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Encyclopedia > Apsis
A diagram of Keplerian orbital elements.

In astronomy, an apsis, plural apsides (IPA: /apsɪdɪːz/) 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. Image File history File links Orbit. ... Image File history File links Orbit. ... The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. ... A giant Hubble mosaic of the Crab Nebula, a supernova remnant. ... For information on how to read IPA transcriptions of English words see here. ... In astrodynamics or celestial mechanics a elliptic orbit is an orbit with the eccentricity greater than 0 and less than 1. ... See also Lists of astronomical objects Category: ... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ...

The point of closest approach is called the periapsis or pericentre and the point of farthest excursion is 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. The ellipse and some of its mathematical properties. ...

Related terms are used to identify the body being orbited. The most common are perigae and apogae, referring to orbits around the Earth, and perihelion and aphelion, referring to orbits around the Sun (Greek ‘ήλιος hēlios sun).

## Contents

There are formulae used to derive apsis and periapsis: In mathematics and in the sciences, a formula (plural: formulae, formulÃ¦ or formulas) is a concise way of expressing information symbolically (as in a mathematical or chemical formula), or a general relatx E=mcÂ² (see special relativity). ...

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

where one easily verifies

$h = sqrt{(1-e^2)mu a}$
$epsilon=-frac{mu}{2a}$

(each the same for both points, like they are for the whole orbit, in accordance with Kepler's laws of planetary motion (conservation of angular momentum) and the conservation of energy) Johannes Keplers primary contributions to astronomy/astrophysics were his three laws of planetary motion. ... This gyroscope remains upright while spinning due to its angular momentum. ...

where:

Properties: The semi-major axis of an ellipse In geometry, the term semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae. ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... In astrodynamics specific relative angular momentum () of orbiting body () relative to central body () is the relative angular momentum of per unit mass. ... In astrodynamics the specific orbital energy (or vis-viva energy) of an orbiting body traveling through space under standard assumptions is the sum of its potential energy () and kinetic energy () per unit mass. ... In astrodynamics, the standard gravitational parameter () of a celestial body is the product of the gravitational constant () and the mass : The units of the standard gravitational parameter are km3s-2 Small body orbiting a central body Under standard assumptions in astrodynamics we have: where: is the mass of the orbiting...

$e=frac{r_mathrm{ap}-r_mathrm{per}}{r_mathrm{ap}+r_mathrm{per}}=1-frac{2}{frac{r_mathrm{ap}}{r_mathrm{per}}+1}$

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

The arithmetic mean of the two distances is the semi-major axis $a!,$. The geometric mean of the two distances is the semi-minor axis $b!,$. In mathematics and statistics, the arithmetic mean (or simply the mean) of a list of numbers is the sum of all the members of the list divided by the number of items in the list. ... The geometric mean of a set of positive data is defined as the nth root of the product of all the members of the set, where n is the number of members. ... In geometry, the semi-minor axis (also semiminor axis) applies to ellipses and hyperbolas. ...

The geometric mean of the two speeds is $sqrt{-2epsilon}$, 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 sum of the squares 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 Geoffrey A. Landis in 1998 before peri/aponigricon (from the Latin) appeared in the scientific literature in 2002. See lists of astronomical objects for a list of the various lists of astronomical objects in Wikipedia. ... Geoffrey A. Landis emerged in the late 1980s as one of the foremost scientist-writers in the science fiction genre. ... 1998 (MCMXCVIII) was a common year starting on Thursday of the Gregorian calendar, and was designated the International Year of the Ocean. ... For album titles with the same name, see 2002 (album). ...

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 Perigae Apogae
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 Apoposedion

• 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)).

Description Role: Earth and Lunar Orbit Crew: 3; CDR, CM pilot, LM pilot Dimensions Height: 36. ... The Birth of Venus, (detail) by Sandro Botticelli, 1485 Aphrodite (Greek: á¼ˆÏ†ÏÎ¿Î´Î¯Ï„Î·, pronounced in English as and in Ancient Greek as ) was the Greek goddess of love, lust, beauty, and sexuality. ...

## Earth's perihelion and aphelion

The Earth is closest to the Sun in early January and furthest in early July. The relation between perihelion, aphelion and the Earth's seasons changes over a 25,765 year cycle. This precession of the equinoxes contributes to periodic climate change, Precession of the equinoxes refers to the precession of the Earths axis of rotation. ... It has been suggested that Global warming in popular culture be merged into this article or section. ...

The day and hour of these events for the next few years are:[1]

Year Perihelion Aphelion
2007 Jan 3 20 Z 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

UTC redirects here. ...

The eccentric anomaly is the angle between the direction of periapsis and the current position of an object on its orbit, projected onto the ellipses circumscribing circle perpendicularly to the major axis, measured at the centre of the ellipse. ... Two bodies with similar mass orbiting around a common barycenter with elliptic orbits. ... Perigee is the point at which an object in orbit around the Earth makes its closest approach to the Earth. ...

## Notes and references

1. ^ Properly pronounced 'affelion' because the Greek is αφήλιον, although the hypercorrection 'ap-helion' is commonly heard.
2. ^ Apsis. Glossary of Terms. National Solar Observatory (February 21, 2005). Retrieved on 2006-09-30.

Hypercorrection comprises two linguistic phenomena: elaborate, prescriptively based correction of common usage, often introduced in an attempt to avoid vulgarity or informality, that results in wording commonly considered clumsier than the usual, colloquial usage. ... For the Manfred Mann album, see 2006 (album). ... September 30 is the 273rd day of the year (274th in leap years) in the Gregorian calendar. ...

Results from FactBites:

 NIPP: Artists: Apsis (143 words) The four Apsis band members originally met during their years at Ponderosa High School in Parker, Colorado. Apsis didn’t come to be until well after High School, in August of 2002. Today, Apsis performs at venues in the Denver metro area, playing a diverse set list of original material.
 Highbeam Encyclopedia - Search Results for apsis (513 words) apsis (pl. apsides) Either of two points in an object's orbit. The closest point to the primary body is known as the periapsis, and the furthest the apapsis. The apsides of the Earth's orbit are its perihelion and aphelion; in the Moon's orbit they are its perigee and apogee.
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