 A comet is a small solar system object made of a mixture of frozen water, ammonia, and various hydrocarbons such as methane. When passing close to the sun, it heats up and begins to out gas, displaying a visible atmosphere or coma. And sometimes a tail, the coma pushed back by the solar wind. You can see the jets in this close-up photo of comet Hartley-2. Comet nuclei range from 100 meters to tens of kilometers across. The coma and tail are much larger, and if sufficiently bright, may be seen from Earth without a telescope, like this image of comet ISON or ISON. From ancient Greece to the middle of the 16th century, comets were thought to be luminous vapors in the Earth's atmosphere. Here's a tapestry that illustrates the 1066 comet. Here's the great comet of 1577, as seen over Prague. Tycho Braugh studied this very bright comet that's shown in the night sky for 74 days. He found that he could not see any parallax. From his data, he concluded that the comet must be at least six times further away from the Earth than the Moon. This took it out of the Earth's atmosphere and started people thinking differently about comets and planets. Remember that the Ptolemy model had rotating spheres holding each planet in place. Tycho's findings would have comets crashing through these crystal spheres. As you can imagine, this tipped the scales in favor of the Copernican sun-centric model and left open the question about just what holds the comets up. Using the great comet of 1680, Edmund Halley worked with Isaac Newton to find out if comets were subject to the same forces as Newton had proposed for the planets. The data showed that the long elliptical paths of the comets fit Newton's theory of gravity perfectly. In 1705 Halley studied the recorded paths of the comets of 1531, 1607, and 1682. He proposed that they were all reappearances of the same comet and that it would be back again in 1758. It was. This was a spectacular vindication of his bold conjecture and of Newton's gravitational theory. For his success, the comet was named after him, Halley's comet. I saw it in 1986. Its orbit goes out past Pluto, so it won't be back again until 2061.