What is the difference between a perihelion and a aphelion?
The distance from the sun.
Aphelion is when the planet is furthest from its parent star, and perihelion is when the planet is closest to its parent star. For example, when the Earth is at perihelion, it is 147.1 million km from the Sun, in early January, and at aphelion, the Earth is 152.1 million km from the Sun in early July.
Perihelion and aphelion refer to the position of a planet relative to it's parent star.
When the planet in it's orbit is at the position closest to it's parent star it is said to be at the perihelion .
When the planet in it's orbit is at the position farthest to it's parent star it is said to be at the aphelion .