# Apogee and perigee

1. Jan 11, 2005

### neiliam

i know the equation for both, apogee=a(1+e) perigee=a(1-e). i know the e stands for eccentricity, but what does the a stand for? thanks.

2. Jan 11, 2005

### Galileo

Since the sum of the apogee and the perigee will give you the length of the major axis of the ellipse, consider a(1+e)+a(1-e)=2a.
So a is the length of the semi-major axis of the ellipse.

It's the same 'a' as in the equation for an ellipse:

$$\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$$
(if b<a).

3. Jan 11, 2005

thank you.