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Apogee (PSF).svg
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]

Contents

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:

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]

Body Closest approach Farthest approach
General Periapsis/Pericentre Apoapsis
Galaxy Perigalacticon Apogalacticon
Star Periastron Apastron
Black hole Perimelasma/Peribothra/Perinigricon Apomelasma/Apobothra/Aponigricon
Sun Perihelion Aphelion[3]
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
Pluto Perihadion Apohadion

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 primarily controlled 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. Perihelion puts Earth at a distance of 91.402505 million miles (147.098074m km, 0.98328989 AU) from the Sun and aphelion is at 94.50913 million miles (152.097701m km, 1.01671033 AU).

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]

Year Perihelion Aphelion
Date Hour Date Hour
2007 January 3 20:00 July 7 00:00
2008 January 3 00:00 July 4 08:00
2009 January 4 15:00 July 4 02:00
2010 January 3 00:00 July 6 11:00
2011 January 3 19:00 July 4 15:00
2012 January 5 00:00 July 5 03:00
2013 January 2 05:00 July 5 15:00
2014 January 4 12:00 July 4 00:00
2015 January 4 07:00 July 6 19:00
2016 January 2 23:00 July 4 16:00
2017 January 4 14:00 July 3 20:00
2018 January 3 06:00 July 6 17:00
2019 January 3 05:00 July 4 22:00
2020 January 5 08:00 July 4 12:00

Planetary perihelion and aphelion

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

See also

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. http://history.nasa.gov/alsj/a15/a15mr-f.htm. 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. http://www.nso.edu/press/glossary.html#apsis. Retrieved 2006-09-30. 
  5. ^ NASA.gov
  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).

External links

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Simple English

The perihelion is the point in the orbit of a planet, asteroid or comet where it is nearest to the sun.

The word perihelion stems from the Greek words "peri" (meaning "near") and "helios" (meaning "sun").

All planets, comets and asteroids in our solar system have elliptical (non-circular) orbits. Thus, they all have a closest and a farthest point from the sun: a perihelion and an aphelion.

Earth comes closest to the sun every year around January 3. It is farthest from the sun every year around July 4. The difference in distance between Earth's nearest point to the sun in January and farthest point from the sun in July is not very great. Earth is about 146 million kilometers from the sun in early January, in contrast to about 150 million kilometers in early July.

When Earth is closest to the sun, it is winter in the northern hemisphere and summer in the southern hemisphere. Thus it is possible to see that Earth's distance from the sun does not cause the season to change. Instead, Earth's seasons come and go because Earth does not orbit exactly upright with respect to the plane of our world’s orbit around the sun. Earth's axis is tilted to that plane by 23-and-a-half degrees. The Earth's tilted axis itself rotates about the notional axis orthogonal (perpendicular) to the orbital plane, almost precisely once per year. Winter falls on that part of the globe where sunlight strikes least directly. Summer falls on that part of the globe where sunlight strikes most directly.


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