wsmith@uiucdcsb.CS.UIUC.EDU (03/31/86)
> Let's go back to the Russell quote: > > his predecessors is merely one of convenience; all motion > is relative, and there is no difference between the two > statements: `the earth rotates once a day' and `the heavens > revolve about the earth once a day.' The two mean exactly the > same thing, just as it means the same thing if I say a certain > length is six feet or two yards. Astronomy is easier if we > take the sun as fixed than if we take the earth, just as > accounts are easier in decimal coinage. {Signet, pp. 13-14} I think there is a more fundamental problem with this statement than the debate of reference frames. The statement ignores the difference between a tropical day and a sidereal day. "the earth rotates once a day" means that in 24 hours the sun is at the (approx) same position in the sky. "the heavens revolve around the earth once a day" means that after a different time period a star returns (approx) to the same position in the sky. Because there are two rotating motions in the standard reference frame: the earth on its axis and the earth around the sun. The two statements do not mean "exactly the same thing" there are a different number of days in a tropical year relative to the sun and sidereal year relative to the stars. I suppose (just to be self-contrary) we could take in the first quote that he is referring to a sidereal day as well although that is a wierd interpretation. If he means by day a typical 24 hour day, than the second statement is false because the heavens do not rotate around the earth in a single 24 hour day. --------- This point is totally irrelevant to the rest of the discussion on the passage being yelled at or from. --------- The terminology in this posting has about a 50% chance of being backwards, confused or otherwise erroneous, so corrections are welcome. Bill Smith ihnp4!uiucdcs!wsmith