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[[File:Apogee_(PSF)_mul.svg|Apogee (PSF) mul|thumb]] [[File:Periapsis_apoapsis.png|Periapsis apoapsis|thumb|left]] [[Image:Angular_Parameters_of_Elliptical_Orbit.png|Angular Parameters of Elliptical Orbit|thumb|left]] [[File:Perihelion-Aphelion.svg|Perihelion-Aphelion|thumb]] [[Image:Inner_Planet_Orbits_02.svg|Inner Planet Orbits 02|thumb]] '''Apsis''' (plural: apsides) refers to either of the two extreme points in the orbit of an astronomical object around another. In the context of a planet orbiting the [[Sun]], these points are known as the perihelion (the closest point to the Sun) and the aphelion (the farthest point from the Sun). Similarly, when discussing the orbit of a moon around a planet, the terms perigee (closest approach to the planet) and apogee (farthest point from the planet) are used. The concept of apsis is fundamental in the field of [[astronomy]] and is crucial for understanding the elliptical nature of orbits as described by [[Kepler's laws of planetary motion]].

==Overview==
The term "apsis" originates from the Greek word "ἁψίς" (hapsis), meaning "loop" or "arch". In an elliptical orbit, which is the shape of most orbits in [[astronomy]], there are two apsides: the periapsis, which is the point of closest approach to the central body, and the apoapsis, which is the point of farthest distance from the central body. The specific names of these points change depending on the central body being orbited. For example, in the orbit of an object around Earth, the terms perigee and apogee are used, derived from the Greek words for "near Earth" and "away from Earth", respectively.

==Significance in Astronomy==
Apsides are significant in astronomy for several reasons. They help astronomers calculate the eccentricity of an orbit, which is a measure of how much an orbit deviates from a perfect circle. Orbits with low eccentricity are nearly circular, while orbits with high eccentricity are more elongated. The positions of the apsides can also influence the climate and seasons on a planet. For instance, Earth's slightly elliptical orbit causes variations in solar radiation received at the perihelion and aphelion, contributing to seasonal changes.

==Calculation of Apsides==
The calculation of the positions of the apsides involves understanding the gravitational forces between the orbiting objects and applying [[Kepler's laws of planetary motion]]. The semi-major axis and the eccentricity of the orbit are crucial parameters in determining the distance of the apsides from the central body. Mathematical formulas and computational models are used to predict the positions of the apsides, which are important for planning space missions and understanding the dynamics of celestial bodies.

==Cultural and Historical Aspects==
The concept of apsides has been known since antiquity, with early astronomers observing the apparent changes in the speed and brightness of planets. The understanding of apsides has evolved over centuries, from the geocentric models of the ancient Greeks to the heliocentric model proposed by [[Nicolaus Copernicus]], and finally to the detailed mathematical descriptions provided by [[Johannes Kepler]] and [[Isaac Newton]]. The study of apsides continues to be a vital part of modern astronomy, contributing to our understanding of the universe and the dynamics of celestial bodies.

==See Also==
* [[Orbit]]
* [[Elliptical orbit]]
* [[Kepler's laws of planetary motion]]
* [[Celestial mechanics]]

[[Category:Astronomy]]
{{Astronomy-stub}}

Apsis - Revision history

Prab: CSV import