An apsis (Greek ἁψίς, gen. ἁψίδος), plural apsides (//; Greek: ἁψίδες), is a point of least or greatest distance of a body in an elliptic orbit about a larger body. For a body orbiting the Sun the points of least and greatest distance are called respectively perihelion (//) and aphelion (//), whereas for any satellite of Earth including the Moon the corresponding points are perigee (//) and apogee. More generally, the prefixes peri- (from περί (peri), meaning "near") and ap-, or apo-, (from ἀπ(ό) (ap(ó)), meaning "away from") can be added to center (of mass) giving pericenter and apocenter. The words periapsis and apoapsis (or apapsis) are also used for these.
A straight line connecting the pericenter and apocenter 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 74% 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).
These formulae characterize the pericenter and apocenter of an orbit:
- Pericenter: maximum speed at minimum (pericenter) distance
- Apocenter: minimum speed at maximum (apocenter) distance
- is the semi-major axis, equal to
- 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 geometric mean of the two limiting speeds is which is the speed of a body in a circular orbit whose radius is .
The words "pericenter" and "apocenter" are often 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. During the Apollo program, the terms pericynthion and apocynthion (referencing Cynthia, an alternative name for the Greek Moon goddess Artemis) were used when referring to the Moon. 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|
Because "peri" and "apo" are Greek, it is considered by some purists more correct to use the Greek form for the body, giving forms such as '-zene' for Jupiter (Zeus) 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 that the generic '-apsis' has become almost universal, with the exceptions being the Sun and Earth.
- 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") 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 pericenter. 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.
The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:
|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|
The dates and times of the perihelions and aphelions vary much more than those of the equinoxes and solstices due to the presence of the Moon. Because perihelion and aphelion are defined by the distance between the center of the Sun and the center of the Earth, the Earth's position in its monthly motion around the Earth-Moon barycenter greatly affects the time when the Earth's is at its shortest or longest distance from the Sun.
Planetary perihelion and aphelion
|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)|
The following chart shows the range of distances of the planets, dwarf planets and Halley's Comet from the Sun.
Distances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively. Long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image.
The images below show the perihelion (green dot) and aphelion (red dot) points of the inner and outer planets.
The perihelion and aphelion points of the inner planets of the Solar System
The perihelion and aphelion points of the outer planets of the Solar System
- "Apollo 15 Mission Report". Glossary. Retrieved October 16, 2009.
- 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.
- Croswell, Ken (1995). The Alchemy of the Heavens. Anchor Books. ISBN 0-385-47214-5.
- "Apsis". Glossary of Terms. National Solar Observatory. 2005-02-21. Archived from the original on 14 October 2006. Retrieved 2006-09-30.
- Merriam–Webster "pericynthion"
- "Solex by Aldo Vitagliano". Retrieved 2012-07-09. (calculated by Solex 11)
- NASA planetary comparison chart http://solarsystem.nasa.gov/planets/compchart.cfm
|Look up apsis in Wiktionary, the free dictionary.|
- Apogee - Perigee Photographic Size Comparison, perseus.gr
- Aphelion - Perihelion Photographic Size Comparison, perseus.gr
- Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000-2020, usno.navy.mil