Here's why:
* Kepler's Laws of Planetary Motion: Kepler's second law states that a planet sweeps out equal areas in equal times. This means that a planet moves faster when it's closer to the sun and slower when it's farther away.
* Conservation of Angular Momentum: Planets maintain a constant angular momentum as they orbit the sun. Since angular momentum is related to both speed and distance from the sun, the planet's speed must change to compensate for its changing distance.
* Elliptical Orbits: Planets don't orbit in perfect circles, but in slightly elliptical paths. Their speed varies depending on their position within the ellipse.
Think of it like this:
Imagine a planet on a circular track around the sun. If the planet were to maintain a constant speed, it would cover the same distance in the same amount of time, regardless of its position. But this isn't how it works. When the planet is closer to the sun, it needs to move faster to cover the same amount of area in the same time. When it's farther away, it moves slower.
Therefore, a planet's orbital speed is constantly changing as it orbits the sun.
