* Planets closer to the Sun move faster.

* Planets farther from the Sun move slower.

This relationship is described by Kepler's Third Law of Planetary Motion. This law states that the square of a planet's orbital period (the time it takes to complete one orbit around the Sun) is proportional to the cube of its average distance from the Sun.

Here's a more detailed explanation:

* Conservation of Energy: A planet's total energy (kinetic + potential) remains constant throughout its orbit. As a planet moves closer to the Sun, its gravitational potential energy decreases (because it's closer to the massive Sun). To conserve total energy, its kinetic energy (and therefore speed) must increase.

* Angular Momentum: Planets also conserve angular momentum. As a planet gets closer to the Sun, the radius of its orbit decreases. To maintain constant angular momentum, its speed must increase.

In simpler terms:

Think of a planet as a spinning top. The closer the top gets to the center of its spin, the faster it rotates. The same concept applies to planets orbiting the Sun.

Important Note: While a planet's speed is not constant throughout its orbit (it's fastest at its closest point to the Sun, called perihelion, and slowest at its farthest point, called aphelion), its average speed is inversely proportional to its distance from the Sun.