What are perihelion and aphelion in a planet's orbit around the Sun?

In the context of the elliptical orbits described by Johannes Kepler, planets do not maintain a constant distance from the Sun. Instead, their paths are defined by two specific points known as apses, which lie on the line of apsides (the major axis of the orbit).123

• Perihelion: This is the point in a planet's orbit where it is at its closest approach to the Sun. Because of the conservation of angular momentum, a planet travels at its highest velocity when passing through this point.234
• Aphelion: This is the point in the orbit where the planet is at its farthest distance from the Sun. Correspondingly, the planet moves at its lowest velocity at this position.53

These terms are specific applications of the more general astronomical concepts of pericentre and apocentre, used specifically when the Sun is the central body of attraction.13