Does the Moon Orbit the Earth or the Sun?

I had never heard the argument that the moon orbits the sun, but it does go around the sun as it revolves around the earth. The question, which does it orbit more? Phil Plait of Bad Astronomy Blog lays out the argument and explains.

Turns out, it orbits the Earth, despite these claims. The above claims are true, but are not important in this argument. Instead, you have to look at something called the Hill sphere. Basically, it’s the volume of space around an object where the gravity of that object dominates over the gravity of a more massive but distant object around which the first object orbits.

OK, in English — and more pertinent to this issue — it’s the volume of space around the Earth where the Earth’s gravity is more important than the Sun’s. If something is orbiting the Earth inside Earth’s Hill’s sphere, it’ll be a satellite of the Earth and not the Sun.

The derivation of the math isn’t terribly important here (and it’s on the Wikipedia page if you’re curious), but when you plug in the numbers, you find the Earth’s Hill sphere has a radius of about 1.5 million kilometers. The Moon’s orbital radius of 400,000 km keeps it well within the Earth’s Hill sphere, so there you have it. The Moon orbits the Earth more than it orbits the Sun. In reality it does both, and saying it orbits one and not the other is silly anyway.

Link

(image credit: NASA)

Newest 5

Newest 5 Comments

So is gravity the only force acting in the solar system keeping planets in their year to year orbit paths?It keeps planets in their orbit paths without flaw every year correct?The Earth,moon and sun orbitung paths are the same yearly correct?Is this correct , The earth is spinning while it moves circular around a spinning moon while both the spinning moon and spinning earth both spinning together around a circular motion around a spinning sun?And they do this every year endlessly?Gravity is acting around several acts of gravity to keep everything in check?

Abusive comment hidden. (Show it anyway.)

the moon and earth orbit the sun together. they probably share a "hillsphere" as a single mass, everything probably shares it but regardless right or wrong the real question is why would we ever be confused to ask? think about who decided gravity be aloud to change graphing rules for three dimensional space?

Abusive comment hidden. (Show it anyway.)

of course it orbits both. and as Xinavera pointed out, it orbits all things up the universal ladder. such is the nature of orbits. However, its 'immediate' orbit is Earth. (just as Earth's is the sun).

On a side note, the Hill sphere is the reason why this is a false claim: "the full moon affect people because people are made mostly of water, and the moon affects tides." (which i've heard from several people. The fact of the matter is that the moon is too far away to affect something with as little mass as a person, compared to, say, another person standing near them. and even if this wasn't true, the phase of the moon (how much light is bouncing off of it from Earth's view) doesn't affect it gravitation pull.