Last Friday I had an argument with two co-workers about planetary motion. Most of the fight, however, was about me trying to get myself understood. Once we had gotten past that, we basically agreed that we just didn’t know. I hope I’ll be able to get my point across more easily now. Here goes:
Assume a sun and a planet. Let the planet move around the sun in such a fashion that the same side keeps facing the sun (like the Moon vs. the Earth). After a year, how many times has the planet turned around its own axis?
The answer seems to depend on your frame of reference. One co-worker said the planet has not turned at all since the same side keeps facing the sun. I said it has turned exactly once in relation to “absolute space”. (Let’s keep this Newtonian; you can replace “absolute space” with “the surrounding stars and galaxies assumed as fixed.”)
Also: How many days have there been for the planet, one or none? Is a day a revolution of the planet around its own axis or is it a full light/dark cycle?
We say that a year on Earth has 365 days (neglecting the .24). Is that true in astronomical terms? Or are there 364 or 366 days/revolutions? (I forget the direction.)
I suppose it all boils down to what definitions astronomers use. Can anyone tell me what they are? I think if you’re observing other objects than just the Earth and the Sun, the “absolute” frame of reference is more useful. But is it used?
Well, I hope my question is clear now. I’d hate to have to shout at you like I did on Friday…