## Earth, Sun, Moon Orbit

Although the Moon has an effect on Earth's movement, it's the Sun's gravitational pull that Earth to orbit it, right?

Recognitions:
Gold Member
 Quote by Natko Although the Moon has an effect on Earth's movement, it's the Sun's gravitational pull that Earth to orbit it, right?
Both the Earth and the Moon are orbiting the Sun. They also both orbit each other around a point called a Barycenter: http://en.wikipedia.org/wiki/Barycen...tes_(astronomy)

The Sun pulls BOTH the Earth and the Moon, keeping us in orbit around it as we both orbit around each other.

Mentor
 Quote by Natko So you're saying that gravity both pushes and pulls at the same time? I don't get this "stretching" concept.
The tidal forces do indeed push and pull at the same time. The following figure (source: wikipedia article on tidal forces) shows the tidal forces exerted by some other object on a planet. Notice that the tidal forces stretch the planet along the line between the center of the planet and the center of the satellite, but squeezes the planet inward on the plane normal to this line.

Quote by tiny-tim
 Quote by Natko How does the Moon affect Earth's orbit around the Sun?
It makes it wobble.

If you watch the Earth and Moon from "above", you'll see that they sort-of dance round each other!
That depends on what you mean by "wobble". The curvature of the Earth's orbit about the Sun is always directed sunward. In fact, even the curvature of the Moon's orbit about the Sun is always directed sunward.

This leads to a devil's advocate question: Does the Moon orbit the Earth or the Sun? Since the curvature is always sunward (this is not the case for a vehicle in low Earth orbit) and the gravitational force exerted by the Sun on the Moon is more than twice that exerted by the Earth on the Moon, why do we say that the Moon orbits the Earth?

Recognitions:
Gold Member
 Quote by D H This leads to a devil's advocate question: Does the Moon orbit the Earth or the Sun? Since the curvature is always sunward (this is not the case for a vehicle in low Earth orbit) and the gravitational force exerted by the Sun on the Moon is more than twice that exerted by the Earth on the Moon, why do we say that the Moon orbits the Earth?
I like to think that an object can have more than 1 orbit at once.

Mentor
 Quote by Drakkith I like to think that an object can have more than 1 orbit at once.
Bingo!

Recognitions:
Gold Member
 Quote by D H ...This leads to a devil's advocate question: Does the Moon orbit the Earth or the Sun? Since the curvature is always sunward (this is not the case for a vehicle in low Earth orbit) and the gravitational force exerted by the Sun on the Moon is more than twice that exerted by the Earth on the Moon, why do we say that the Moon orbits the Earth?
Another way to look at it is to consider the Moon another solar-orbiting object that is in a 1:1 resonance with Earth. The set of 1:1 orbits include horseshoe orbits (L4 to L3 to L5 and back), tadpole orbits (L4 or L5), quasi orbits (circles the Earth in a rotating frame of reference), and stuff that actually does orbit the Earth, regardless of what direction its sunward curvature points.

In the case of the Moon, you can picture it orbiting the Sun. Anytime it gets too far ahead of the Earth, the Earth pulls it into a slower orbit where it ultimately falls behind the Earth. The Earth then pulls it forward until its solar orbit is faster than the Earth's and it gets ahead of the Earth. Repeat indefinately.

If the Sun were to suddenly disappear (yes I know its hypothetical, but I can ponder that anyway), horseshoe, tadpole and quasi orbits would cease to exist, while an object in a direct orbit would fly away from the solar system, bound to its planet.

Unlike the horseshoe, tadpole and quasi orbits, a direct orbit has an energy PE+KE < 0, an eccentricity < 1, and a positive semi-major axis.

 Quote by D H This leads to a devil's advocate question: Does the Moon orbit the Earth or the Sun? Since the curvature is always sunward (this is not the case for a vehicle in low Earth orbit) and the gravitational force exerted by the Sun on the Moon is more than twice that exerted by the Earth on the Moon, why do we say that the Moon orbits the Earth?
We could take it even further and argue that actually the moon orbits the galactic centre as it rotates on the spiral arm! I dont know how far we could take these orbital frames but ultimately they are all orbits.

So the moon does orbit the erath, but it also orbits the sun and the galactic centre. This is all quite interesting!

Cosmo

Recognitions:
Gold Member
 Quote by Cosmo Novice We could take it even further and argue that actually the moon orbits the galactic centre as it rotates on the spiral arm! I dont know how far we could take these orbital frames but ultimately they are all orbits. So the moon does orbit the earth, but it also orbits the sun and the galactic centre. This is all quite interesting! Cosmo
An interesting side note is that Jupiter's large moons do not always simultaneously orbit both Jupiter and the Sun. There are portions of their orbits where their velocity with respect to the Sun is greater than solar escape velocity. It's as if during each orbit, Jupiter captures Io, Europa, Ganymede, and Callisto from interstellar trajectories, then ejects them back onto an interstellar trajectories.
 So the Moon orbits the Earth, which orbits the Sun, so the Moon ultimately orbits the Sun as well.

Recognitions:
Gold Member
 Quote by Natko So the Moon orbits the Earth, which orbits the Sun, so the Moon ultimately orbits the Sun as well.
Sure.

 Tags earth, gravity, horseshoe orbit, moon, orbit, sun, tadpole orbit