Apsis

File:Angular Parameters of Elliptical Orbit.png
A diagram of Keplerian orbital elements. F Periaps, H Apoapsis and the red line between them is the line of apsides

In astronomy, an apsis, plural apsides (pronounced /ˈæpsɪdɪːz/) is the point of greatest or least distance of the elliptical orbit of an astronomical object from its center of attraction, which is generally the center of mass of the system. The point of closest approach is called the periapsis or pericentre and the point of farthest excursion is called the apoapsis (Greek από, from, which becomes απ before a vowel, and αφ before rough breathing), apocentre or apapsis (the latter term, although etymologically more correct, is much less used). A straight line drawn through the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, the line through the longest part of the ellipse.

Related terms are used to identify the body being orbited. The most common are perigee and apogee, referring to orbits around the Earth, and perihelion and aphelion, referring to orbits around the Sun (Greek ‘ήλιος hēlios sun). During the Apollo program, the terms pericynthion and apocynthion were used when referring to the moon.

Formula

These formulae characterize the periapsis and apoapsis of an orbit:

• Periapsis: maximum speed ${\displaystyle v_{\mathrm {per} }={\sqrt {\frac {(1+e)\mu }{(1-e)a}}}\,}$ at minimum (periapsis) distance ${\displaystyle r_{\mathrm {per} }=(1-e)a\!\,}$
• Apoapsis: minimum speed ${\displaystyle v_{\mathrm {ap} }={\sqrt {\frac {(1-e)\mu }{(1+e)a}}}\,}$ at maximum (apoapsis) distance ${\displaystyle r_{\mathrm {ap} }=(1+e)a\!\,}$

while, in accordance with Kepler's laws of planetary motion (conservation of angular momentum) and the conservation of energy, these quantities are constant for a given orbit:

where:

• ${\displaystyle a\!\,}$ is the semi-major axis
• ${\displaystyle \mu \!\,}$ is the standard gravitational parameter
• ${\displaystyle e\!\,}$ is the eccentricity, defined as ${\displaystyle e={\frac {r_{\mathrm {ap} }-r_{\mathrm {per} }}{r_{\mathrm {ap} }+r_{\mathrm {per} }}}=1-{\frac {2}{{\frac {r_{\mathrm {ap} }}{r_{\mathrm {per} }}}+1}}}$

Note that for conversion from heights above the surface to distances between an orbit and its primary, the radius of the central body has to be added, and conversely.

The arithmetic mean of the two limiting distances is the length of the semi-major axis ${\displaystyle a\!\,}$. The geometric mean of the two distances is the length of the semi-minor axis ${\displaystyle b\!\,}$.

The geometric mean of the two limiting speeds is ${\displaystyle {\sqrt {-2\epsilon }}}$, the speed corresponding to a kinetic energy which, at any position of the orbit, added to the existing kinetic energy, would allow the orbiting body to escape (the square root of the product of the two speeds is the local escape velocity).

Terminology

The words "pericentre" and "apocentre" are occasionally seen, although periapsis/apoapsis are preferred in technical usage.

Various related terms are used for other celestial objects. The '-gee', '-helion' and '-astron' and '-galacticon' forms are frequently used in the astronomical literature, while the other listed forms are occasionally used, although '-saturnium' has very rarely been used in the last 50 years. The '-gee' form is commonly (although incorrectly) used as a generic 'closest approach to planet' term instead of specifically applying to the Earth. The term peri/apomelasma (from the Greek root) was used by physicist Geoffrey A. Landis in 1998 before peri/aponigricon (from the Latin) appeared in the scientific literature in 2002[citation needed].

Body Closest approach Farthest approach
Galaxy Perigalacticon Apogalacticon
Star Periastron Apastron
Black hole Perimelasma/Perinigricon Apomelasma/Aponigricon
Sun Perihelion Aphelion[1]
Mercury Perihermion Apohermion
Venus Pericytherion/Pericytherean/Perikrition Apocytherion/Apocytherean/Apokrition
Earth Perigee Apogee
Moon Periselene/Pericynthion/Perilune Aposelene/Apocynthion/Apolune
Mars Periareion Apoareion
Jupiter Perizene/Perijove Apozene/Apojove
Saturn Perikrone/Perisaturnium Apokrone/Aposaturnium
Uranus Periuranion Apouranion
Neptune Periposeidion Apoposeidion

Since "peri" and "apo" are Greek, it is considered by some purists[2] more correct to use the Greek form for the body, giving forms such as '-zene' for Jupiter and '-krone' for Saturn. The daunting prospect of having to maintain a different word for every orbitable body in the solar system (and beyond) is the main reason why the generic '-apsis' has become the almost universal norm.

• In the Moon's case, in practice all three forms are used, albeit very infrequently. The '-cynthion' form is, according to some, reserved for artificial bodies, whilst others reserve '-lune' for an object launched from the Moon and '-cynthion' for an object launched from elsewhere. The '-cynthion' form was the version used in the Apollo Project, following a NASA decision in 1964.
• For Venus, the form '-cytherion' is derived from the commonly used adjective 'cytherean'; the alternate form '-krition' (from Kritias, an older name for Aphrodite) has also been suggested.
• For Jupiter, the '-jove' form is occasionally used by astronomers whilst the '-zene' form is never used, like the other pure Greek forms ('-areion' (Mars), '-hermion' (Mercury), '-krone' (Saturn), '-uranion' (Uranus), '-poseidion' (Neptune) and '-hadion' (Pluto)).

Earth's perihelion and aphelion

The Earth is closest to the Sun in early January and farthest in early July. The relation between perihelion, aphelion and the Earth's seasons changes over a 21,000 year cycle. This anomalistic precession contributes to periodic climate change (see Milankovitch cycles).

The day and hour of these events for the next few years are:[3]

Year Perihelion Aphelion
2007 Jan 3 20Z July 7 00Z
2008 Jan 3 00Z July 4 08Z
2009 Jan 4 15Z July 4 02Z
2010 Jan 3 00Z July 6 11Z
2011 Jan 3 19Z July 4 15Z
2012 Jan 5 00Z July 5 03Z
2013 Jan 2 05Z July 5 15Z
2014 Jan 4 12Z July 4 00Z
2015 Jan 4 07Z July 6 19Z
2016 Jan 2 23Z July 4 16Z