# Orbit of Moon and the earth around thier COM

1. Dec 7, 2011

### hms.tech

1. The problem statement, all variables and given/known data

does the moon orbit the earth or the Center of mass of the earth-moon system.

2. Relevant ideas
well, i think that it orbits the COM of the earth-moon system.

If this is the case then is the radius of the moon from the earth always constant.
ie, if the moon orbits the COM of this system then the earth must also orbit the COM of the system, hence the distance of earth from the moon would always be changing and therefore the force applied would be changing too.

2. Dec 7, 2011

### okielogger78

I think your in the right mindframe. They both orbit the COM. It is just that the COM is located within the Earth, so that makes the moon orbit at different distances. Hence, from that deduction, this would also cause a small womble in the Earth as its orbit around the COM would be a smaller tighter orbit. You are right about the different forces. This is apparent with the different tidal forces that cause the rise and fall of the oceans on Earth. Any other questions or discussions?

3. Dec 7, 2011

### armin.hodaie

the distances are not changing,they have the same distance from COM and from eachother.both of them are rotating around COM with same angular velocity,so their distance wont change,
go0d luck

4. Dec 7, 2011

### Stonebridge

The moon's orbit is not regular. There is a perigee and apogee whereby the distance between the Earth and Moon varies considerably during its orbit. This in turn means the distance of both Earth and Moon from the CoM varies.
The c of m is in orbit about the Sun which gives rise to a slight wobble in the Earth's path.

5. Dec 7, 2011

### hms.tech

if u are refering to the equation : F=mrw^2

then even if the angular velocity (w) is constant(ie moon taking a constant no. of days to complete it's different phases) but still we can conclude (using simple maths) that distances are changing: