# Why does the moon orbit the Earth, and not the other way around?

• accelerate23
In summary, the moon orbits the Earth because the Earth has more momentum (m*v). The Earth also has more mass (m).
accelerate23
I know it has something to do with gravitational pull, but I don't know why.

accelerate23 said:
I know it has something to do with gravitational pull, but I don't know why.

Newton's first law of inertia states that all object in motion will continue in a straight line at a constant acceleration until acted upon by an unbalanced and outside force. In space, there is no friction to slow down and stop bodies such as the moon.

Gravity tries to pull the Earth and the Moon together. So, the moon is trying to move forward, while it is moving towards the Earth. Swing a string with a weight attached to the end of it around. The weight is trying to move forward, but the string prevents it. The end result: the weight orbits your hand.

A fun way to visualize this is to stretch a rubber sheet and pin it down, but it can't touch the ground (support it with books and such). Now put a marble on it. The marble will distort the rubber around it and all smaller marbles that touch the rubber and are close enough to this depression will go towards it. Einstein said that this shows that the first marble's gravity attracted the second. Now pick the second marble back up and roll it forward on the sheet. It will now "orbit" the first marble. Gravity!

thanks!

accelerate23 said:
thanks!

No problem. Always trying to help

accelerate23 said:
Why does the moon orbit the Earth, and not the other way around?
Both the moon and the Earth orbit around a common center of mass (barycentre or barycenter), which happens to be about 4700 km from the center of the earth, about 3/4 of the way to the surface of the Earth from the center.

The larger body has momentum (m*v) at the same speed, so it takes the trajectory closest the center of mass of the two body system (barycenter).

center of mass of the two bodies is going to form that eliptical trajectory, the barycenter will be inside of the larger body.

See another thread here in physics forums.

Thanks(:

I am no expert, but I had the same question and researched the answer.

## 1. Why does the moon orbit the Earth, and not the other way around?

The moon orbits the Earth because of the force of gravity. Gravity is a force that pulls objects towards each other, and the strength of this force depends on the mass and distance between the objects. The Earth is much more massive than the moon, so it exerts a stronger gravitational pull on the moon. This causes the moon to orbit around the Earth.

## 2. Can the moon ever orbit the Earth in the opposite direction?

No, the moon cannot orbit the Earth in the opposite direction. This is because the moon's orbit is determined by the Earth's gravity, and this force always pulls the moon towards the Earth in a specific direction. It would require a significant external force to change the moon's orbit direction, which is not possible in our solar system.

## 3. Why is the moon's orbit around the Earth not a perfect circle?

The moon's orbit around the Earth is not a perfect circle because of the Earth's gravitational pull varies depending on the distance between the two objects. This is known as the inverse square law, which states that the force of gravity decreases with the square of the distance between the objects. As the moon moves closer or further away from the Earth, the strength of Earth's gravity changes, causing the moon's orbit to become slightly elliptical.

## 4. How does the moon's orbit affect the Earth's tides?

The moon's orbit around the Earth plays a significant role in creating the tides on Earth. The moon's gravitational pull causes the Earth's oceans to bulge out towards the moon, creating high tides. As the Earth rotates, the bulge moves with it, causing a second high tide on the opposite side of the Earth. The moon's gravitational pull also causes a slight bulge in the solid Earth, resulting in small land tides.

## 5. Will the moon always orbit the Earth?

Yes, the moon will always orbit the Earth. However, the distance between the two may change over time due to tidal forces. The Earth's gravity causes the moon's orbit to slowly expand, and as a result, the moon is moving away from the Earth at a rate of about 1.5 inches per year. This process is very slow, and it will take millions of years for the moon to significantly affect the Earth's orbit.

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