• Planets describe elliptical orbits around the Sun, with the  at one focus of the ellipse. This law applies just as well to satellites orbiting a planet.
• The radius vector from the Sun to a planet sweeps out equal areas in equal times.
• The period of a planet T (how long it takes to go round the Sun once) is related to the semi-major axis of the orbit a (very roughly, the average distance between the planet and the Sun) by T ² µ a³.

The third point is the subject of this learning outcome.