Friday, August 19, 2011

How often does the moon rotate around the earth?

Connolley makes a curious objection here to my claim that the moon rotates around the earth once a day. What is wrong with that? Doesn’t it? From the beach on holiday this effect was clearly visible. I concede that it does not do this exactly once a day. But simplification is a virtue, as Connolley himself shows when he refers to ‘two tides a day’ (actually there are two tides every 24h and 50 minutes).  So what, broadly speaking, is wrong with my claim?


AC said...

Technically, the answer to "how often does the Moon rotate around the Earth?" is "the Moon doesn't rotate around the Earth".

Celestial bodies rotate around themselves and, possibly, revolve around other bodies.

That minor technical quibble aside, it is not correct to say that, relative to the Earth, the Moon revolves around once a day.

Rather, relative to you, on the surface of the Earth, the Moon revolves around once a day.

Edward Ockham said...

Ah yes. Revolve vs rotate. What a plebian mistake.

William M. Connolley said...

You already know the answer (and what is actually going on depends on your reference frame, of course, but assuming you're thinking of the one moving with the solid earth): because the moon orbits the earth, the rotation "seen" isn't exactly once per day.

Edward Ockham said...

>>the rotation "seen" isn't exactly once per day.

That is a quibble worthy of a scholastic philosopher.