Geography Study Materials and Hand-outs

Perihelion and Aphelion According to Kepler's first law of planetary motion, all planets, comets, and asteroids in the Solar System have approximately elliptical orbits around the Sun. Hence, an orbiting body has a closest and a farthest point from its parent object, that is, a perihelion and an aphelion, known collectively as apsides. Orbital eccentricity measures the flatness (departure from a perfect circle) of the orbit. Perihelion The word "perihelion" stems from the Ancient Greek words "peri", meaning "near", and "helios", meaning "the Sun". The perihelion is the point in the orbit of a planet, minor planet, or comet, where it is nearest to its orbital focus, generally a star.   Aphelion "Aphelion" derives from the preposition "apo", meaning "away, off, apart". (The similar words "perigee" and "apogee" refer to the nearest and furthest points in some object's orbit around t...