Caption: The Sun and a general (Solar System) planet in an elliptical orbit.

The mean orbital radius (AKA semi-major axis) r of the planet is the average of the distances from the Sun to perihelion and aphelion.

That planets orbit the Sun in elliptical orbits is historically speaking Kepler's 1st law of planetary motion. Actually, their orbits are elliptical orbits only to 1st order due to astronomical perturbations, overwhelmingly just gravitational perturbations.

Credit/Permission: © David Jeffery, 2003 / Own work.
Image link: Itself.
Local file: local link: sun_planet.html.
File: Orbit file: sun_planet.html.