* Kepler's Laws: Johannes Kepler, through observation and calculations, discovered that planets move in elliptical orbits, not perfect circles.
* Conservation of Energy: As a planet orbits the sun, its distance from the sun changes. When it's closer, it has more gravitational potential energy converted to kinetic energy, meaning it moves faster. When it's farther away, the opposite happens - it has more potential energy and less kinetic energy, causing it to move slower.
* Angular Momentum: A planet's angular momentum (a measure of its rotational inertia) remains constant throughout its orbit. This means that as the planet's distance from the sun changes, its velocity changes to compensate.
Think of it like a swing:
* When you're at the highest point of the swing, you move slowly.
* As you swing downwards, you gain speed.
* At the bottom, you're moving the fastest.
* As you swing back up, you slow down again.
In summary: A planet's speed in its orbit is constantly changing due to the elliptical nature of the orbit and the laws of physics governing energy and momentum.
