Kepler's First Law8.4 -Understand Kepler's laws of planetary motion
Law #1. Planets move in elliptical orbits around the Sun.
They do not move in circular orbits. Before Kepler astronomers did not recognise this and so their computations were flawed.
An ellipse is an oval shape with two foci (plural of focus). The Sun is at one focus and the other is in space.