 This History of Science video will be mostly about the Copernican Revolution in astronomy, or rather about several common misunderstandings about the Copernican Revolution. So the Copernican Revolution was introduced by Copernicus around about 1514 and was opposed to the system due to Potolomy which had been around since about for about 1500 years. So the Copernican system had the sun at the centre of the solar system with the earth moving around it and the Potolomy system had the earth at the centre of the solar system with the sun and all planets moving around it. And to reason, the Copernicus was simpler and gave a better fit to observation and was more correct because it predicted the earth was moving whereas Potolomy's system incorrectly predicted the earth wasn't moving. Well almost everything I've said about Copernicus's system is wrong as I will now explain. First of all it wasn't really due to Copernicus. It was first introduced by Aristarchus or possibly by somebody even before that probably about 16 or 17 centuries before Copernicus. So Aristarchus was a Greek astronomer and the Greeks had managed to measure the size of the earth and the distance to the moon and the distance to the sun and they got the size of the earth pretty much correct and they got the distance to the moon and the size of the moon pretty much correct and they got the distance to the sun rather wrong because measuring the distance to the sun was a very very difficult measurement. They had to measure the angle between the earth, moon axis and the earth, sun axis at a very precise time when the moon was shadow of the moon covered exactly half the moon and this involves measuring an angle extremely close to 90 degrees and they couldn't really quite get it right. It's a very difficult measurement. Anyway, although they didn't get the distance to the sun right they got it right enough that they figured out the sun was very much bigger than the earth and Aristarchus had this idea that if the sun was much bigger than the earth then maybe it was the earth that was orbiting the sun rather than the sun that was orbiting the earth. Anyway this was sort of more or less forgotten for many centuries until Copernicus came along Copernicus incidentally correctly credited Aristarchus with this idea Copernicus didn't claim his ideas being original he was reviving an old idea due to Greek astronomers. So the first problem is did Copernicus's theory give a better fit to observation than Potolomy's theory and the answer is very clear it gave a worse fit to observation. The problem is stellar parallax suppose you've got a star now if the earth is moving around the sun the angle from the earth to the star should vary with a sort of yearly period because the earth is sort of moving backwards and forwards by a few hundred million kilometers and this angle is changing the angle of the stars called stellar parallax and there wasn't any stellar parallax people looked for it and it just wasn't there. It wasn't discovered wasn't measured until more than three centuries after Copernicus came up with this theory Bessel he's the guy who invented Bessel functions in 1838 finally managed to measure stellar parallax. The problem is it's very small and need very sensitive instruments to measure it. Galileo did correctly suggest that the reason why stellar parallax wasn't observed is that the stars were so far away that it was too small to measure but you could perfectly well say that Galileo was just inventing this ad hoc excuse for why Copernicus's theory failed. Incidentally as everybody knows Galileo got into serious trouble with the Catholic Church for claiming the earth went round the sun. Well that's probably not the real reason he got into trouble. I mean Copernicus had claimed the earth went round the sun and no one was really that bothered by it. The real reason Galileo got into trouble appears to be that he sort of publicly mocked and insulted some rather powerful people who disagreed with him and as these powerful people were also rather powerful people in the Catholic Church they were able to use the Catholic Church in order to get at their enemies so he seems to have, one gets the impression he was in trouble more because of scientific rivalry rather than because of any theological problems. But anyway, the next question is was Copernicus's theory simpler than Potolomy's theory and the answer is no it wasn't really. So Potolomy's theory needed a lot of epicycles in order to make sense of observations. So what is an epicycle? Well if you take the earth, well an epicycle you have you think of a circle going round the earth and then on this circle there's another thing going round in a circle around that point and then on this there's another thing going round in a circle about that so it's very complicated. You have this thing going round in a circle and this thing going round in a circle maybe a planet is going round on this thing so the planet is going round on some complicated path and Potolomy needed quite a lot of epicycles in order to fit the movements of planets so this whole system looked like a rather complicated mess and you can say isn't Copernicus's system much simpler because all he had was the sun and a lot of planets moving in circles around the sun except that didn't work. If you take the sun as being in the centre of the solar system and planets moving around on circles it simply didn't fit observations. What Copernicus had to do is he had to reintroduce epicycles again in order to make his model fit with observations so it wasn't really any simpler than Potolomy's theory both of them used large numbers of epicycles. Copernicus's system only became better than Potolomy's system when Kepler came along and pointed out that the planets seemed to be moving around not in circles around the sun but on ellipses around the sun with the sun at one of the two focuses of the ellipse and once you've done that Copernicus's system finally becomes better than Potolomy's system. It's still not completely correct of course because Newton pointed out that the planets are all perturbing each other and the sun isn't quite stationary because it's perturbed by Jupiter and Saturn and so on. So once you've added Kepler and Newton then Copernicus's theory finally becomes a reasonable fit to observations. Incidentally there's a problem that John Conway liked to ask people which is what is the orbit of the moon around the sun? So the problem is you should try and sketch roughly what the moon's orbit around the sun looks like. So you remember the moon is moving in an ellipse around the earth and meanwhile the earth is also moving in an ellipse around the sun. It looks like the orbit is going to be kind of complicated and most people when asked this question