Which law states that each planet revolves so that an imaginary line connecting it to the sun sweeps over equal areas in equal time intervals?

Kepler’s Laws are three mathematic laws to describe the movement of the planets around the Sun, but it can be generalized for the movement of any body orbiting a bigger one, for example, The Moon orbiting the Earth.

These laws were formulated by the astronomer Johannes Kepler from observations made by the Danish astronomer Tycho Brahe of the orbit of Mars.

Now, according to the Second Kepler’s Law of Planetary motion:

In equal times, the areas swept by the planet in its orbit around the Sun are equal.  

For this to be possible, the speed of the planet must vary. Hence, the planet will move rapidly near the Sun (perihelion) and move slowly when it is away from the Sun (aphelion).