Kepler’s laws of planetary motion

Johannes Kepler set out his first and second laws of planetary motion in 1609 and the third in 1618.

The first states that the orbit of a planet is an ellipse with the Sun at one of its foci. The foci of an ellipse are hard to define but easy to show with a nursery experiment. Push two nails into a surface maybe 20cm apart, throw a loop of string round them, put a pencil in the loop and run it round. The shape you'llĀ have drawn is an ellipse and the nails are the foci. The Earths orbit is nearly circular, but its slight eccentricity – the difference between its shape and an exact circle – has tangible effects, as the text explains.

The second law states that a line from a planet to the Sun sweeps out equal areas in equal intervals of time. In other words, the closer a planet gets to the Sun the faster it goes. Again, this has real effects for the Earth despite the low eccentricity of the Earth's orbit.

The third law states that the period a planet takes to orbit the Sun squared is proportional to the size of its orbit (to be exact, the long radius of the ellipse) cubed. In other words, planets deeper in the solar system move much slower.