Apsis - Wikipedia, The Free Encyclopedia

Apsis

Illustration of Closest Approachof 2 celestial bodies

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For Edenbridge's album, see Aphelion (album).For the literary journal Perigee, see Perigee: Publication for the Arts.

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

The point of closest approach (the point at which two bodies arethe closest) is called the periapsis or pericentre, from Greekπερὶ, peri, around. The point of farthest excursion is called theapoapsis (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 morecorrect, is much less used). A straight line drawn through theperiapsis and apoapsis is the line of apsides. This is the major axis ofthe ellipse, the line through the longest part of the ellipse.

Derivative terms are used to identify the body being orbited. The mostcommon are perigee and apogee, referring to orbits around theEarth (Greek γῆ, gê, "earth"), and perihelion and aphelion, referringto orbits around the Sun (Greek ἥλιος, hēlios, "sun"). During theApollo program, the terms pericynthion and apocynthion were usedwhen referring to the moon.

Contents1 Formula2 Terminology3 Earth's perihelion and aphelion4 Planetary perihelion and aphelion5 See also6 Notes and references7 External links

Formula

Apsis - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Perihelion

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Keplerian orbital elements: F is theperiapsis, H the apoapsis and the red line

between them the line of apsides

These formulae characterize the periapsis and apoapsis of anorbit:

Periapsis: maximum speed at

minimum (periapsis) distance

Apoapsis: minimum speed at

maximum (apoapsis) distance

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

specific relative angular momentum

specific orbital energy

where:

is the semi-major axis is the standard gravitational parameter

is the eccentricity, defined as

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 . The geometricmean of the two distances is the length of the semi-minor axis .

The geometric means of the two limiting speeds is , the speed corresponding to a kinetic energywhich, at any position of the orbit, added to the existing kinetic energy, would allow the orbiting body toescape (the square root of the product of the two speeds is the local escape velocity).

TerminologyThe words "pericenter" and "apocenter" are occasionally seen, although periapsis/apoapsis arepreferred 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 areoccasionally used, although '-saturnium' has very rarely been used in the last 50 years. The '-gee' form iscommonly (although incorrectly) used as a generic 'closest approach to planet' term instead ofspecifically applying to the Earth. The term peri/apomelasma (from the Greek root) was used by physicistGeoffrey A. Landis in 1998 before peri/aponigricon (from the Latin) appeared in the scientific literature in2002 [1].

Body Closest approach Farthest approachGalaxy Perigalacticon ApogalacticonStar Periastron Apastron


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Black hole Perimelasma/Peribothra/Perinigricon Apomelasma/Apobothra/AponigriconSun Perihelion Aphelion[2]

Mercury Perihermion ApohermionVenus Pericytherion/Pericytherean/Perikrition Apocytherion/Apocytherean/ApokritionEarth Perigee ApogeeMoon Periselene/Pericynthion/Perilune Aposelene/Apocynthion/ApoluneMars Periareion ApoareionJupiter Perizene/Perijove Apozene/ApojoveSaturn Perikrone/Perisaturnium Apokrone/AposaturniumUranus Periuranion ApouranionNeptune Periposeidion ApoposeidionPluto Perihadion Apohadion

Since "peri" and "apo" are Greek, it is considered by some purists[3] more correct to use the Greek formfor the body, giving forms such as '-zene' for Jupiter and '-krone' for Saturn. The daunting prospect ofhaving to maintain a different word for every orbitable body in the solar system (and beyond) is the mainreason 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 objectlaunched 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'; thealternate 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 neverused, like the other pure Greek forms ('-areion' (Mars), '-hermion' (Mercury), '-krone' (Saturn),'-uranion' (Uranus), '-poseidion' (Neptune) and '-hadion' (Pluto)).

Earth's perihelion and aphelionFor the Earth's orbit around the sun, the time of apsis is most relevantly expressed in terms of a timerelative 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'srotation axis relative to the plane of the orbit. Currently, perihelion occurs about 14 days after thenorthern hemisphere's winter solstice, making its winters milder than they would be otherwise, andsouthern hemisphere winters more extreme. 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 ofthe ice ages, known as precession.

A common convention is to express the timing of perihelion relative to the vernal equinox not in days, butas an angle of orbital displacement, a longitude of the periapsis. For Earth's orbit, this would be alongitude of perihelion, which in 2000 AD was 282.895 degrees [4].

The day and hour[A] (UT) of perihelion and aphelion for the next few years are:[5]

YearPerihelion Aphelion

Date Hour Date Hour2007 January 3 20:00 July 7 00:00


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2008 January 3 00:00 July 4 08:002009 January 4 15:00 July 4 02:002010 January 3 00:00 July 6 11:002011 January 3 19:00 July 4 15:002012 January 5 00:00 July 5 03:002013 January 2 05:00 July 5 15:002014 January 4 12:00 July 4 00:002015 January 4 07:00 July 6 19:002016 January 2 23:00 July 4 16:002017 January 4 14:00 July 3 20:002018 January 3 06:00 July 6 17:002019 January 3 05:00 July 4 22:002020 January 5 08:00 July 4 12:00

Planetary perihelion and aphelionThe images below show the Perihelion and Aphelion points of the inner and outer planets respectively.

Perihelion and aphelion points

Inner planets Outer planets

See alsoEccentric anomalyElliptic orbit


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This page was last modified on 5 July 2009 at 09:06.Text is available under the Creative Commons Attribution/Share-Alike License; additional termsmay apply. See Terms of Use for details.Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.

Notes and references^ The source data is specific only to the hour; the table value minutes are placeholders only.A.

^ 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 SeenOrbiting the Supermassive Black Hole at the Centre of the Milky Way," Nature 419, 694-696 (17 October2002), doi:10.1038/nature01121.

1.

^ Properly pronounced 'affelion' because the (neo) Greek is αφήλιον, although the hypercorrection 'ap-helion'is commonly heard.

2.

^ "Apsis ". Glossary of Terms. National Solar Observatory. 2005-02-21. http://www.nso.edu/press/glossary.html#apsis . Retrieved on 2006-09-30.

3.

^ http://aom.giss.nasa.gov/srorbpar.html4.^ Earth's Seasons Equinoxes, Solstices, Perihelion, and Aphelion - 2000-2020 (http://aa.usno.navy.mil/data/docs/EarthSeasons.php) — U.S. Naval Observatory, Astronomical Applications Department;2003-10-30 (accessed 2007-05-06).

5.

External linksApogee - Perigee (http://www.perseus.gr/Astro-Lunar-Scenes-Apo-Perigee.htm) PhotographicSize ComparisonAphelion - Perihelion (http://www.perseus.gr/Astro-Solar-Scenes-Aph-Perihelion.htm) Photographic Size ComparisonAphelion - Perihelion Dates and Times (http://www.islandnet.com/~see/weather/almanac/seasondate.htm)

Retrieved from "http://en.wikipedia.org/wiki/Apsis"Categories: Celestial mechanics | Orbits | Astrodynamics | EarthHidden categories: Articles containing Ancient Greek language text


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