# How is perihelion calculated?

Nov 30, 2015

Perihelion can be calculated if you know the eccentricity of the orbit and the Semi-major axis of the elliptical orbit.

#### Explanation:

Perihelion can be calculated if you know the eccentricity of the orbit and the Semi-major axis of the elliptical orbit.

Perihelion = a (1 - e)

Where,

a is the semi-major axis
e is the eccentricity

Consider the orbit of Earth around the Sun.

The eccentricity of Earth's orbit is about 0.0173 and the semi-major axis is 149 million km. In other words the semi-major axis is actually the average distance between Earth and the Sun.

SO,

Perihelion = 1.49 x ${10}^{8}$ * (1 - 0.0173)

Perihelion = 146 million km

Also,

Aphelion = a (1 + e)

Aphelion = 1.49 x ${10}^{8}$ * (1 + 0.0173)

Aphelion = 152 million km.