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Q & A: Moon Orbiting the Sun

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Most recent answer: 08/06/2017
Q:
The earth revolves round the sun because the sun attracts the earth. The sun also attracts the moon and this force is about twice as large as the attraction of the earth on the moon. Why does the moon not revolve round the sun ?
- Kelvin (age 19)
India
A:

Now that you ask, I'm surprised this question hasn't come up before.
The answer is that the moon does orbit the sun. Think of the path of the moon over a year. It stays close to the earth, which goes around the sun. So the moon also goes around the sun. In addition to that, it makes smaller, quicker loops around the earth. The velocities of those loops are smaller than the main orbital speed around the Sun, so they show up as wiggles around the main orbital path, not as actual loops. If you could remove the earth while leaving the moon in its current position with its current velocity, it would orbit the sun without those extra little wiggles.

Mike W.


(published on 01/24/2011)

Follow-Up #1: moon's path around Sun

Q:
A correction: Your answer states that the moon makes loops in its path through space. It doesn't. It's path through space is always concave toward the sun. You can see some explanation about this here: https://www.wired.com/2012/12/does-the-moon-orbit-the-sun-or-the-earth/ Feel free to contact me for more information about this.
- Dr. Matt Bobrowsky (age 61)
Dover, DE, USA
A:

Thanks, you're right. "Looping" would mean that the tangential motion of the Moon around the Sun sometimes went backwards due to its orbit around the Earth. The scale of those two orbital velocities determines whether that's true. We don't have to worry about various factors that are the same for either orbit.

Sun orbit velocity proportional to 93,000,000 miles/365 days

Earth orbit velocity proportional to 240,000 miles/28 days. 

So the main Sun orbit velocity is always bigger and there are no actual backward loops.

Mike W.

I've made the corrections to the original answer.


(published on 08/06/2017)

Follow-up on this answer.