NavList:

Gary, you wrote:
" To the ancients the heavens seemed immutable and constant and made out of the fifth element. But comparing the positions of the stars over a relatively short period of 25 years shows that they move considerably."

Ah, but the precession of the equinoxes, which is nearly all of what you're seeing there, is just a "coordinate rotation" in the modern view. The positions of the stars, one relative to another, do not change. The angular distances between the stars do not change under precession. In that sense, it still fits in very nicely with the conception of an immutable sphere made of some "quintessence". And that's why the classical astronomers had no problem with precession conceptually. If you have a star globe, locate the north ecliptic pole on it. It's in Draco, at 18h right ascension, 23.45 degrees from the north celestial pole. The ecliptic is the equator of those coordinates. Now imagine separating the star globe from the RA, Dec coordinate globe. Imagine the RA, Dec coordinate globe as a separate transparent sphere with the star globe just inside of it. Rotate the star globe around the the axis through the north ecliptic pole at a rate of 50 arcseconds per century. That's it. That's precession. The "firmament" consisting of the sphere of stars remains immutable.

Nutation, which is really a type of precession, also can keep the stars on a fixed sphere. It's only when we deal with proper motion, especially, and parallax, on a smaller scale, that we have to give up the idea (even in mathematical principle) of stars on a fixed celestial sphere. Of course the early astronomers had reasoned away the celestial sphere and recognized that the stars must be other suns at varying distances from the Earth well before they had the proof of the changing positions. That proper motion, by the way, wipes out the simple notion of the precession of the pole star and nearby constellations that you'll see in many astronomy textbooks. The constellations change significantly on a time scale of tens of thousands of years. In addition, the tilt of the Earth's axis varies on a similar time scale so, again, the simple picture is lost.

Incidentally, if you want to see changes among the navigational stars that go beyond precession, check the coordinates of Arcturus or alpha Centauri (Rigil Kentauri). For example, try calculating the angle between Spica and Arcturus over the past couple of centuries.

-FER

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