The speed of planets orbiting the Sun increases as they get closer to the Sun. This is due to a fundamental principle in physics called conservation of angular momentum.

Here's a simplified explanation:

* Angular Momentum: Think of it as the "spinningness" of an object. It depends on both the object's mass, velocity, and its distance from the axis of rotation (in this case, the Sun).

* Conservation: This principle states that the total angular momentum of a system remains constant unless acted upon by an external force.

Applying it to planets:

1. Farther from the Sun: When a planet is farther from the Sun, it has a larger orbital radius. To maintain its angular momentum, it moves slower.

2. Closer to the Sun: As a planet gets closer, its orbital radius decreases. To conserve its angular momentum, it must speed up to compensate for the smaller radius.

Think of a figure skater:

* When a figure skater pulls in their arms while spinning, they get faster. They're essentially reducing their radius (like the planet getting closer to the Sun).

* To keep their angular momentum the same, they need to spin faster.

This is the reason why planets like Mercury, which is closest to the Sun, have the highest orbital speeds, while planets like Neptune, which is farthest out, move much slower.