# Homework Help: What is this formula?

1. Oct 22, 2011

### Kyle91

Hey guys

Well I'm doing a group assignment based on orbital mechanics and my partner found a formula for eccentricity which is:

e = h2 / μEarth / (radius at perigee) - 1

Anyway when calculating h he said it was angular momentum:

h = (radius at perigee)*(velocity at perigee)

But it can't be because there's no mass component, so I'm worried we used the eccentricity formula wrong. Is the above formula for h correct? If so, what is it called?

Thanks!

2. Oct 22, 2011

### Staff: Mentor

h is specific angular momentum. That is, the angular momentum per unit mass of the orbiting object. Multiply h by mass to find the actual angular momentum.

It's often more convenient to work with specific angular momentum because in almost all cases the various characteristics of an orbit don't depend upon the orbiting body's mass when it's negligible compared to its primary.

Last edited: Oct 22, 2011