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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.
An event called a "supermoon" occurs when the full Moon is closest to Earth (perigee). The largest possible apparent diameter of the Moon is the same 12% larger (as perigee versus apogee distances) than the smallest; the apparent area is 25% more and so is the amount of light it reflects toward Earth.
Astrophysicist Fred Espenak uses Nolle's definition but preferring the label of full Moon at perigee, and using the apogee and perigee nearest in time rather than the greatest and least of the year. [13] Wood used the definition of a full or new moon occurring within 24 hours of perigee and also used the label perigee-syzygy. [7]
(When orbiting the Earth, the last two terms are known as the apogee and perigee.) It is common to specify the period instead of the semi-major axis a in Keplerian element sets, as each can be computed from the other provided the standard gravitational parameter, GM, is given for the central body.
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.
The nearer to perigee the moon is, the bigger it appears to us; the nearer to apogee, the smaller it appears. Read More : How to See the First Solar Eclipse of 2024
r a is defined as apogee of phasing orbit; r p is defined as perigee of phasing orbit; Finally, the phasing orbit's angular momentum can be found from the equation: = + where h 2 is defined as angular momentum of phasing orbit; r a is defined as apogee of phasing orbit
At perigee (closest), since the Moon is up to 14% closer to Earth than at apogee (most distant), it subtends a solid angle which is up to 30% larger. Consequently, given the same phase, the Moon's brightness also varies by up to 30% between apogee and perigee. [208] A full (or new) moon at such a position is called a supermoon. [202] [203] [209]