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

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Discussion Overview

The discussion revolves around the question of why the Moon orbits the Earth instead of the other way around, exploring concepts related to gravitational pull, inertia, and the dynamics of celestial bodies. The scope includes theoretical explanations and conceptual clarifications related to orbital mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants note that gravitational pull is a key factor in the Moon's orbit, although they express uncertainty about the underlying reasons.
  • One participant references Newton's first law of inertia, suggesting that in the absence of friction in space, objects like the Moon continue in motion unless acted upon by an external force.
  • A participant describes a model using a rubber sheet to visualize how gravity affects the motion of celestial bodies, indicating that the Moon's orbit can be understood through this analogy.
  • Another participant mentions that both the Earth and the Moon orbit around a common center of mass (barycenter), which is located about 4700 km from the Earth's center.
  • It is proposed that the larger body (Earth) has momentum and follows a trajectory closest to the barycenter, which influences the elliptical nature of the orbit.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanics of the Moon's orbit, with no consensus reached on the explanations provided. Multiple competing models and analogies are presented without resolution.

Contextual Notes

Some statements rely on assumptions about gravitational interactions and the definitions of motion and inertia, which may not be universally agreed upon. The discussion does not resolve the complexities of orbital dynamics.

Who May Find This Useful

This discussion may be of interest to individuals curious about gravitational physics, celestial mechanics, and the dynamics of orbital systems.

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

Link

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.:smile:
 

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