1. Elliptical Orbits: Planets don't orbit the Sun in perfect circles, but rather in ellipses. This means the distance between a planet and the Sun varies throughout its orbit.
2. Conservation of Energy: The total energy of a planet in orbit is constant. This energy is the sum of its kinetic energy (due to motion) and potential energy (due to its position in the Sun's gravitational field).
3. Velocity Changes:
* Perihelion (Closest to Sun): At the point in its orbit closest to the Sun (perihelion), a planet has its highest kinetic energy (fastest velocity) and its lowest potential energy.
* Aphelion (Farthest from Sun): At the point in its orbit farthest from the Sun (aphelion), a planet has its lowest kinetic energy (slowest velocity) and its highest potential energy.
4. Kepler's Second Law: This law states that a planet sweeps out equal areas in equal times. Think of a line connecting the planet to the Sun. This line sweeps out a certain area as the planet moves in its orbit. The faster the planet moves, the smaller the area swept out in a given time. To keep this area consistent, the planet must move faster near perihelion and slower near aphelion.
In summary:
* A planet's velocity is constantly changing as it orbits the Sun.
* It moves faster when closer to the Sun (perihelion) and slower when farther away (aphelion).
* This is due to the conservation of energy and Kepler's Second Law, which dictate that the planet's total energy and the area swept out by its orbit remain constant.
