Planets orbit the Sun in elliptical paths, not perfect circles. This means that the distance between a planet and the Sun varies throughout the planet's orbit.

* Perihelion: This is the point in a planet's orbit where it is closest to the Sun.

* Aphelion: This is the point in a planet's orbit where it is farthest from the Sun.

The difference in distance between perihelion and aphelion is called orbital eccentricity. Some planets have more eccentric orbits than others, meaning their distance to the Sun varies more dramatically. For example, Pluto has a very eccentric orbit, with a large difference between its closest and farthest distances from the Sun.