APSIS
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:''In architecture, apsis is a synonym for apse.''
:'''Aphelia' redirects here. For the moth genus, see Aphelia (moth).''
:'''Apogee' redirects here. For the video game publishing and developing company, see Apogee Software. For the album by Bongzilla, see Apogee (album).''
In astronomy, an 'apsis', plural ''apsides'' (IPA: //) 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.
There are formulae used to derive apsis and periapsis:
★ Periapsis: maximum speed at minimum distance (periapsis distance)
★ Apoapsis: minimum speed at maximum distance (apoapsis distance)
where one easily verifies
:
:
(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)
where:
★ is the semi-major axis
★ is the eccentricity
★ is the specific relative angular momentum
★ is the specific orbital energy
★ is the standard gravitational parameter
Properties:
:
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 .
The geometric mean of the two distances is the semi-minor axis .
The geometric mean of the two speeds is , 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).
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.
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)).
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]
★ Eccentric anomaly
★ Elliptic orbit
★ Perigee
1. Properly pronounced 'affelion' because the (neo) Greek is αφήλιον, although the hypercorrection 'ap-helion' is commonly heard.
2. Apsis
3. Earth's Seasons Equinoxes, Solstices, Perihelion, and Aphelion - 2000-2020 — U.S. Naval Observatory, Astronomical Applications Department; 2003-10-30 (accessed 2007-05-06).
★ Apogee - Perigee Photographic Size Comparison
★ Aphelion - Perihelion Photographic Size Comparison
★ Aphelion - Perihelion Dates and Times
:'''Aphelia' redirects here. For the moth genus, see Aphelia (moth).''
:'''Apogee' redirects here. For the video game publishing and developing company, see Apogee Software. For the album by Bongzilla, see Apogee (album).''
A diagram of Keplerian orbital elements.
In astronomy, an 'apsis', plural ''apsides'' (IPA: //) 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.
| Contents |
| Formula |
| Terminology |
| Earth's perihelion and aphelion |
| See also |
| Notes and references |
| External links |
Formula
There are formulae used to derive apsis and periapsis:
★ Periapsis: maximum speed at minimum distance (periapsis distance)
★ Apoapsis: minimum speed at maximum distance (apoapsis distance)
where one easily verifies
:
:
(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)
where:
★ is the semi-major axis
★ is the eccentricity
★ is the specific relative angular momentum
★ is the specific orbital energy
★ is the standard gravitational parameter
Properties:
:
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 .
The geometric mean of the two distances is the semi-minor axis .
The geometric mean of the two speeds is , 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 physicist Geoffrey A. Landis in 1998 before ''peri/aponigricon'' (from the Latin) appeared in the scientific literature in 2002.
| 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 | Apoposedion |
| Pluto | Perihadion | Apohadion |
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 |
See also
★ Eccentric anomaly
★ Elliptic orbit
★ Perigee
Notes and references
1. Properly pronounced 'affelion' because the (neo) Greek is αφήλιον, although the hypercorrection 'ap-helion' is commonly heard.
2. Apsis
3. 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
★ Apogee - Perigee Photographic Size Comparison
★ Aphelion - Perihelion Photographic Size Comparison
★ Aphelion - Perihelion Dates and Times
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psst.. try this: add to faves
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