|In the late 16th century, Tycho Brahe measured the positions of the planets almost every day. His assistant, Johannes Kepler, later used Brahe's observations to improve the Copernican system, which Brahe himself had rejected. Kepler was the first to search for a physical explanation for planetary orbits, and he stated his findings in three laws.|
The First Law states that the orbits of the planets around the Sun are ellipses with the Sun at one focus. One way to draw an ellipse is to pin down the ends of a string, then use a pencil to stretch out the string. The curve drawn is an ellipse and the positions of the two pins are the foci.
The Second Law states that a line from a planet to the Sun sweeps over equal areas in equal intervals of time. This means when the planet is nearer to the Sun, it moves faster.
The Third Law states that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun.