# What do the terms perihelion and aphelion mean and how are they related?

Jan 29, 2016

Perihelion and aphelion are the points in a body's orbit where it is closest to and furthest from its parent body respectively.

#### Explanation:

Planets and other bodies orbit around the Sun in elliptical orbits. The Sun is at one of the foci of the ellipse.

Perihelion is the point where the planet is closest to its sun and aphelion is the point where it is furthest from its sun.

The shape of the ellipse is defined by two parameters, one is the semi-major axis distance $a$, the other being either the semi-minor axis distance $b$ or the eccentricity $e$.

The relationship between the parameters is ${b}^{2} = {a}^{2} \left(1 - {e}^{2}\right)$. The perihelion distance is $a \left(1 - e\right)$ and the aphelion distance is $a \left(1 + e\right)$.