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An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault'; pl. apsides /ˈæpsɪˌdiːz/ AP-sih-deez)[1][2] is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values.
Apsides pertaining to orbits around the Sun have distinct names to differentiate themselves from other apsides; these names are aphelion for the farthest and perihelion for the nearest point in the solar orbit.[3] The Moon's two apsides are the farthest point, apogee, and the nearest point, perigee, of its orbit around the host Earth. Earth's two apsides are the farthest point, aphelion, and the nearest point, perihelion, of its orbit around the host Sun. The terms aphelion and perihelion apply in the same way to the orbits of Jupiter and the other planets, the comets, and the asteroids of the Solar System.
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- ^ "apsis". Dictionary.com Unabridged (Online). n.d.
- ^ "apsis". The American Heritage Dictionary of the English Language (5th ed.). HarperCollins.
- ^ Joe Rao (July 6, 2023). "Happy Aphelion Day! Earth is at its farthest from the sun for 2023 today". Space.com. Retrieved April 22, 2024.