered the best opportunity for such measurement, especially as it was thought that as Venus entered on the solar disk, the sweep of light round the dark disk of Venus would enable a very precise observation to be made, the transit of Venus in 1874, in which the present writer assisted overthrew this delusion. In 1877, Sir David Gill used Lord Crawford's heliometer at the Island of Ascension to measure the parallax of Mars in opposition, and found the Sun's distance 93,080,000 miles. He considered that while the superiority of the heliometer had been proved, the results would still be better with the points of light shown by minor planets, rather than with the disk of Mars. In 1888 through 1889, at the Cape, he observed the minor planets, Iris, Victoria, and Sappho, and secured the cooperation of four other heliometers. His final result was 92,870,000 miles, that parallax being 8.802 minutes. So delicate were these measures that Gill detected a minute periodic error of theory of 27 days, owing to a periodically erroneous position of the center of gravity of the Earth and Moon, to which the position of the observer was referred. This led him to correct the mass of the Moon, and to fix its ratio to the Earth's mass, at 0.012240. Another method of getting the distance from the Sun is to measure the velocity of the Earth's orbital motion, giving the circumference traversed in a year, and so the radius of the orbit. This has been done by comparing observation and experiment. The aberration of light is an angle 20.48 minutes, giving the ratio of the Earth's velocity to the velocity of light. The velocity of light is 186,000 miles a second, whence the distance to the Sun is 92,780,000 miles. There seems however to be some uncertainty about the true value of the aberration. Any determination of which is subject to irregularities due to the seasonal errors. The velocity of light was experimentally found in 1862 by Fidso and Fokol, each using an independent method. These methods have been developed, and new values found by Cornu, Michelson, Newcomb, and the present writer. Quite lately, Holm, at the Cape of Good Hope, measured spectroscopically the velocity of the Earth to and from a star, by observations taken six months apart, thence he obtained an accurate value of the Sun's distance. But the remarkably erratic minor planet, Eros, discovered by Witt in 1898, approaches the Earth within 15 million miles at rare intervals, and with the Eta photography, will certainly give us the best result. A large number of observatories combined to observe the opposition of 1900. Their results are not yet completely reduced, but the best value deduced so far for the parallax is 8.807 minutes. Give or take 0.0028. End of Chapter 10 Chapter 11 of History of Astronomy This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by Jennifer Henry. History of Astronomy by George Forbes Chapter 11 History of the Telescope and Spectroscope Accounts of wonderful optical experiments by Roger Bacon, who died in 1292, and in the 16th century by Diggs, Baptiste Porta, and Antonio de Domini's, have led some to suppose that they invented the telescope. The writer considers that it is more likely that these notes refer to a kind of camera obscura in which a lens throws an inverted image of a landscape on the wall. The first telescopes were made in Holland, the originator being either Henry Liberhé, Zacharias Jansen, or James Metius, and the date 1608 or earlier. In 1609 Galileo, being in Venice, heard of the invention, went home and worked out the theory and made a similar telescope. These telescopes were all made with a convex object glass and a concave eye lens. And this type is spoken of as the Galilean telescope. Its defects are that it has no real focus where cross wires can be placed, and that the field of view is very small. Kepler suggested the convex eye lens in 1611. Unshiner claimed to have used one in 1617, but it was Huygens who really introduced them. In the 17th century telescopes were made of great length, going up to 300 feet. Huygens also invented the compound eyepiece that bears his name, made of two convex lenses to diminish spherical aberration. But the defects of colour remained, although their cause was unknown until Newton carried out his experiments on dispersion and the solar spectrum. To overcome the spherical aberration, James Gregory of Aberdeen and Edinburgh in 1663, in his Optica Promota, proposed a reflecting speculum of parabolic form. But it was Newton about 1666 who first made a reflecting telescope, and he did it with the object of avoiding colour dispersion. Some time elapsed before reflectors were much used. Pound and Bradley used one presented to the Royal Society by Hadley in 1723. Hawksby, Bradley, and Malinot made some, but James Short of Edinburgh made many excellent Gregorian reflectors from 1732 till his death in 1768. Newton's trouble with refractors, chromatic aberration, remained insurmountable until John Dolan, born 1706, died 1761, after many experiments found out how to make an acromatic lens out of two lenses, one of crown glass, the other of flint glass, to destroy the colour. In a way originally suggested by Euler. He soon acquired a great reputation for his telescopes of moderate size, but there was a difficulty in making flint glass lenses of large size. The first actual inventor and constructor of an acromatic telescope was Chester Moore Hall, who was not in trade and did not patent it. Towards the close of the 18th century, a Swiss named Genemd at last succeeded in producing larger flint glass discs free from stray. Fraunhofer of Munich took him up in 1805 and soon produced, among others, Struve's Dorpat Refractor of 9.9 inches diameter and 13.5 feet focal length, and another of 12 inches diameter and 18 feet focal length for Le Mans of Munich. In the 19th century, gigantic reflectors have been made. LaSalle's two foot reflector made by himself did much good work and discovered four new satellites. But Lord Ross's six foot reflector, 54 feet focal length, constructed in 1845, is still the largest ever made. The imperfections of our atmosphere are against the use of such large apertures, unless it be on high mountains. During the last half century, excellent specula have been made of silvered glass, and Dr. Common's five foot speculum, removed since his death to Harvard, has done excellent work. Then there are the five foot Urquese reflector at Chicago and the four foot by Grubb at Melbourne. Passing now from these large reflectors to refractors, further improvements have been made in the manufacture of glass by chance of Birmingham, Fail and Mantois of Paris, and Shot of Jenna, while specialists in grinding lenses like Alvin Clark of the USA and others have produced many large refractors. Cook of York made an object glass 25 inch diameter for Newell of Gateshead, which has done splendid work at Cambridge. We have the Washington 26 inch by Clark, the Vienna 27 inch by Grubb, the Nice 29 and a half inch by Gutierre, the Polkoa 30 inch by Clark. Then there was the sensation of Clark's 36 inch for the Lick Observatory in California and finally his tour de force, the Urquese 40 inch refractor for Chicago. At Greenwich there is the 28 inch photographic refractor and the Thompson Equatorial by Grubb carrying both the 26 inch photographic refractor and the 30 inch reflector. At the Cape of Good Hope we find Mr. Frank McLean's 24 inch refractor with an object glass prism for spectroscopic work. It would be out of place to describe here the practical adjuncts of a modern equatorial, the adjustments for pointing it, the clock for driving it, the position micrometer and various eyepieces, the photographic and spectroscopic attachments, the revolving domes, observing seats and rising floors and different forms of mounting, the ciderostats and sealostats and other convenient adjuncts besides the registering chronograph and numerous facilities for aiding observation. On each of these a chapter might be written but the most important part of the whole outfit is the man behind the telescope and it is with him that a history is more especially concerned. Since the invention of the telescope no discovery has given so great an impetus to astronomical physics as the spectroscope and in giving us information about the systems of stars and their proper motions it rivals the telescope. Frauenhofer at the beginning of the 19th century while applying Dollard's discovery to make large acromatic telescopes studied the dispersion of light by a prism, admitting the light of the sun through a narrow slit in a window shutter an inverted image of the slit can be thrown by a lens of suitable focal length on the wall opposite. If a wedge or prism of glass be interposed the image is deflected to one side but as Newton had shown the images formed by the different colors of which white light is composed are deflected to different extents the violet most the red least the number of colors forming images is so numerous as to form a continuous spectrum on the wall with all the colors red orange yellow green blue indigo and violet but Frauenhofer found with a narrow slit well focused by the lens that some colors were missing in the white light of the sun and these were shown by dark lines across the spectrum these are the Frauenhofer lines some of which he named by the letters of the alphabet the D line is a very marked one in the yellow these dark lines in the solar spectrum had already been observed by Walliston on examining artificial lights it was found that incandescent solids and liquids including the carbon glowing in a white gas flame give continuous spectra gases except under enormous pressure give bright lines if sodium or common salt be thrown on the colorless flame of a spirit lamp it gives a yellow color and its spectrum is a bright yellow line agreeing in position with line D of the solar spectrum in 1832 Sir David Brewster found some of the solar black lines increased in strength towards sunset and attributed them to absorption in the earth's atmosphere he suggested that the others were due to absorption in the sun's atmosphere thereupon professor J.D. Forbes pointed out that during a nearly total eclipse the lines ought to be strengthened in the same way as that part of the sun's light coming from its edge passes through a great distance in the sun's atmosphere he tried this with the annular eclipse of 1836 with a negative result which has never been accounted for and which seemed to condemn Brewster's view in 1859 Kerchoff on repeating Frauhoffer's experiment found that if a spirit lamp with salt in the flame were placed in the path of the light the black D line is intensified he also found that if he used a limelight instead of the sunlight and passed it through the flame with salt the spectrum showed the D line black or the vapor of sodium absorbs the same light that it radiates this proved to him the existence of sodium in the sun's atmosphere iron calcium and other elements were soon detected in the same way extensive laboratory researches still incomplete have been carried out to catalog according to their wavelength on the undulatory theory of light all the lines of each chemical element under all conditions of temperature and pressure at the same time all the lines have been catalogued in the light of the sun and the brighter of the stars another method of obtaining spectra had long been known by transmission through or reflection from a grating of equidistant lines ruled upon glass or metal ha Rowland developed the art of constructing these gratings which requires great technical skill and for this astronomers owe him a debt of gratitude in 1842 Doppler proved that the color of a luminous body like the pitch or note of a sounding body must be changed by velocity of approach or recession everyone has noticed on a railway that on meeting a locomotive whistling the note is lowered after the engine has passed the pitch of a sound or the color of a light depends on the number of waves striking the ear or eye in a second this number is increased by approach and lowered by recession thus by comparing the spectrum of a star alongside a spectrum of hydrogen we may see all the lines and be sure that there is hydrogen in the star yet the lines in the star spectrum may be all slightly displaced to one side of the lines of the comparison spectrum if towards the violet end it means mutual approach of the star and earth if to the red end it means recession the displacement of lines does not tell us whether the motion is in the star the earth or both the displacement of the lines being measured we can calculate the rate of approach or recession in miles per second in 1868 Huggins succeeded in thus measuring the velocities of stars in the direction of the line of sight in 1873 Vogel compared the spectra of the sun's east approaching limb and west receding limb and the displacement of lines endorsed the theory this last observation was suggested by Zollner end of chapter 11 chapter 12 of history of astronomy this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org recording by Justin Ordway history of astronomy by George Forbes chapter 12 the sun one of Galileo's most striking discoveries when he pointed his telescope to the heavenly bodies was that of the irregularly shaped spots on the sun with the dark central umbra and the less dark but more extensive penumbra surrounding it sometimes with several umbrae in one penumbra he has left us many drawings of these spots and he fixed their period of rotation as a lunar month it is not certain whether Galileo Fabricius or Schermer was the first to see the spots they all did good work the spots were found to be ever varying in size and shape sometimes when a spot disappears at the western limb of the sun it is never seen again in other cases after a fortnight it reappears at the eastern limb the faculae or bright areas which are seen all over the sun's surface but especially in the neighborhood of spots and most distinctly near the sun's edge were discovered by Galileo a high telescopic power resolves their structure into an appearance like willow leaves or rice grains fairly uniform in size and more marked than on other parts of the sun's surface speculations as to the cause of sunspots has never ceased from Galileo's time to ours he supposed them to be clouds shinor said they were the indications of tumultuous movements occasionally agitating the ocean of liquid fire of which he supposed the sun to be composed a wilson of glasgow in 1769 noticed a movement of the umbra relative to the penumbra in the transit of the spot over the sun's surface exactly as if the spot were a hollow with a black base and gray shelving sides this was generally accepted but later investigations have contradicted its universality regarding the cause of these hollows wilson said quote whether their first production and subsequent numberless changes depend upon the erectation of elastic vapors from below or upon eddies or whirlpools commencing at the surface or upon the dissolving of the luminous matter in the solar atmosphere as clouds are melted and again given out by our air or if the reader pleases upon the annihilation and reproduction of parts of this resplendent covering is left for theory to guess at end quote ever since that date theory has been guessing at it the solar astronomer is still applying all the instruments of modern research to find out which of these suppositions or what modification of any of them is nearest the truth the obstacle one that is perhaps fatal to a real theory lies in the impossibility of reproducing comparative experiments in our laboratories or in our atmosphere sir william herschel propounded an explanation of wilson's observation which received much notice but which out of respect for his memory is not now described as it violated the elementary laws of heat sir john herschel noticed that the spots are mostly confined to two zones extending to about 35 degrees on each side of the equator and that a zone of equatorial comms is free from spots but it was rc carrington who by his continuous observations at redhill and surrey established the remarkable fact that while the rotation period in the highest latitudes fifth degrees where the spots are seen is 27 and a half days near the equator the period is only 25 days his splendid volume of observations of the sun led to much new information about the average distribution of spots at different epochs schwaab of desau began in 1826 to study the solar surface and after many years of work arrived at a law of frequency which has been more fruitful of results than any discovery in solar physics in 1843 he announced the decennial period of maxima and minima of sun spot displays in 1851 it was generally accepted and although a period of 11 years has been found to be more exact all later observations besides the earlier ones which have been hunted up for the purpose go to establish a true periodicity in the number of sunspots but quite lately shester has given reasons for admitting a number of coexistent periods of which the 11 year period was predominant in the 19th century in 1851 lament a scotchman at munich found a decennial period in the daily range of magnetic declination in 1852 sir edward sabine announced a similar period in the number of quote magnetic storms and quote affecting all of the three magnetic elements declination dip and intensity australian and canadian observations both showed the decennial period in all three elements wolf of xeric and gothy a of geneva each independently arrived at the same conclusion it took many years before this coincidence was accepted as certainly more than an accident by the old-fashioned astronomers who want rigid proof for every new theory but the last doubts have been long vanished and a connection has been further traced between the violent outburst of solar activity and simultaneous magnetic storms the frequency of the aurora borealis was found by wolf to follow the same period in fact it is closely allied in its cause to terrestrial magnetism wolf also collected old observations tracing the periodicity of sunspots back to about 1700 ad sporer deduced a law of dependence of the average latitude of sunspots on the phase of the sunspot period all modern total solar eclipse observations seem to show that the shape of the luminous corona surrounding the moon at the moment of totality has a special distinct character during the time of a sunspot maximum and another totally different during a sunspot minimum a suspicion is entertained that the total quantity of heat received by the earth from the sun is subject to the same period this would have far-reaching effects on storms harvests vintages floods and droughts but it is not safe to draw conclusions of this kind except from a very long period of observations solar photography has deprived astronomers of the type of kerrington of the delight in devoting a life's work to collecting data it has now become part of the routine work of an observatory in 1845 focalt and physo took a daguerreotype photograph of the sun in 1850 bond produced one of the moon of great beauty draper having made some attempts at an even earlier date but astronomical photography really owes its beginning to delaru who used the collodion process for the moon in 1853 and constructed the coup photo heliograph in 1857 from which date these instruments have been multiplied and have given us an accurate record of the sun's surface gelatin dry plates were first used by huggins in 1876 it is noteworthy that from the outset delaru recognized the value of stereoscopic vision which is now known to be of supreme accuracy in 1853 he combined pairs of photographs of the moon in the same phase but under different conditions regarding liberation showing the moon from slightly different points of view these in the stereoscope exhibited all the relief resulting from binocular vision and looked like a solid globe in 1860 he used successive photographs of the total solar eclipse stereoscopically to prove that the red prominences belong to the sun and not to the moon in 1861 he similarly combined two photographs of a sunspot the perspective effect showing the umbra like a floor at the bottom of a hollow penumbra and in one case the faculae were discovered to be sailing over a spot apparently at some considerable height these appearances may be partly due to a proper motion but so far as it went this was a beautiful confirmation of wilson's discovery hulet however in 1894 after 30 years of work showed that the spots are not always depressions being very subject to disturbance the coup photographs contributed a vast amount of information about sunspots and they showed that the faculae generally follow the spots in their rotation around the sun the constitution of the sun's photosphere the layer which is the principal light source of the sun has always been a subject of great interest and much was done by men with exceptionally keen eyesight like mr. dawes but it was a difficult subject owing to the rapidity of the changes in appearance of the so-called rice grains about one inch in diameter the rapid transformations and circulations of these rice grains if thoroughly studied might lead to a much better knowledge of solar physics this seemed almost hopeless as it was found impossible to identify any rice grain in the turmoil after a few minutes but m handski of bokowa whose recent death is deplored introduced successfully a scheme of photography which might also be called a solar cinematograph he took photographs of the sun at intervals of 15 or 30 seconds and then enlarged selected portions of these 200 times giving a picture corresponding to a solar disk of six meters diameter in these enlarged pictures he was able to trace the movements and changes of shape and brightness of individual rice grains some granules become larger or smaller some seem to rise out of a mist as it were and to become clearer others grow feebler some are split in two some are rotated through a right angle in a minute or less although each of the grains may be the size of great britain generally they move together in groups of very various velocities up to 40 kilometers a second these movements seem to have definite relation to any sunspots in the neighborhood from the results already obtained it seems certain that if this method of observation be continued it cannot fail to supply facts of the greatest importance it is quite impossible to do justice here to the work of all those who are engaged on astronomical physics the utmost that can be attempted is to give a fair idea of the directions of human thought and endeavor during the last half century america has made splendid progress and an entirely new process of studying the photosphere has been independently perfected by professor hail at chicago and elandra at paris they have succeeded in photographing the sun's surface in monochromatic light such as the light given off as one of the bright lines of hydrogen or of calcium by means of the spectrogelograph the spectroscope is placed with its slit in the focus of an equatorial telescope pointed to the sun so that the circular image of the sun falls on the slit at the other end of the spectroscope is the photographic plate just in front of this plate there is another slit parallel to the first in the position where the image of the first slit formed by the k line of calcium falls thus is obtained a photograph of the section of the sun made by the first slit only in k light as the image of the sun passes over the first slit the photographic plate is moved at the same rate and in the same direction behind the second slit and as successive sections of the sun's image in the equatorial enter the apparatus so are these sections successively thrown in their proper place on the photographic plate always in k light by using a high dispersion the faculae which give off k light can be correctly photographed not only at the sun's edge but all over his surface the actual mechanical method of carrying out the observation is not quite so simple as what is here described by choosing another line of the spectrum instead of calcium k for example the hydrogen line h subscript 3 we obtain two photographs one showing the appearance of the calcium flocculi and the other of the hydrogen flocculi on the same part of the solar surface and nothing is more astonishing than to note the total want of resemblance in the form shown on the two this mode of research promises to afford many new and useful data the spectroscope has revealed the fact that broadly speaking the sun is composed of the same materials as the earth angstrom was the first to map out all of the lines to be found in the solar spectrum but raland of baltimore after having perfected the art of making true gradings with equidistant lines ruled on metal for producing spectra then proceeded to make a map of the solar spectrum on a large scale in 1866 lock year threw an image of the sun upon the slit of a spectroscope and was thus enabled to compare the spectrum of a spot with that of the general solar surface the observation proved the darkness of a spot to be caused by increased absorption of light not only in the dark lines which are widened but over the entire spectrum in 1883 young resolved this continuous obscurity into an infinite number of fine lines which have all been traced in a shadowy way onto the general solar surface lock year also detected displacements of the spectrum lines in the spots such as would be produced by a rapid motion in the line of sight it has been found that both uprushes and downrushes occur but there is no marked predominance of either in a sunspot the velocity of motion thus indicated in the line of sight sometimes appears to amount to 320 miles a second but it must be remembered that pressure of a gas has some effect in displacing the spectral lines so we must go on collecting data until a time comes when the meaning of all the facts can be made clear total solar eclipses during total solar eclipses the time is so short and the circumstances so impressive that drawings of the appearance could not always be trusted the red prominences of jagged form that are seen around the moon's edge and the corona with its streamers radiating or interlacing have much detail that can hardly be recorded in a sketch by the aid of photography a number of records can be taken during the progress of totality from a study of these the extent of the corona is demonstrated in one case to extend to at least six diameters of the moon though the eye has traced it farther this corona is still one of the wonders of astronomy and leads to many questions what is its consistency if it extends many million miles from the sun's surface how is it that it opposed no resistance to the motion of comets which have almost grazed the sun's surface is this the origin of the zodiacal light the character of the corona and photographic records has been shown to depend upon the phase of the sunspot period during the sunspot maximum the corona seems most developed over the spot zones i.e. neither at the equator nor the poles the four great sheaves of light give it a square appearance and are made up of rays or plumes delicate like the petals of a flower during a minimum the nebulous ring seems to be made of tuffs of fine hairs with a grits or radiations from both poles and streamers from the equator on september 19 1868 eclipse spectroscopy began with the indian eclipse in which all observers found that the red prominences showed a bright line spectrum indicating the presence of hydrogen and other gases so bright was it that jansen exclaimed quote je vais les ses lignes en deux eau d'éclipse and the next day he observed the lines at the edge of the une eclipse sun huggins had suggested this observation in february 1868 his idea being to use prisms of such great dispersive power that the continuous spectrum reflected by our atmosphere should be greatly weakened while a bright line would suffer no diminution by the high dispersion on october 20 lock year having news of the eclipse but not of jansen's observations the day after was able to see these lines this was a splendid performance for enabled the prominences to be observed not only during eclipses but every day moreover the next year huggins was able by using a wide slit to see the whole of a prominence and note its shape prominences are classified according to their form into flame and cloud prominences the spectrum of the latter showing calcium hydrogen and helium that of the former including a number of metals the d line of sodium is a double line and in the same eclipse 1868 an orange line was noticed which was afterwards found to lie close to the two components of the d line it did not correspond with any known terrestrial element and the unknown element was called helium it was not until 1895 that sir william ramsey found this element as a gas in the mineral cleavite the spectrum of the corona is partly continuous indicating light reflected from the sun's body but it also shows a green line corresponding with no known terrestrial element and the name coronium has been given to the substance causing it a vast number of facts have been added to our knowledge about the sun by photography and the spectroscope speculations and hypotheses in plenty have been offered but it may be long before we have a complete theory evolved to explain all the phenomena of the storm swept metallic atmosphere of the sun the proceedings of scientific societies team with such facts and working hypotheses and the best of them have been collected by miss clerk and her history of astronomy during the 19th century as to established facts we learn from the spectroscopic research is that the continuous spectrum is derived from the photosphere where solar gaseous material compressed almost to liquid consistency that the reversing layer surrounds it and gives rise to black lines in the spectrum that the chromosphere surrounds this is compressed mainly of hydrogen and is the cause of the red prominences in eclipses and that the gaseous corona surrounds all of these and extends to vast distances outside the sun's visible surface end of chapter 12 the sun recording by Justin Ordway chapter 13 of history of astronomy this is a libra vox recording all libra vox recordings are in the public domain for more information or to volunteer please visit libra vox.org recording by Courtney Miller history of astronomy by George Forbes the moon in planets the moon telescopic discoveries about the moon commence with Galileo's discovery that her surface has mountains and valleys like the earth he also found that while she always turns the same face to us there is periodically a slight twist to let us see a little round the eastern or western edge this was called liberation and the explanation was clear when it was understood that in showing always the same face to us she makes one revolution a month on her axis uniformly and that her revolution around the earth is not uniform Galileo's said that the mountains on the moon showed greater differences of level than those on the earth Schroeder supported this opinion W. Herschel opposed it but beer and mildler measured the heights of lunar mountains by their shadows and found four of them over 20 000 feet above the surrounding plains land greenest was the first to do serious work on selenography and named the lunar features after eminent men ritchole also made lunar charts in 1692 Cassini made a chart of the full moon since then we have the charts of schroeder beer and modler 1837 and of schmitt of Athens 1878 and above all the photographic atlas by loy and pusa the details of the moon's surface require for their discussion a whole book like that of nissan or the one by nasmyth and carpenter here a few words must suffice mountain ranges like our andes or himalayas are rare instead of that we see an immense number of circular cavities with rugged edges and flat interior often with a cone in the center reminding one of instantaneous photographs of the spash of a drop of water falling into a pool many of these are 50 or 60 miles across some more they are generally spoken of as resembling craters of volcanoes active or extinct on the earth but some of those who have most fully studied the shapes of craters denial together their resemblance to the circular objects on the moon these so-called craters in many parts are seen to be closely grouped especially in the snow white parts of the moon but there are great smooth dark spaces like the clear black ice on a pond more free from craters to which the equally inappropriate name of seas has been given the most conspicuous crater taiko is near the south pole at full moon there are seen to radiate from taiko numerous streaks of light or rays cutting through all the mountain formations and extending over fully half the lunar disk like the star-shaped cracks made on a sheet of ice by a blow similar cracks radiate from other large craters it must be mentioned that these white rays are well seen only in full light of the sun at full moon just as the white snow in the crevasses of a glacier is seen bright from a distance only when the sun is high and disappears at sunset then there are deep narrow crooked rills which may have been water courses also clefts about half a mile wide and often hundreds of miles long like deep cracks in the surface going straight through mountain and valley the moon shares with the sun the advantage of being a good subject for photography though the planets are not this is owing to her larger apparent size and the abundance of illumination the consequence is that the finest details of the moon as seen in the largest telescope in the world may be reproduced at a cost within the reach of all no certain changes have ever been observed but several suspicions have been expressed especially as to the small crater line in the mayor's serenity tattis it is now generally agreed that no certainty can be expected from drawings and that for real evidence we must await the verdict of photography no trace of water or of an atmosphere has been found on the moon it is possible that the temperature is too low in any case no displacement of a star by atmospheric refraction at all cultivation has been surely recorded the moon seems to be dead the distance of the moon from the earth is just now the subject of remeasurement the baseline is from grenich to cape of good hope and the new feature introduced is the selection of a definite point on a crater moosting a instead of the moon's edge as the point whose distance is to be measured the inferior planets when the telescope was invented the faces of venus attracted much attention but the brightness of this planet and her proximity to the sun as with mercury also seemed to be a bar to the discovery of markings by which the axis in period of rotation could be fixed casini gave the rotation as 23 hours by observing a bright spot on her surface shrewder made it 23 hours 21 minutes and 19 seconds this value was supported by others in 1890 scaparelli announced that venus rotates like our moon once in one of her revolutions and always directs the same face to the sun this property has also been described to mercury but in neither case has the evidence been generally accepted 24 hours is probably about the period of rotation for each of these planets several observers have claimed to have seen a planet within the orbit of mercury either in transit over the sun's surface or during an eclipse it has even been named vulcan these announcements would have received little attention but for the fact that the motion of mercury has irregularities which have not been accounted for by known planets and laverier has stated that an intramercurial planet or ring of asteroids would account for the unexplained part of the motion of the line of apsis of mercury's orbit amounting to 38 inches per century mars the first study of the appearance of mars by morality led him to believe that there were changes proceeding in the two white caps which are seen at the planet's poles w hershel attributed these caps to ice and