Apsis
- "Apogee", "Aphelion", "Perigee" and "Perihelion" redirect here. For the literary journal, see Perigee: Publication for the Arts. For Edenbridge's Album, see Aphelion (album). For the architectural term, see Apse. For other uses, see Apogee (disambiguation) and Perihelion (disambiguation).
A straight line connecting the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the system lies on this line at one of the two foci of the ellipse. When one body is sufficiently larger than the other it may be taken to be at this focus. However whether or not this is the case, both bodies are in similar elliptical orbits each having one focus at the system's center of mass, with their respective lines of apsides being of length inversely proportional to their masses. Historically, in geocentric systems, apsides were measured from the center of the Earth. However in the case of the Moon, the center of mass of the Earth-Moon system or Earth-Moon barycenter, as the common focus of both the Moon's and Earth's orbits about each other, is about 75% of the way from Earth's center to its surface.
In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are commonly used to describe the orbital altitude of the spacecraft from the surface of the central body (assuming a constant, standard reference radius)
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Contents
Mathematical formulae
These formulae characterize the periapsis and apoapsis of an orbit:- Periapsis: maximum speed at minimum (periapsis) distance
- Apoapsis: minimum speed at maximum (apoapsis) distance
where:
- is the semi-major axis, equal to
- is the standard gravitational parameter
- is the eccentricity, defined as
The arithmetic mean of the two limiting distances is the length of the semi-major axis . The geometric mean of the two distances is the length of the semi-minor axis .
The geometric mean of the two limiting speeds is which is the speed of a body in a circular orbit whose radius is .
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/Pericenter | Apoapsis/Apocenter |
Galaxy | Perigalacticon[3] | Apogalacticon |
Star | Periastron | Apastron |
Black hole | Perimelasma/Peribothra/Perinigricon | Apomelasma/Apobothra/Aponigricon |
Sun | Perihelion | Aphelion |
Mercury | Perihermion | Aphermion |
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 |
- In the Moon's case, in practice all three forms are used, albeit very infrequently. The '-cynthion' form (from the moon goddess Artemis' Ancient Greek epithet "Cynthia")[5] is, according to some[who?], 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/Ares), '-hermion' (Mercury/Hermes), '-krone' (Saturn/Kronos), '-uranion' (Uranus), '-poseidion' (Neptune/Poseidon) and '-hadion' (Pluto/Hades)).
Perihelion and aphelion of the Earth
For the orbit of the Earth around the Sun, the time of apsis is often expressed in terms of a time relative to seasons, since this determines the contribution of the elliptical orbit to seasonal variations. The variation of the seasons is primarily controlled by the annual cycle of the elevation angle of the Sun, which is a result of the tilt of the axis of the Earth measured from the plane of the ecliptic. The Earth's eccentricity and other orbital elements are not constant, but vary slowly due to the perturbing effects of the planets and other objects in the solar system. See Milankovitch cycles.Currently, the Earth reaches perihelion in early January, approximately 14 days after the December Solstice. At perihelion, the Earth's center is about 0.98329 astronomical units (AU) or 147,098,070 kilometers (about 91,402,500 miles) from the Sun's center.
The Earth reaches aphelion currently in early July, approximately 14 days after the June Solstice. The aphelion distance between the Earth's and Sun's centers is currently about 1.01671 AU or 152,097,700 kilometers (94,509,100 mi).
On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in 22,000 to 26,000 years. There is a corresponding movement of the position of the stars as seen from Earth that is called the apsidal precession. (This is closely related to the precession of the axis.)
Astronomers commonly express the timing of perihelion relative to the vernal equinox not in terms of days and hours, but rather as an angle of orbital displacement, the so-called longitude of the periapsis. For the orbit of the Earth, this is called the longitude of perihelion, and in 2000 was about 282.895 degrees. By the year 2010, this had advanced by a small fraction of a degree to about 283.067 degrees.[6]
The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:[7]
Year | Perihelion | Aphelion | ||
---|---|---|---|---|
Date | Time (UT) | Date | Time (UT) | |
2007 | January 3 | 19:43 | July 6 | 23:53 |
2008 | January 2 | 23:51 | July 4 | 07:41 |
2009 | January 4 | 15:30 | July 4 | 01:40 |
2010 | January 3 | 00:09 | July 6 | 11:30 |
2011 | January 3 | 18:32 | July 4 | 14:54 |
2012 | January 5 | 00:32 | July 5 | 03:32 |
2013 | January 2 | 04:38 | July 5 | 14:44 |
2014 | January 4 | 11:59 | July 4 | 00:13 |
2015 | January 4 | 06:36 | July 6 | 19:40 |
2016 | January 2 | 22:49 | July 4 | 16:24 |
2017 | January 4 | 14:18 | July 3 | 20:11 |
2018 | January 3 | 05:35 | July 6 | 16:47 |
2019 | January 3 | 05:20 | July 4 | 22:11 |
2020 | January 5 | 07:48 | July 4 | 11:35 |
Planetary perihelion and aphelion
The following table shows the distances of the planets and dwarf planets from the Sun at their perihelion and aphelion.[8]Type of body | Body | Distance from Sun at perihelion | Distance from Sun at aphelion |
---|---|---|---|
Planet | Mercury | 46,001,009 km (28,583,702 mi) | 69,817,445 km (43,382,549 mi) |
Venus | 107,476,170 km (66,782,600 mi) | 108,942,780 km (67,693,910 mi) | |
Earth | 147,098,291 km (91,402,640 mi) | 152,098,233 km (94,509,460 mi) | |
Mars | 206,655,215 km (128,409,597 mi) | 249,232,432 km (154,865,853 mi) | |
Jupiter | 740,679,835 km (460,237,112 mi) | 816,001,807 km (507,040,016 mi) | |
Saturn | 1,349,823,615 km (838,741,509 mi) | 1,503,509,229 km (934,237,322 mi) | |
Uranus | 2,734,998,229 km (1.699449110×109 mi) | 3,006,318,143 km (1.868039489×109 mi) | |
Neptune | 4,459,753,056 km (2.771162073×109 mi) | 4,537,039,826 km (2.819185846×109 mi) | |
Dwarf planet | Ceres | 380,951,528 km (236,712,305 mi) | 446,428,973 km (277,398,103 mi) |
Pluto | 4,436,756,954 km (2.756872958×109 mi) | 7,376,124,302 km (4.583311152×109 mi) | |
Makemake | 5,671,928,586 km (3.524373028×109 mi) | 7,894,762,625 km (4.905578065×109 mi) | |
Haumea | 5,157,623,774 km (3.204798834×109 mi) | 7,706,399,149 km (4.788534427×109 mi) | |
Eris | 5,765,732,799 km (3.582660263×109 mi) | 14,594,512,904 km (9.068609883×109 mi) |
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The perihelion and aphelion points of the inner planets of the Solar System
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The perihelion and aphelion points of the outer planets of the Solar System
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