# How do astronmers find perihelion and aphelion distances?

##### 1 Answer

Feb 7, 2016

From the orbital parameters a (semi-major axis) and e (eccentricity) of the elliptic orbit,: perihelion = a(1 - e) and aphelion = a(1 + e)

#### Explanation:

At perihelion, distance from Sun is least but angular and orbital speeds about Sun are greatest. At aphelion, the distance is greatest and the speeds are least.