Here's why:
* Conservation of Energy: A planet's total energy (kinetic + potential) remains constant throughout its orbit.
* Potential Energy: At aphelion, the planet has the highest potential energy due to its greater distance from the sun.
* Kinetic Energy: To maintain constant total energy, its kinetic energy (related to its speed) must be lower at aphelion.
Therefore, a planet moves slowest when it's farthest from the sun and fastest when it's closest (at perihelion). This is due to the balance between gravitational potential energy and kinetic energy.