Based on Kepler’s first law, the path of a planet is an ellipse. More precisely, each planet follows an elliptical orbit with the Sun at one of the ellipse’s two foci.
This was a major change from the older belief that heavenly bodies had to move in perfect circles. Johannes Kepler used highly accurate observations of Mars made by the astronomer Tycho Brahe and published his first law in Astronomia nova in 1609. The elliptical model explained Mars’s motion more accurately than circular arrangements with added epicycles.
An ellipse is a stretched circle described by its eccentricity. A perfectly circular orbit has zero eccentricity, while a more elongated ellipse has a higher value. The planets’ actual orbits are usually only modestly elongated; Earth’s orbit, for example, is close to circular.
The Sun is not at the centre of a planetary ellipse. It occupies one focus, which means a planet’s distance from the Sun changes during its orbit. The nearest point is perihelion, and the farthest is aphelion. Kepler’s later laws describe how orbital speed and period vary along that path.