snow and the dates of his observations indicated a melting of these ice caps in the martian summer shrewder attributed the other markings on mars to drifting clouds but bier and modler in 1830 to 39 identified the same dark spots as being always in the same place though sometimes blurred by mist in the local winter a spot sketched by hoggins in 1672 one frequently seen by w hershel in 1783 another by orago in 1813 and nearly all the markings recorded by bier and modler in 1830 were seen and drawn by f kaiser in laden during 17 nights of the opposition of 1862 asked noct number 1468 when he deduced the period of rotation to be 24 hours 37 minutes 22 seconds 62 or one tenth of a second less than the period deduced by r.a proctor from a drawing by hook in 1666 it must be noted that if the periods of rotation both of mercury and venus be about 24 hours as seems probable all the four planets nearest to the sun rotate in the same period while the great planets rotate in about 10 hours new anus and neptune being still indeterminate the general service of mars is a deep yellow but there are dark gray or greenish patches sir john hershel was the first to attribute the ready color of mars to its soil rather than to its atmosphere the observations of that keen-sighted observer does led to the first good map of mars in 1869 in the 1877 opposition scaparelli revived interest in the planet by the discovery of canals uniformly about 60 miles wide running generally on great circles some of them being three or four thousand miles long during the opposition of 1881 to 82 the same observer re-observe the canals and in 20 of them he found the canals duplicated the second canal being always 200 to 400 miles distant from its fellow the existence of these canals has been doubted mr. loll has now devoted years to the subject has drawn them over and over again and has photographed them and accepts the explanation that they are artificial and that vegetation grows on their banks thus is revived the old controversy between wool and booster as to the habitability of the planets the new arguments are not yet generally accepted loll believes he has with the spectroscope proved the existence of water on mars one of the most unexpected and interesting of all telescopic discoveries took place in the opposition of 1877 when mars was unusually near to the earth the washington observatory had acquired the fine 26-inch refractor and as of hall search for satellites concealing the planet's disk to avoid the glare on august 11th he had a suspicion of a satellite this was confirmed on the 16th and on the following night a second one was added they are exceedingly faint and can be seen only by the most powerful telescopes and only at the times of opposition their diameters are estimated at six or seven miles it was soon found that the first dimus completes its orbit in 30 hours and 18 minutes but the other fubos at first was a puzzle owing to its incredible velocity being unsuspected later it was found that the period of revolution was only seven hours 39 minutes and 22 seconds since the martian day is 24 and a half hours this leads to remarkable results obviously the easterly motion of the satellite overwhelms the diurnal rotation of the planet and fubos must appear to the inhabitants if they exist to rise in the west and set in the east showing two or even three full moons in a day so that sufficiently well for the ordinary purposes of life the hour of the day can be told by its phases the discovery of these two satellites is perhaps the most interesting telescopic visual discovery made with the large telescopes of the last half century photography having been the means of discovering all the other new satellites except jupiter's fifth in order of discovery jupiter galileo's discovery of jupiter's satellites was followed by the discovery of his belts zuki and torricelli seem to have seen them fontana in 1633 reported three belts in 1648 grimaldi saw a bit too and noticed that they lay parallel to the ecliptic dusky spots were also noticed as transient hook measured the motion of one in 1664 in 1665 casini with a fine telescope 35 feet focal length observed many spots moving from east to west once he concluded that jupiter rotates on an axis like the earth he watched an unusually permanent spot during 29 rotations and fixed the period at nine hours 56 minutes later he inferred that spots near the equator rotate quicker than those in higher latitudes the same as kerington found for the sun and w hershel confirmed this in 1778 to nine jupiter's rapid rotation ought according to newton's theory to be accompanied by a great flattening at the poles casini had noted an oval form in 1691 this was confirmed by lahir rumor and picard pound measured the ellipticity equals 1 over 13.25 w hershel supposed the spots to be masses of cloud in the atmosphere an opinion still accepted many of them were very permanent casini's great spot vanished and reappeared nine times between 1665 and 1713 it was close to the northern margin of the southern belt hershel supposed the belts to be the body of the planet and the lighter parts to be clouds confined to certain latitudes in 1665 casini observed transits of the four satellites and also saw their shadows on the planet and worked out a lunar theory for jupiter mathematical astronomers have taken great interest in the perturbations of the satellites because their relative periods introduce peculiar effects airy in his delightful book gravitation has reduced these investigations to simple geometrical explanations in 1707 and 1713 morality noticed that the fourth satellite varies much in brightness w hershel found this variation to depend upon its position in its orbit and concluded that in the positions of feebleness it is always presenting to us a portion of its surface which does not well reflect the sun's light proving that it always turns the same face to jupiter as is the case with our moon this fact has also been established for Saturn's fifth satellite and may be true for all satellites in 1826 struve measured the diameters of the four satellites and found them to be 2429 2180 3561 and 3046 miles in modern times much interest has been taken in watching a rival to casini's famous spot the great red spot was first observed by niesten pritchett and temple in 1878 as a rosy cloud attached to a whitish zone beneath the dark southern equatorial band shaped like the new war balloons 30 000 miles long and 7 000 miles across the next year it was brick red a white spot beside it completed a rotation in less time by five and a half minutes than the red spot a difference of 260 miles an hour thus they came together again every six weeks but the motions did not continue uniform the spot was feeble in 1882.4 brightened in 1886 and after many changes is still visible galileo's great discovery of jupiter's four moons was the last word in this connection until september 9th 1892 when barnard using the 36 inch refractor of the lick observatory detected a tiny spot of light closely following the planet this proved to be a new satellite fifth nearer to the planet than any other and revolving around it in 11 hours 57 minutes and 23 seconds between its rising and setting there must