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# definitions

## perihelion(n.)

1.periapsis in solar orbit; the point in the orbit of a planet or comet where it is nearest to the sun

# analogical dictionary

astronomy[Domaine]

Region[Domaine]

perihelion (n.)

# Apsis

(Redirected from Perihelion)
Illustration of closest approach of two celestial bodies

In celestial mechanics, an apsis, plural apsides (pronounced /ˈæpsɨdiːz/) is the point of greatest or least distance of the elliptical orbit of an object from its center of attraction, which is usually the center of mass of the system.

The point of closest approach (the point at which two bodies are the closest) is called the periapsis or pericentre, from Greek περὶ, peri, around. The point of farthest excursion is called the apoapsis (ἀπό, apó, "from", which becomes ἀπ-, ap- or ἀφ-, aph- before an unaspirated or aspirated vowel, respectively), 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.

Derivative terms are used to identify the body being orbited. The most common are perigee and apogee, referring to orbits around the Earth (Greek γῆ, , "earth"), and perihelion and aphelion, referring to orbits around the Sun (Greek ἥλιος, hēlios, "sun"). During the Apollo program, the terms pericynthion and apocynthion were used when referring to the moon.[1]

## Formula

Keplerian orbital elements: F is the periapsis, H the apoapsis and the red line between them the line of apsides

These formulae characterize the periapsis and apoapsis of an orbit:

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

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:

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

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 $a$.The geometric mean of the two distances is the length of the semi-minor axis $b$.

The geometric means of the two limiting speeds is $\sqrt{-2\epsilon}$, 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 "pericenter" and "apocenter" 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 [2].

BodyClosest approachFarthest approach
GeneralPeriapsis/PericentreApoapsis
GalaxyPerigalacticonApogalacticon
StarPeriastronApastron
Black holePerimelasma/Peribothra/PerinigriconApomelasma/Apobothra/Aponigricon
SunPerihelionAphelion[3]
MercuryPerihermionApohermion
VenusPericytherion/Pericytherean/PerikritionApocytherion/Apocytherean/Apokrition
EarthPerigeeApogee
MoonPeriselene/Pericynthion/PeriluneAposelene/Apocynthion/Apolune
MarsPeriareionApoareion
JupiterPerizene/PerijoveApozene/Apojove
SaturnPerikrone/PerisaturniumApokrone/Aposaturnium
UranusPeriuranionApouranion
NeptunePeriposeidionApoposeidion

Since "peri" and "apo" are Greek, it is considered by some purists[4] 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

For the Earth's orbit around the sun, the time of apsis is most relevantly expressed in terms of a time relative to seasons, for that will determine the contribution of the elliptic orbit to seasonal forcing, meaning the annual variation in insolation at the top of the atmosphere. This forcing is primarilycontrolled by the annual cycle of the declination of the sun,a consequence of the tilt of the Earth's rotation axis relative to the plane of the orbit. Currently, perihelion occurs about 14 days after the northern hemisphere's winter solstice of December 21, thus making January 4 the perihelion mean.The time of perihelion progresses through the seasons, making one complete cycle in 22,000 to 26,000 years, a contribution to Milankovitch cycles, a forcing of the ice ages, known as precession.

A common convention is to express the timing of perihelion relative to the vernal equinox not in days, but as an angle of orbital displacement, a longitude of the periapsis. For Earth's orbit, this would be a longitude of perihelion, which in 2000 was 282.895 degrees[5].

The day and hour[A] (UT) of perihelion and aphelion for the next few years are:[6]

YearPerihelionAphelion
DateHourDateHour
2007January 320:00July 700:00
2008January 300:00July 408:00
2009January 415:00July 402:00
2010January 300:00July 611:00
2011January 319:00July 415:00
2012January 500:00July 503:00
2013January 205:00July 515:00
2014January 412:00July 400:00
2015January 407:00July 619:00
2016January 223:00July 416:00
2017January 414:00July 320:00
2018January 306:00July 617:00
2019January 305:00July 422:00
2020January 508:00July 412:00

## Planetary perihelion and aphelion

The images below show the perihelion and aphelion points of the inner and outer planets respectively.

## Notes and references

1. ^ The source data is specific only to the hour; the table value minutes are placeholders only.
1. ^ "Apollo 15 Mission Report". Glossary. Retrieved October 16 2009.
2. ^ R. Schodel, T. Ott, R. Genzel, R. Hofmann, M. Lehnert, A. Eckart, N. Mouawad, T. Alexander, M.J. Reid, R. Lenzen, M. Hartung, F. Lacombe, D. Rouan, E. Gendron, G. Rousset, A.-M. Lagrange, W. Brandner, N. Ageorges, C. Lidman, A.F.M. Moorwood, J. Spyromilio, N. Hubin, and K.M. Menten, "Closest Star Seen Orbiting the Supermassive Black Hole at the Centre of the Milky Way," Nature 419, 694-696 (17 October 2002), doi:10.1038/nature01121.
3. ^ Properly pronounced 'affelion' because the (neo) Greek is αφήλιον, although the hypercorrection 'ap-helion' is commonly heard.
4. ^ "Apsis". Glossary of Terms. National Solar Observatory. 2005-02-21. Retrieved 2006-09-30.
5. ^ http://aom.giss.nasa.gov/srorbpar.html
6. ^ Earth's Seasons Equinoxes, Solstices, Perihelion, and Aphelion - 2000-2020 —U.S. Naval Observatory, Astronomical Applications Department; 2003-10-30 (accessed 2007-05-06).