Is the moon's speed greater than Earth's? I thought about this for a bit and I believe it to be so.

The moon has a certain speed from its orbit around the Earth. Let's call this x.

The Earth has a certain speed from its orbit around the Sun. Let's call this y.

Since the moon is "stuck" to the Earth and moves with it, it also has speed y from going around the Sun at the same rate.

Therefore, the moon's speed is equal to x + y.

Is my reasoning correct? If so, I assume this would apply to all other moons and planets?

  • $\begingroup$ Where are you measuring from? $\endgroup$ Aug 31, 2017 at 22:48
  • $\begingroup$ +A. C. A. C. What do you mean? If you're talking about relative velocity, then I'm measuring from the solar system. $\endgroup$ Aug 31, 2017 at 22:52

2 Answers 2


What @A.C.A.C is getting at is (I think) is speed relative to what?

The Earth moves with a speed around the Sun, but that is not an absolute speed, that is a speed relative to the Sun. The Moon has a speed that it orbits the Earth, and another at which it orbits the Sun. I think you are attempting those two together, but those are two different references. You can somewhat do so, and say that you come up with a speed that the moon is orbiting the Sun.

We will ignore that in reality the Earth also orbits the Moon part as it it relatively unimportant here, the Moon is at times effective going around the Sun faster than the Earth, but only while its orbital direction matches the Earth's orbital direction around the Sun. Half the time it does not, half the time it is in the other direction, which means at those times it is orbiting the Sun slower than the Earth. If you are talking average speed relative to the Sun, then the Moon has the same average orbital speed on the Sun as the Earth, however, because it is also orbiting Earth it ends up with a slightly longer orbital path, so a slightly higher average orbital speed relative to the Sun.

But the, the entire Solar System is also moving in the Galaxy because it is spinning at a much higher speed than the relative orbital speed of the Earth or Moon. Is that in a direction that adds to of subtracts from the Orbital speed? Oh, Wait, not only is the Galaxy is also moving through space at a much higher rate as well, that really makes it complicated in deciding, relative it what is anything moving faster than anything else. You need to set a point of reference first.

ETA: This link may help to the clarified question: https://en.wikipedia.org/wiki/Talk%3AOrbit_of_the_Moon

This shows a highly exagerated scale what the Earth-Moon orbit around the sun looks like in this image:

Image of Earth-Moon orbit from Wikipedia

Note that the smooth orbit we think of as Earths is not either, it is a two body system orbiting the sun and the moon and Earth actually orbit a center of gravity between them, it is just much closer to the Earth. The picture again is grossly out of scale.

  • $\begingroup$ Thanks for the reply. Yes, I know that the solar system and galaxy move at far higher speeds around the universe. For this scenario my reference point would be the Sun, like you said. On average, could I figure out the moon's total orbital speed (relative to the Sun) by adding up x and y? x would be the moon's orbital speed around the Earth, and y would be the Earth's orbital speed around the Sun. Since, like you said, the moon has a slightly longer orbital path around the Sun due to also orbiting the Earth, but it completes the path around the Sun in the same time as the Earth does. $\endgroup$ Aug 31, 2017 at 23:08
  • $\begingroup$ @InertialIgnorance If you add the stipulation of relative to the Sun, which really does need to be stated in the question to be valid, then the answer as stated is that the moon orbits the sun with a trivially faster average speed than the Earth, but not even close to your X+Y value. It is Y + a very tiny amount. $\endgroup$
    – dlb
    Aug 31, 2017 at 23:13
  • $\begingroup$ Alright, that's what I assumed. I know that X would be a very tiny amount (something like 1/30th of Y). But when you say "not even close to your X+Y value. It is Y + a very tiny amount." are you saying that doing the X+Y operation would not work? Or just that X is a very tiny amount? $\endgroup$ Aug 31, 2017 at 23:17
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    $\begingroup$ The X+Y does not work at all. Relative to the Sun the Moon is not orbiting the Earth. Perspective must be accounted for and scale. The distance the of the moons orbital length about the Sun is only trivially longer than the Earths when compared to the orbital radius and period. Hopefully the edit will help. $\endgroup$
    – dlb
    Aug 31, 2017 at 23:28
  • $\begingroup$ Even though the image is an exaggeration, the moon does seem to be moving more around the Sun than the Earth is (due to its actual "half orbit" around Earth?). Is there a way to mathematically calculate this extra distance, or is it so trivial it doesn't matter? The reason I asked my question in the first place was because I wanted to figure out the total kinetic energy of all bodies in the solar system. To find the Moon's Kinetic Energy, would this be close enough: Moon_KE = Moon's mass * Earth's orbital velocity around the Sun? I would be ignoring the moon's "orbit" around Earth. $\endgroup$ Aug 31, 2017 at 23:45

A simplistic answer: based on the above diagram the moon appears to be travelling considerably further than the earth in roughly the same time, so therefore has to be moving faster.


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