Here's what they are:
* Perihelion: The point in a planet's orbit where it is closest to the Sun.
* Aphelion: The point in a planet's orbit where it is furthest from the Sun.
Why do planets have perihelion and aphelion?
Planets don't orbit the Sun in perfect circles, they orbit in ellipses. An ellipse is like a slightly squashed circle. Because of this elliptical shape, the distance between the planet and the Sun changes throughout the orbit.
Example:
Earth's orbit is slightly elliptical. At perihelion (around January 3rd), Earth is about 147 million kilometers from the Sun. At aphelion (around July 4th), Earth is about 152 million kilometers from the Sun. The difference is about 5 million kilometers!
Important Note: While all planets have a perihelion and aphelion, the difference in distance between these points varies greatly depending on the planet's orbit. For example, Mercury has a much more eccentric (elongated) orbit than Earth, so its difference between perihelion and aphelion is much larger.
