An apsis (Greek: ἁψίς; plural apsides /ˈæpsɪdiːz/, Greek: ἁψῖδες) is an extreme point in an object's orbit. The word comes via Latin from Greek and is cognate with apse.[1] For elliptic orbits about a larger body, there are two apsides, named with the prefixes peri (from περί (peri), meaning 'near') and ap, or apo (from ἀπ(ό) (ap(ó)), meaning 'away from') added to a reference to the thing being orbited. For a body orbiting the Sun, the point of least distance is the perihelion (/ˌpɛrɪˈhiːliən/), and the point of greatest distance is the aphelion (/æpˈhiːliən/).[2] The terms become periastron and apastron when discussing orbits around other stars. For any satellite of Earth, including the Moon, the point of least distance is the perigee (/ˈpɛrɪdʒiː/) and greatest distance the apogee. For objects in lunar orbit, the point of least distance is the pericynthion (/ˌpɛrɪˈsɪnθiən/) and the greatest distance the apocynthion (/ˌæpəˈsɪnθiən/). Perilune and apolune are also used.[3] For any orbit around a center of mass, there are the terms periapsis and apoapsis (or apapsis). Pericenter and apocenter are equivalent alternatives. 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 twobody 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
Contents 1 Mathematical formulae 2 Terminology 2.1 Terminology summary 3
Perihelion and aphelion
Mathematical formulae[edit] Keplerian orbital elements: point F is at the pericenter, point H is at the apocenter, and the red line between them is the line of apsides These formulae characterize the pericenter and apocenter of an orbit: Pericenter: maximum speed v p e r = ( 1 + e ) μ ( 1 − e ) a displaystyle v_ mathrm per = sqrt tfrac (1+e)mu (1e)a , at minimum (pericenter) distance r p e r = ( 1 − e ) a displaystyle r_ mathrm per =(1e)a!, Apocenter: minimum speed v a p = ( 1 − e ) μ ( 1 + e ) a displaystyle v_ mathrm ap = sqrt tfrac (1e)mu (1+e)a , at maximum (apocenter) distance r a p = ( 1 + e ) a displaystyle r_ mathrm ap =(1+e)a!, while, in accordance with
Kepler's laws of planetary motion
specific relative angular momentum h = ( 1 − e 2 ) μ a displaystyle h= sqrt left(1e^ 2 right)mu a specific orbital energy ε = − μ 2 a displaystyle varepsilon = frac mu 2a where: a is the semimajor axis, equal to r p e r + r a p 2 displaystyle frac r_ mathrm per +r_ mathrm ap 2 μ is the standard gravitational parameter e is the eccentricity, defined as e = r a p − r p e r r a p + r p e r = 1 − 2 r a p r p e r + 1 displaystyle 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 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 semimajor axis a. The geometric mean of the two distances is the length of the semiminor axis b. The geometric mean of the two limiting speeds is − 2 ε = μ a displaystyle sqrt 2varepsilon = sqrt frac mu a which is the speed of a body in a circular orbit whose radius is a displaystyle a .
Terminology[edit]
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', 'astron' and 'galacticon' forms are frequently
used in the astronomical literature when referring to the Earth, Sun,
stars and the Galactic Center respectively. The suffix 'jove' is
occasionally used for Jupiter, while 'saturnium' has very rarely been
used in the last 50 years for Saturn. The 'gee' form is commonly 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
Solar System
Astronomical object Sun Mercury Venus Earth Moon Mars Jupiter Saturn Uranus Neptune Pluto Suffix helion hermion cytherion gee lune[3] cynthion selene[3] areion zene jove chron[3] krone saturnium uranion poseidon hadion Origin of the name Helios Hermes Cytherea Gaia Luna Cynthia Selene Ares Zeus Jupiter Cronos Saturn Uranus Poseidon Hades Other objects Astronomical object Star Galaxy Barycenter Black hole Suffix astron galacticon center focus apsis bothron nigricon
Perihelion and aphelion
This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2017) (Learn how and when to remove this template message) For the orbit of the
Earth
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[edit]
The following table shows the distances of the planets and dwarf
planets from the
Sun
Type of body
Body
Distance from
Sun
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) Haumea 5,157,623,774 km (3.204798834×109 mi) 7,706,399,149 km (4.788534427×109 mi) Makemake 5,671,928,586 km (3.524373028×109 mi) 7,894,762,625 km (4.905578065×109 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
Distances of selected bodies of the
Solar System
The images below show the perihelion (green dot) and aphelion (red dot) points of the inner and outer planets.[1]
Perihelion and aphelion
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[edit] Eccentric anomaly Perifocal coordinate system Solstice References[edit] ^ a b "the definition of apsis". Dictionary.com.
^ Since the Sun, Ἥλιος in Greek, begins with a vowel (H is
considered a vowel in Greek), the final o in "apo" is omitted from the
prefix. The pronunciation "Aphelion" is given in many dictionaries
[1], pronouncing the "p" and "h" in separate syllables. However, the
pronunciation /əˈfiːliən/ [2] is also common (e.g., McGraw Hill
Dictionary of Scientific and Technical Terms, 5th edition, 1994, p.
114), since in late Greek, 'p' from ἀπό followed by the 'h' from
ἥλιος becomes phi; thus, the Greek word is αφήλιον. (see,
for example, Walker, John, A Key to the Classical Pronunciation of
Greek, Latin, and Scripture Proper Names, Townsend Young 1859 [3],
page 26.) Many [4] dictionaries give both pronunciations
^ a b c d "Basics of Space Flight". NASA. Retrieved 30 May 2017.
^ "Apollo 15 Mission Report". Glossary. Retrieved October 16,
2009.
^ R. Schödel, 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, K. M. Menten (17 October 2002). "A star in a 15.2year orbit
around the supermassive black hole at the centre of the Milky Way".
Nature. 419: 694–696. arXiv:astroph/0210426 .
Bibcode:2002Natur.419..694S. doi:10.1038/nature01121. CS1 maint:
Uses authors parameter (link)
^ Koberlein, Brian (20150329). "Peribothron –
Star
External links[edit] Look up apsis in Wiktionary, the free dictionary. Apogee – Perigee Photographic Size Comparison, perseus.gr
Aphelion
v t e Gravitational orbits Types General Box Capture Circular Elliptical / Highly elliptical Escape Graveyard Horseshoe Hyperbolic trajectory Inclined / Noninclined Osculating Parabolic trajectory Parking Synchronous semi sub Transfer orbit Geocentric Geosynchronous
Geostationary
Sunsynchronous
Low Earth
Medium Earth
High Earth
Molniya
Nearequatorial
Orbit
About other points Areosynchronous Areostationary Halo Lissajous Lunar Heliocentric Heliosynchronous Parameters Shape Size e Eccentricity a Semimajor axis b Semiminor axis Q, q Apsides Orientation i Inclination Ω Longitude of the ascending node ω Argument of periapsis ϖ Longitude of the periapsis Position M Mean anomaly ν, θ, f True anomaly E Eccentric anomaly L Mean longitude l True longitude Variation T Orbital period n Mean motion v Orbital speed t0 Epoch Maneuvers Collision avoidance (spacecraft)
Deltav
Deltav
Orbital mechanics Celestial coordinate system
Characteristic energy
Escape velocity
Ephemeris
Equatorial coordinate system
Ground track
Hill sphere
Interplanetary Transport Network
Kepler's laws of planetary motion
Lagrangian point
nbody problem
Orbit
