# Orbital Reference Frames

1. Oct 10, 2007

### cellery

1. The problem statement, all variables and given/known data
There is a planet of mass m_1 orbiting a star of mass m_2. One question is "What is the semimajor axis of the planet's orbit in the coordinate system centered at the star's center", and another is "What is the semimajor axis of the planet's orbit centered at the center of mass of the system.

2. Relevant equations
t^2/A^3 = 4pi^2/G(m_1+m_2)
^Kepler's Third Law

3. The attempt at a solution
Basically, as far as I can visualize this problem, the semimajor axis does not change when you switch the center of the system. The planet still has to make the same orbit, so the center of said orbit should still be the same. I have an answer, but I can't tell if I'm somehow supposed to modify it for one of these scenarios.

2. Oct 10, 2007

### D H

Staff Emeritus
Think about this again. The orbit will have the same size and shape in any inertial frame, but the sun-centered frame is not inertial. The sun is orbiting the center of mass as well. What is the distance between the planet and the sun at apofucus and perifocus?

Last edited: Oct 10, 2007
3. Oct 10, 2007

### cellery

Alright, thanks, I'll give it a try. The problem is I just can't visualize what orbits look like if I have to account for a moving frame of reference as well. Everything changes. I don't quite see how to encorporate it into any equation, either.

But I'll see what I can do. The amount of help I need may be beyond the scope of what helpers are supposed to do.

4. Oct 10, 2007

### D H

Staff Emeritus