# Truth of this statement?

hello, could someone help me explain the truth of this statement:

"The moon moves in a near- circular orbit around earth. Because the earth is so much more massive, its own motion is not appreciably affected by the moons precense"

Thank you.

## Answers and Replies

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Danger
Gold Member
It's apparently true, but not exactly. As with any other pair of objects, they both orbit a point midway between their centres of mass. That point, however, is so close to the centre of the Earth that the difference is very difficult to detect. Proper instrumentation can detect a distinct 'wobble' to Earth's orbit.

Mk
Both of the masses pull each other. The moon's pull is much lesser than the Earth's.

rbj
the better "truth" is that the moon and the earth move in a nearly circular elliptical orbit around their common center of mass.

ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
Mk said:
Both of the masses pull each other. The moon's pull is much lesser than the Earth's.
Whoa! You may want to think about this again. Look at how one would calculate such a force using $F = G M_1 M_2/r^2$. You'll notice that it doesn't matter which mass you assign to the earth and which one you assigned to the moon. The force on the earth due to the moon is equal to the force on the moon due to the earth. This is Newton's 3rd law.

Zz.

Sounds to me like it all comes back to inertia. Since Zz points out the forces are the same, then the variable is mass. Since the Earth has much greater mass, it has much greater inertia, therefore the Moon "succumbs" most obviously to the force and orbits the Earth in the layman sense.

Danger's reply could have been worded better, but it is correct.

In layman terms, the Moon orbits the Earth, and the Earth orbits the Sun but wobbles a little due to the effect of the Moon.