Expert: Courtney Seligman - 2/12/2010

Question
hello
i'm 17 and curious about astronomy.
I'm wondering: does the Sun an influence on planets' satellites speed (according to respective planet frame of reference). For instance, if Sun weren't there, would the moon run slower than 1km/sec around the Earth?

thank you

Answer
Basically, the answer is no, for three reasons:

(1) The Earth's gravity is the main cause of the Moon's motion around the Earth, and the Sun's effect is too small to make any significant difference, so whether the Sun exists or not would have very little effect on the Moon's orbit.

(2) The Earth and Moon are so close together that the difference in the Sun's pull on each of us is practically zero, and it is only that nearly zero difference that could have any effect on the Moon's motion around us.

(3) Even if the Sun's effect weren't small, it is different when the Moon is on one side of its orbit, and pretty much cancels out the effect when the Moon is on the other side.

The net result of these factors is that the Moon's average speed is not affected by the presence (or absence) of the Sun.