Does the moon change orbit?

Question

you have a perfectly spherical planet circled by a perfectly spherical moon in a perfectly circular orbit. Now introduce the sun. Does the moon stay in a perfectly circular orbit or does the sun pull the moon away from the planet when the moon gets closer to the sun?

Answer 1

Yes, the sun will pull the moon out from a perfectly spherical orbit. In this make-believe problem, does the moon orbit around the planet's equator? And does the planet orbit the sun around the Sun's equator? Also, is the planet's spin axis perpendicular to it's solar orbit, or is it tilted?

Unless everything is perfectly aligned, the orbits will be "chaotic". This does NOT mean that it goes wild all over the place. It just means that the orbit will be elliptical, and the amount of ellipticalness will very over some (small) range, in an unpredictable way (slow changes over time). Also the orbits' obliquity will change chaotically.

To prove this would take more than just Newtonian physics. You would need some general relativity also.

Answer 2

The Earth is not perfectly spherical. And the moon's orbit is elliptical, not circular (eccentricity of .0549).

The moon doesn't orbit around Earth's centre, the moon and Earth orbit around a common centre of gravity (which, due to the difference in mass is located about 1700 km beneath the surface of the Earth).

This, in part, contributes to the moon's elliptical orbit.
The sun also contributes - the sun is much more massive but also much farther away, so its gravitational effect on the moon is less than the Earth's effect on the moon.

Answer 3

Neither the Earth nor its Moon's orbit are perfectly circular.
The Moon's orbital numbers are changing all the time, because the Moon is receeding from the Earth at a rate of ~4.5cm/yr.
This is known from very exact laser measurements utilizing the reflectors left there by the Apollo astronauts,

Answer 4

Almost all solutions to gravitational n-body problems (n>2) turn out to be chaotic. So the answer is that, indeed, the orbit of the moon is rather complicated.

http://inkido.indiana.edu/a100/earthmoon5.html
http://ircamera.as.arizona.edu/astr_250/Lectures/Lec_03sml.htm

Answer 5

I'm not sure how much, but I do know that every year the moon gets farther and farther away from the earth. Luckily its a small number.

Answer 6

the moon is not in a perfect orbit around the earth. it is slowly moving away from the earth and will eventually break beyond earths gravitational pull.