# Two moons

1. Oct 20, 2009

### Want to learn

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

Two moons orbit a planet in nearly circular orbits. Moon D has orbital radius r, and moon E has orbital radius 4r. Moon D takes 20 days to complete one orbit. How long does it take for moon E to complete one orbit

2. Relevant equations

None - I think

3. The attempt at a solution

I am hoping that this problem is not complicated. Since it takes 20 days for moon D with radius r, it would take 80 days for moon E with radius 4r?

Is my reasoning correct ??

2. Oct 20, 2009

### D H

Staff Emeritus

3. Oct 20, 2009

### Want to learn

Is it just 20 days the same as moon D?

A line drawn from the sun to any planet sweeps out equal areas in equal time intervals.

Does the size - radius matter then?

4. Oct 20, 2009

### Want to learn

Ok the above doesn't make sense.

Then I don't know, really.

5. Oct 20, 2009

### willem2

It's not just 20 days.

Keplers second law is only valid for the same planet or moon (and it's trivial for a circular orbit)
Try the third law.