An apsis (Greek gen. ) plural apsides ( /psdiz/; Greek: ) is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction which is usually the center of mass of the system. Historically in geocentric systems apsides were measured from the center of the Earth.

The point of closest approach (the point at which two bodies are the closest) is called the periapsis or pericentre from Greek peri around and . The point of farthest excursion is called the apoapsis ( ap "from" apocentre or apapsis from - ap- before an unaspirated or - aph- before an aspirated vowel respectively) (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.

Derivative terms are used to identify the body being orbited. The most common are perigee ( /prdi/) and apogee ( /pdi/) referring to orbits around the Earth (Greek g "earth") and perihelion ( /prihilin/) and aphelion ( /filin/) referring to orbits around the Sun (Greek hlios "sun"). During the Apollo program the terms pericynthion and apocynthion were used when referring to the Moon.1 Contents 1 Mathematical formulas 2 Terminology 3 The perihelion and aphelion of the Earth 4 Planetary perihelion and aphelion 5 See also 6 Notes and references 7 External links Mathematical formulas Keplerian orbital elements: F is the periapsis H the apoapsis and the red line between them the line of apsides

These formulas characterize the periapsis and apoapsis of an orbit: Periapsis: maximum speed at minimum (periapsis) distance Apoapsis: minimum speed at maximum (apoapsis) distance

while in accordance with Kepler's laws of planetary motion (based on the conservation of angular momentum) and the conservation of energy these two 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 a. The geometric mean of the two distances is the length of the semi-minor axis b.

The geometric mean of the two limiting 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 product of the two speeds is the local escape velocity). 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/Pericentre Apoapsis Galaxy Perigalacticon Apogalacticon Star Periastron Apastron Black hole Perimelasma/Peribothra/Perinigricon Apomelasma/Apobothra/Aponigricon Sun Perihelion Aphelion 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 Apoposeidion Pluto Perihadion Apohadion Since "peri" and "apo" are Greek it is considered by some purists3 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 cases other than the Sun and Earth. 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 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. Currently the annual perihelion happens at about 14 days after the December Solstice thus making January 4 the average date of perihelion. The perihelion that currently occurs in early January places the Earth at a distance of about 147098070 kilometers (about 91402500 miles) from the sun which can also be expressed as about 0.98329 astronomical units (AU). (The eccentricity of the orbit also varies slowly over many millennia.) Likewise the annual aphelion that currently occurs in early July happens about 14 days after the June Solstice. At this time the distance of the aphelion is currently about about 152097700 kilometers (94509130 miles) which can also be expressed as about 1.01671 AU. 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 22000 to 26000 years. There is a corresponding movement of the position of the stars as seen from Earth that is called the precession of the orbit. (This is not the precession of the axis.) A common thing that astronomers do is to 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 the year 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.4 The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:5 Year Perihelion Aphelion Date HourA (UT) Date HourA (UT) 2007 January 3 20:00 July 7 00:00 2008 January 3 00:00 July 4 08:00 2009 January 4 15:00 July 4 02:00 2010 January 3 00:00 July 6 12:00 2011 January 3 19:00 July 4 15:00 2012 January 5 01:00 July 5 04:00 2013 January 2 05:00 July 5 15:00 2014 January 4 12:00 July 4 00:00 2015 January 4 07:00 July 6 20:00 2016 January 2 23:00 July 4 16:00 2017 January 4 14:00 July 3 20:00 2018 January 3 06:00 July 6 17:00 2019 January 3 05:00 July 4 22:00 2020 January 5 08:00 July 4 12:00 Planetary perihelion and aphelion The following table shows the distances of the planets and dwarf planets from the Sun at their perihelion and aphelion.6 Type of body Body Distance from Sun at perihelion Distance from Sun at aphelion Planet Mercury 46001009 km (28583702 mi) 69817445 km (43382549 mi) Venus 107476170 km (66782600 mi) 108942780 km (67693910 mi) Earth 147098291 km (91402640 mi) 152098233 km (94509460 mi) Mars 206655215 km (128409597 mi) 249232432 km (154865853 mi) Jupiter 740679835 km (460237112 mi) 816001807 km (507040016 mi) Saturn 1349823615 km (838741509 mi) 1503509229 km (934237322 mi) Uranus 2734998229 km (1.699449110109 mi) 3006318143 km (1.868039489109 mi) Neptune 4459753056 km (2.771162073109 mi) 4537039826 km (2.819185846109 mi) Dwarf planet Ceres 380951528 km (236712305 mi) 446428973 km (277398103 mi) Pluto 4436756954 km (2.756872958109 mi) 7376124302 km (4.583311152109 mi) Makemake 5671928586 km (3.524373028109 mi) 7894762625 km (4.905578065109 mi) Haumea 5157623774 km (3.204798834109 mi) 7706399149 km (4.788534427109 mi) Eris 5765732799 km (3.582660263109 mi) 14594512904 km (9.068609883109 mi) The following chart shows the range of distances of the planets dwarf planets and Halley's Comet from the Sun. The images below show the perihelion and aphelion points of the inner and outer planets. Perihelion and aphelion points 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 See also Eccentric anomaly Elliptic orbit Notes and references The source data is specific only to the hour; the table value minutes are placeholders only. "Apollo 15 Mission Report". Glossary. http://history.nasa.gov/alsj/a15/a15mr-f.htm. 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. "Apsis". Glossary of Terms. National Solar Observatory. 2005-02-21. http://www.nso.edu/press/glossary.html#apsis. Retrieved 2006-09-30.  NASA.gov Earth's Seasons: Equinoxes Solstices Perihelion and Aphelion - 2000-2020 U.S. Naval Observatory Astronomical Applications Department (accessed 2010-07-06). 1 External links 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 v d eOrbits  Types General Box  Capture  Circular  Elliptical / Highly elliptical  Escape  Graveyard  Hyperbolic trajectory  Inclined / Non-inclined  Osculating  Parabolic trajectory  Parking  Synchronous (semi  sub) Geocentric Geosynchronous  Geostationary  Sun-synchronous  Low Earth  Medium Earth  High Earth  Molniya  Near-equatorial  Orbit of the Moon  Polar  Tundra  Two-line elements About other points Areosynchronous  Areostationary  Halo  Lissajous  Lunar  Heliocentric  Heliosynchronous  Parameters Classical  Inclination   Longitude of the ascending node   Eccentricity   Argument of periapsis   Semi-major axis   Mean anomaly at epoch Other  True anomaly   Semi-minor axis   Linear eccentricity   Eccentric anomaly   Mean longitude   True longitude   Orbital period  Maneuvers Bi-elliptic transfer  Delta-v budget  Geostationary transfer  Gravity assist  Gravity turn  Hohmann transfer  Low energy transfer  Oberth effect   Inclination change  Phasing  Rendezvous  Transposition docking and extraction  Collision avoidance (spacecraft)  Other orbital mechanics topics Apsis  Celestial coordinate system  Characteristic energy   Direct motion  Epoch  Ephemeris  Equatorial coordinate system  Ground track  Interplanetary Transport Network  Kepler's laws of planetary motion  Lagrangian point  n-body problem  Orbit equation  Orbital speed  Orbital state vectors  Perturbation  Retrograde motion  Specific orbital energy  Specific relative angular momentum List of orbits