* Perihelion: At one point in their orbit, they are closest to the sun.
* Aphelion: At another point in their orbit, they are farthest from the sun.
This difference in distance is due to Kepler's laws of planetary motion, specifically Kepler's First Law:
* Planets orbit the Sun in elliptical paths with the Sun at one focus.
Here's why this happens:
1. Gravity: The Sun's gravity is the dominant force in the solar system. It pulls on planets, keeping them in orbit.
2. Velocity: Planets are also moving, with a velocity determined by their initial conditions.
3. Balance: The balance between the Sun's gravity and the planet's velocity determines the planet's orbit.
4. Elliptical path: This balance doesn't result in a circular path, but rather an elliptical one.
Think of it like this: Imagine a planet moving in a circle around the Sun. It has a constant velocity and is always the same distance from the Sun. Now imagine that the planet gets a slight "kick" in its velocity. This causes it to move faster at some points in its orbit and slower at others. This variation in velocity results in an elliptical path where the planet is closer to the Sun at its fastest point (perihelion) and farther away at its slowest point (aphelion).
Key point: While a planet's distance from the Sun varies throughout its orbit, it always remains bound by the Sun's gravity. The slight change in distance is simply a consequence of the elliptical nature of the orbit.