be an interval of two and a half jovian days and two or three full moons the sixth and seventh satellites were found by the examination of photographic plates at the lick observatory in 1905 since which time they have been continuously photographed and their orbits traced at greenwich on examining these plates in 1908 mr. malott detected the eighth satellite which seems to be revolving in a retrograde orbit three times as far from its planet as the next one seventh in these two points agreeing with the outermost of satan satellites phoebe saturn this planet with its marvelous ring was perhaps the most wonderful object of those first examined by galileo's telescope he was followed by dominique casini who detected bands like jupiter's belts hershal established the rotation of the planet in 1775 to 94 from observations during 100 rotations he found the period to be 10 hours 16 minutes 0 seconds 44 hershal also measured the ratio of the polar to the equatorial diameter as 10 to 11 the ring was a complete puzzle to galileo most of all when the planet reached a position where the plane of the ring was in line with the earth and the ring disappeared december 4 1612 it was not until 1656 that hoggins in his small pamphlet de saturni luna observatio nova was able to suggest in a cipher the ring form and in 1659 in his system a saturnium he gave his reasons and translated the cipher the planet is surrounded by a slender fat ring everywhere distinct from its surface and inclined to the ecliptic this theory explained all the phases of the ring which had puzzled others this ring was then and has remained ever since a unique structure we in this age have got accustomed to it but hoggins discovery was received with amazement in 1675 casini found the ring to be double the concentric rings being separated by a black band a fact which was placed beyond dispute by hershal who also find that the thickness of the ring subtends an angle less than zero inches 0.3 shrewder estimated its thickness at 500 miles many speculations have been advanced to explain the origin and constitution of the ring de sejour said that it was thrown off from Saturn's equator as a liquid ring and afterwards solidified he noticed that the outside would have a greater velocity and be less attracted to the planet than the inner parts and that equilibrium would be impossible so he supposed it to have solidified into a number of concentric rings the exterior ones having the least velocity clerk Maxwell in the atoms prize essay gave a physical mathematical demonstration that the rings must be composed of meteoritic matter like gravel even so there must be collisions absorbing the energy of rotation and tending to make the rings eventually fall into the planet the slower motion of the external parts has been proved by the spectroscope in keeler's hands 1895 saturn has perhaps received more than its share of attention owing to these rings this led to other discoveries hoggins in 1655 and JD Cassini in 1671 discovered the sixth and eighth satellites titan and japetus Cassini lost his satellite and in searching for it found rhea the fifth in 1672 besides his old friend whom he lost again he added the third and fourth in 1684 teethus and dyouni the first and second memos and insulatus were added by her shall in 1789 and the seventh hyperion simultaneously by lasso and bond in 1848 the ninth phoebe was found on photographs by pickering in 1898 with retrograde motion and he has lately added a tenth the occasional disappearance of cassini's japetus was found on investigation to be due to the same causes as that of jupiter's fourth satellite and proves that it always turns the same face to the planet uranus and Neptune the splendid discoveries of uranus and two satellites by Sir William Herschel in 1787 and of Neptune by atoms and laverier in 1846 have been already described lasso added two more satellites to uranus in 1851 and found Neptune satellite in 1846 all of the satellites of uranus have retrograde motion and their orbits are inclined about 80 degrees to the ecliptic the spectroscope has shown the existence of an absorbing atmosphere on jupiter and saturn and there are suspicions that they partake something of the character of the sun and emit some light besides reflecting solar light on both planets some absorption lines seem to agree with the aqueous vapor lines of our own atmosphere while one which is a strong band in the red common to both planets seems to agree with a line in the spectrum of some reddish stars uranus and neptune are difficult to observe spectroscopically but appear to have peculiar spectra agreeing together sometimes uranus shows frownhofer lines indicating reflected solar light but generally these are not seen and six broad bands of absorption appear one is the f of hydrogen another is the red starline of jupiter and saturn neptune is a very difficult object for the spectroscope quite lately p lull has announced that vm slifer at flake staff observatory succeeded in 1907 in rendering some plates sensitive far into the red a reproduction is given of photograph spectra of the four outermost planets showing one a great number of new lines and bands two intensification of hydrogen f and c lines three a steady increase of effects one and two as we pass from jupiter and saturn to uranus and a still greater increase in neptune asteroids the discovery of these new planets has been described at the beginning of the last century it was an immense triumph to catch a new one since photography was called into the service by wolf they have been caught every year in shoals it is like the difference between sea fishing with the line and using a steam trawler in the 1908 almanacs nearly 700 asteroids are included the computation of their perturbations and ephemerides by eulers and lagrange's method of variable elements became so laborious that enki devised a special process for these which can be applied to many other disturbed orbits when a photograph is taken of a region of the heavens including an asteroid the stars are photographed as points because the telescope is made to follow their motion but the asteroids by their proper motion appear short lines the discovery of eros and the photographic attack upon its path have been described in their relation to finding the sun's distance a group of four asteroids has lately been found with a mean distance in period equal to that of jupiter to three of these masculine names have been given hector patrickless achilles the other has not yet been named end of chapter 13 recording by courtney miller chapter 14 of history of astronomy this is a libre vox recording all libre vox recordings are in the public domain for more information or to volunteer please visit libre vox.org history of astronomy by george forbs chapter 14 comments and meteors ever since hailey discovered that the comet of 1682 was a member of the solar system these wonderful objects have had a new interest for astronomers and a comparison of orbits has often identified the return of a comet and led to the detection of an elliptic orbit where the difference from a parabola was imperceptible in the small portion of the orbit visible to us a remarkable case in point was the comet of 1556 of whose identity were the comet of 1264 there could be little doubt hide wanted to compute the orbit more exactly than hailey had done he knew that observations had been made but they were lost having expressed his desire for a search all the observations of fabrisias and of heller and also a map of the comet's path among the stars were eventually unearthed in the most unlikely manner after being lost nearly 300 years hind and others were certain that this comet would return between 1844 and 1848 but it never appeared when the spectroscope was first applied to finding the composition of the heavenly bodies there was a great desire to find out what comets are made of the first opportunity came in 1864 when donati observed the spectrum of a comet and saw three bright bands thus providing that it was a gas in at least partly self-luminous in 1868 huggins compared the spectrum of winnicki's comet with that of a with that of a geisler tube containing olefiant gas and found exact agreement nearly all comets have shown the same spectrum a very few comets have given bright band spectra differing from the normal type also a certain kind of continuous spectrum as well as reflected solar light showing frauenhafer lines have been seen when well's comet in 1882 approached very close indeed to the sun the spectrum changed to a monochromatic yellow color due to sodium for a full account of the wonders of the cometary world the reader is referred to books on descriptive astronomy or to monographs on comets nor can the very uncertain speculations about the structure of comets tails be given here a new explanation has been proposed almost every time that a great discovery has been made in the theory of light heat chemistry or electricity hailey's comet remain the only one of which a prediction of the return had been confirmed till the orbit of the small ill-defined comet found by ponds in 1819 was computed by enki and found to have a period of three and one-third years it was predicted to return in 1822 and was recognized by him as identical with many previous comets this comet called after enki has shown in each of its returns an inexplicable reduction of mean distance which led to the assertion of a resisting medium in space until a better explanation could be found since that date 14 comets have been found with elliptical orbits whose appealing distances are all about the same as jupiter's mean distance and six have an appealing distance about 10 percent greater than neptune's mean distance other comets are similarly associated with the planet Saturn and Uranus the physical transformations of comets are among the most wonderful of unexplained phenomena in the heavens but for physical astronomers the greatest interest attaches to the reduction of radius vector of enki's comet the splitting of biela's comet into two comets in 1846 and the somewhat similar behavior of other comets it must be noted however that comets have a sensible size that all their parts cannot travel in exactly the same orbit under the sun's gravitation that their mass is not sufficient to retain the parts together very forcibly also that the inevitable collision of particles or else fluid friction is absorbing energy and so reducing the comet's velocity in 1770 lexel discovered a comet which as was afterwards proved by investigations of lexel burkhard and leplace had in 1767 been deflected by jupiter out of an orbit in which it was invisible from the earth into an orbit with a period of five and a half years enabling it to be seen in 1779 it again approached jupiter closer than some of his satellites and was sent off in another orbit never to be again recognized but our interest in cometary orbits has been added to by the discovery that owing to the causes just cited a comment if it does not separate into discrete parts like biela's must in time have its parts spread out so as to cover a sensible part of the orbit and that when the earth passes through such part of a comet's orbit a meteor shower is the result a magnificent meteor shower was seen in america on november 12 13 1833 when the pass of the meteors all seemed to radiate from a point in the constellation leo a similar display had been witnessed in mexico by humble and bomb plan on november 12 1799 ha newton traced such records back to october 13 ad 902 the orbital motion of a cloud or stream of small particles was indicated the period favored by ha newton was 354 and a half days another suggestion was 375 and a half days and another 33 and one fourth years he noticed that the events of the date of the shower between 902 and 1833 at the rate of one day in 70 years meant a progression of the node of the orbit adams undertook to calculate what the amount would be on all the five suppositions that had been made about the period after a laborious work he found that none gave one day in 70 years except the 33 and one fourth year period which did so exactly ha newton predicted a return of the shower on the night of november 13 14 1866 he is now dead but many of us are alive to recall the wonder and enthusiasm with which we saw this prediction being fulfilled by the grandest display of meteors ever seen by anyone now alive the progression of the nodes proved the path of a meteor stream to be retrograde the radiant had almost the exact longitude of the point towards which the earth was moving this proved that the meteor cluster was at perihelion the period being known the eccentricity of the orbit was obtainable also the orbital velocity of the meteors in perihelion and by comparing this with the earth's velocity the latitude of the radiant enabled the inclination to be determined while the longitude of the earth that night was the longitude of the node in such a way she apparently was able to find first the elements of the orbit of the august meteor shower pursuits and show its identity with the orbit of tuddles comet 1862 then in january 1867 la verie gave the elements of the november meteor shower leonides and peters of altona identified these with opulsors elements for temples comet 1866 she apparently having independently attained both of these results subsequently weiss of vienna identified the meteor shower of april 20th lyrets with a comet 1861 finally that indefatigable worker on meteors as hershel added to the number and in 1878 gave a list of 76 coincidences between cometary and meteoric orbits cometary astronomy is now largely indebted to photography not merely for accurate delineations of shape but actually for the discovery of most of them the art has also been applied to the observation of comets at distances from their perihelia so great as to prevent their visual observation thus says wolf of heidelberg found upon old plate's position of comets 1905 as a star of the 15.5 magnitude 783 days before the date of its discovery from the point of view of the importance of finding out the divergence of a cometary orbit from a parabola its period and its and its appealing distance this increase of range attains the very highest value the present astronomer royal appreciating this possibility has been searching by photography for haley's comet since november 1907 although its perihelium passage will not take place until april 1910 end of chapter 14