Question

What are aphelion and perihelion?

Answer 1

Perihelion and aphelion are the closest and furthest distances for an orbit.

Explanation:

When a moon orbits a planet or a planet orbits a star, the orbit is elliptical.

Kepler's three laws describe planetary orbits quite accurately as ellipses.

An ellipse can be defined by two parameters, the semi-major axis distance `a` and the eccentricity `0<=e<1`. An eccentricity of zero is a circle.

The closest point the body comes to its parent is called perihelion.

`d_p=a(1-e)`

The farthest point the body gets to its parent is called aphelion.

`d_p=a(1+e)`

For the Earth, perihelion is around 3 January and aphelion is around 3 July. There is not a big difference between the two distances as the Earth's orbit is nearly circular.

For a comet the perihelion distance can be in the inner solar system and the aphelion distance in the outer solar system or even further.