Motion in the Heavens: Stars, Sun, Moon, Planets

Michael Fowler Physics Department, UVa.


The purpose of this lecture is just to review the various motions observed in the heavens in the simplest, most straightforward way. We shall ignore for the moment refinements like tiny deviations from simple motion, but return to them later.

It is illuminating to see how these observed motions were understood in early times, and how we see them now. Of course, you know the Earth rotates and orbits around the Sun. However, I want you to be bilingual for this session: to be able to visualize also the ancient view of a fixed Earth, and rotating heavens, and be able to think from both points of view.

This is really largely an exercise in three-dimensional visualization--that's the hard part! But without some effort to see the big picture, you will not be able to appreciate some really nice things, like the phases of the moon, eclipses, and even just the seasons. You really need to have a clear picture of the Earth orbiting around the Sun and at the same time rotating about an axis tilted relative to the plane the orbit lies in, with the axis of rotation always pointing at the same star, and not changing its direction as the Earth goes around the Sun. Then you must add to your picture the Moon orbiting around the Earth once a month, the plane of its orbit tilted five degrees from the plane of the Earth's orbit around the Sun. Then we add in the planets....

Some of these topics are treated nicely in Theories of the World from Antiquity to the Copernican Revolution, by Michael J. Crowe, Dover.