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Lecture: Star catalogues: Narrator: Listen to part of a lecture in an astronomy class. Professor: Okay, we're going to be looking through a number of star catalogues throughout this course because they're important tools for astronomers. A star catalogue is a list of information about stars, information like its position in the sky, of course, and magnitude, which is its brightness. From historical perspectives, star catalogues are interesting because they tell us how much past astronomers knew and what they saw. One of the first star catalogues was produced by the Greek astronomer Hipparchus. This was around 129 B.C.E. That was an amazing accomplishment at the time and incredibly influential. Unfortunately, the Hipparchus star catalogue was lost along with most of his other writings. But we still know something about him. Hipparchus, if you still remember from our other discussions, was credited with a number of other important discoveries including precession of the equinoxes. Now, if you remember what precession is, yes, Jane? Jane: It's the idea that the stars shift positions in the sky over thousands of years. Professor: As you'd commerce, yes. Why? Jane: Because the Earth doesn't rotate perfectly. It wobbles a little, right? Professor: Yeah, because the Earth wobbles on its axis. Over time, the stars appear to move across the sky in relation to some fixed point. For instance, Polaris, what we call the North Star, is just above the North Pole now, but it will appear further and further away from it over time. It will, however, be back to the same position when the cycle completes in twenty-six thousand years. So, it was quite a discovery considering what Hipparchus had available to him. Anyway, precession of the equinoxes is important because using calculations based on precession, we can determine the position of the stars for any given date in history, but as I've said, we thought Hipparchus star catalogue was lost. Well now, it appears that the catalogue may be represented on the globe on an ancient Roman statue called Farnese Atlas, which is a copy of an even older Greek statue. In ancient Greek mythology, atlas is the God that holds up the world, right? Well, the globe that Atlas is holding up here shows the ancient Greek constellation. Not the individual stars, but the figures they represent like Aries the ram and Hercules the hero. Anyway, the big news now is that well, some researchers have concluded that the original sculptor may have use Hipparchus' star catalogue as a source of information in creating the sculpture. How did they figure this out? Well, we know the position of the stars where they are in the sky today, right? Well, by mentioning star positions on this globe, and then making calculations based on the precession of the equinoxes ... Male Student: Wait, wasn't that Hipparchus' discovery? You know, precession? Professor: That's right. And by measuring the star positions on the globe and making the calculations, researchers have determined that those star positions correspond to the way the sky looked around 125 B.C.E. Now, that date, 125 B.C.E., corresponds to when Hipparchus created his star catalogue, around 129 B.C.E. which is definitely within the margin of error. Now, here is something else about the Farnese Atlas. The constellations on the globe appear on a grid of circle, the standard lines we see on modern globes. So there's of course the Equator, the Tropics of Cancer and Capricorn, as well as the Arctic and Antarctic circles. Male Student: And that helped us tell where the constellations are, or were, right? Professor: Yes, of course. The constellations are placed according to these. But we can also pretty much place the observer's position on Earth based on the latitude of the constellation and their positions on the globe and guess what? After we make those calculations, this places us just where? Jane: Well, in Greece of course. Professor: Yes, but in the specific part of Greece where Hipparchus lived. Now, it's exciting to think that even though his catalogue was lost, we may have at least a concrete record of it. As I've said, this is big news because until now, the theory, the general consensus, was that the sculpture had relied on information about the constellation found in a famous poem, a poem that predates Hipparchus by more than a hundred year so this discovery may change all that. But I suppose I should stress, this is only a theory and there are skeptics. But if the globe does turn out to be connected to Hipparchus, well, the beautiful part about this is that it will have been Hipparchus' own discovery of precession of the equinoxes as Jim pointed out that will have made it possible to trace the connection.