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The apsides refer to the farthest (2) and nearest (3) points reached by an orbiting planetary body (2 and 3) with respect to a primary, or host, body (1). An apsis (from Ancient Greek ἁψίς (hapsís) 'arch, vault'; pl. apsides / ˈ æ p s ɪ ˌ d iː z / AP-sih-deez) [1] [2] is the farthest or nearest point in the orbit of a planetary body about its primary body.
At θ = 0°, perihelion, the distance is minimum = + At θ = 90° and at θ = 270° the distance is equal to . At θ = 180°, aphelion, the distance is maximum (by definition, aphelion is – invariably – perihelion plus 180°)
aphelion The point at which a body orbiting the Earth's Sun is furthest from the Sun. Contrast perihelion. apoapsis The point at which an orbiting body is furthest from its primary. Contrast periapsis. apogee The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is furthest from the Earth. Contrast perigee.
The ancient Greek astronomer Hipparchus noted the apsidal precession of the Moon's orbit (as the revolution of the Moon's apogee with a period of approximately 8.85 years); [4] it is corrected for in the Antikythera Mechanism (circa 80 BCE) (with the supposed value of 8.88 years per full cycle, correct to within 0.34% of current measurements). [5]
A clear example of this is the eccentricities of the two known Interstellar objects as of October 2019, 1I/'Oumuamua. and 2I/Borisov. 'Oumuamua had an incoming V inf of 26.5 kilometres per second (59,000 mph), but due to its low perihelion distance of only 0.255 au, it had an eccentricity of 1.200.
The Earth spends less time near perihelion and more time near aphelion. This means that the lengths of the seasons vary. [ 14 ] Perihelion currently occurs around 3 January, so the Earth's greater velocity shortens winter and autumn in the northern hemisphere, and summer and spring in the southern hemisphere.
r a is the radius at apoapsis (also "apofocus", "aphelion", "apogee"), i.e., the farthest distance of the orbit to the center of mass of the system, which is a focus of the ellipse. r p is the radius at periapsis (or "perifocus" etc.), the closest distance.
Near-Earth asteroids are divided into groups based on their semi-major axis (a), perihelion distance (q), and aphelion distance (Q): [2] [26] The Atiras or Apoheles have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU).