# How is the semi-major axis found if perihelion and aphelion distance are known?

##
For example, if an asteroid has a perihelion distance of 2.0 and an aphelion distance of 4.0 AU, what is the semi-major axis?

For example, if an asteroid has a perihelion distance of 2.0 and an aphelion distance of 4.0 AU, what is the semi-major axis?

##### 1 Answer

Nov 29, 2015

The semi major axis distance is half the sum of the perihelion and aphelion distances.

#### Explanation:

Given that the orbit is an ellipse with semi major axis a and eccentricity e, then the aphelion distance is

Therefor the semi major axis distance is half the sum of the perihelion and aphelion distances.