sort of get the answer quite wrong. You think it's going to be something like this. So here's the sun and maybe the moon well the earth is going to be moving around on an ellipse and the moon is going to be moving around the earth so maybe it should be looking something like this or maybe the wiggles aren't quite that big and it should be moving in a sort of bumpy curve like this and both of these are completely wrong. The moon's orbit looks nothing like that or like that. This is what the moon's orbit actually looks like. The moon's orbit looks like this. Here we have a circular template and this is the moon's orbit with the sun almost but not quite in the center. So what happened to the moon orbiting the earth? Shouldn't there be some bumps in this because the moon is going around the earth? Well, no. The distance from the moon to the earth is about a thousandth of the distance from the earth to the sun so if the earth is moving around like that the moon it will be moving in almost exactly the same path the difference will be a thousandth of this distance here which as you can see is much less than the thickness of this line so the orbit of the moon is indistinguishable from this circle. So the next question is why is this a circle not an ellipse? Didn't Kepler teach us that the earth orbits in an ellipse around the sun? Well, sort of. However, this ellipse has very small eccentricity and ellipses of small eccentricity are indistinguishable from circles to the native eye. The only difference you might be able to see is that the sun is not quite at the center but the difference is very tiny. You could probably just about make it out. It would be a fraction of a millimeter off at this scale. So if you're a really good eyesight you might just about be able to detect the sun isn't quite at the center. So the moon's orbit around the sun is for all practical purposes a circle, not some funny wiggly line. Kepler discovered planets moving in ellipses rather than circles because he was looking at the orbit of Mars and Mars's orbit has a much higher eccentricity than the earth's orbits and you really can tell it's not moving in a circle. Incidentally, the reason why Ptolemy's theory about epicycles work pretty well is because the earth's orbit being an ellipse of small eccentricity is very close to being circular and you can make it almost exact by adding another small epicycle to account for the fact the sun isn't quite at the center of the ellipse. So the earth's orbit around the sun is actually fitted very, very well by an epicycle as used by Ptolemy. The final problem with Copernicus's theory is this claim that the earth is moving. Now, saying that the earth is moving isn't really true. It isn't really false either. It's not a matter of something being absolutely true. It's a matter of which coordinate system you take. So let me give an example. Suppose you're trying to describe a spiral. Now, if you're trying to describe a spiral, it's usually easiest to use a polar coordinate system where you measure points on the spiral by radius and angle. If you try describing a spiral in a rectangular coordinate system, the equations describing it are going to be a bit of a mess. However, anybody who said a polar coordinate system is true and a rectangular coordinate system is false would be kind of stupid. Coordinate systems aren't true or false. They can be convenient or inconvenient. And you could say a polar coordinate system is convenient and a rectangular coordinate system is inconvenient, but you can't say this one is true and this one is false. And the same is true for the difference between Copernicus and botolomies theory. They're just different coordinate systems. So two coordinate systems. The first one is you take the Earth fixed. And if the Earth is fixed, the Earth isn't moving and the Sun is moving around the Earth. If you take the Sun to be fixed, then the Earth is moving in a circle and the Sun is moving around it. Sorry, the Earth is moving in a circle around the Sun. So if you want to describe positions of planets, then the coordinate system with the Earth fixed is really rather a mess. It's kind of like trying to describe spirals in rectangular coordinate systems. It's just unnecessarily complicated. So if you're an astronomer, you should use the coordinate system with the Sun being fixed. If you're trying to send a probe to Mars, so suppose you're NASA and sending a probe to Mars, then again, it's much easier if you choose a coordinate with the Sun fixed and the Earth and Mars moving. On the other hand, if you're a normal person, say you're a sailor in a sailing boat, then, and you're trying to work at which direction to go in by looking at the Sun, it's much easier to imagine the Earth being fixed and the Sun moving around it. This doesn't mean that your coordinate system with the Earth being fixed is true. It just means it's easier to work with, just as for an astronomer, this coordinate system is easier to work with. Well, you could say, well, you know, you should take a coordinate system with the Sun is the centre of mass of the solar system, so it's better to use a coordinate system at the centre of mass of the solar system. Well, why? Well, first of all, the centre of mass of the solar system isn't quite at the centre of the Sun. It's actually near the surface of the Sun because of Jupiter's influence. Secondly, what's the great thing about the Sun? Why not take the centre of mass of the entire galaxy, in which case you find a yet another coordinate system, or you could take our local galactic cluster and you can keep going up until you reach, say, the cosmic microwave background. Now, in the cosmic microwave background, you can actually detect the Earth and the Sun moving with respect to it. And the speed of the Earth and Sun with respect to the cosmic microwave background is about 10 times the speed of the Earth with respect to the Sun. So here, the Earth is going to about 30 kilometres per second. And here, there's nothing special about saying the Earth orbits the Sun. This coordinate system is no better than the coordinate system saying the Earth is fixed, or the coordinate system saying the cosmic microwave background is fixed and the Earth is moving along the straight line. And which of these coordinate systems do you use? Depends on who you are. So a cosmologist studying the expansion of the universe would use this coordinate system. An astronomer would use this coordinate system and a sailor would use this coordinate system and none of these systems are either right or wrong. And arguing about whether the Earth moves or not is as silly as arguing about whether a polar coordinate system is true or not, or for that matter arguing about whether the metric system is true or not.