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| : ''"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)]].''
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| [[Image:Apogee (PSF).png|thumb|right|250px|'''Apsides''' 1) Apoapsis; 2) Periapsis; 3) Focus]]
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| An '''apsis''' ([[Greek language|Greek]] ἁψίς, gen. ἁψίδος), plural '''apsides''' ({{IPAc-en|ˈ|æ|p|s|ɨ|d|iː|z}}; Greek: ἁψίδες), is a point of greatest or least distance of a body in an [[elliptic orbit]] about a larger body. For a body orbiting the [[Sun]] these points are called respectively '''aphelion''' ({{IPAc-en|@|ˈ|f|i:|l|i|ə|n}} or {{IPAc-en|æ|p|'|h|i:|l|i|ə|n}}) and '''[http://simple.wikipedia.org/wiki/Perihelion perihelion]''' {{IPAc-en|ˌ|p|Er|i-|ˈ|h|i:|l|i|ə|n}}, while for any satellite of [[Earth]] including the Moon the corresponding points are '''apogee''' and '''perigee''' {{IPAc-en|ˈ|p|Er|i-|dZ|i:}}. The generic suffix, independent of the particular central body, can be either ''apsis'' or ''centre'', hence '''apoapsis''', '''apocentre''' or '''apapsis''' ({{ety||''ἀπ(ό)'' (ap(ó))|from}}), and '''periapsis''' or '''pericentre''' ({{ety||''περί'' (peri)|around}}). 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]].<ref>{{cite web | title = Apollo 15 Mission Report | work= Glossary | url= http://history.nasa.gov/alsj/a15/a15mr-f.htm | accessdate = October 16, 2009 }}</ref>
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| 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 [[conic section|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 [[Similarity (geometry)|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 model|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 [[Barycentric coordinates (astronomy)|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.
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| In [[orbital mechanics]], the apsis technically refers to the distance measured between the [[center of mass|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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| ==Mathematical formulae==
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| [[Image:Angular Parameters of Elliptical Orbit.png|thumb|250px|[[Johannes Kepler|Keplerian]] [[orbital elements]]: point F is at the periapsis, point H is at the apoapsis, and the red line between them is the line of apsides]]
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| These [[formula]]e characterize the periapsis and apoapsis of an orbit:
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| * Periapsis: maximum speed <math> v_\mathrm{per} = \sqrt{ \tfrac{(1+e)\mu}{(1-e)a} } \,</math> at minimum (periapsis) distance <math>r_\mathrm{per}=(1-e)a\!\,</math>
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| * Apoapsis: minimum speed <math> v_\mathrm{ap} = \sqrt{ \tfrac{(1-e)\mu}{(1+e)a} } \,</math> at maximum (apoapsis) distance <math>r_\mathrm{ap}=(1+e)a\!\,</math>
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| 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:
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| * [[specific relative angular momentum]] <math>h = \sqrt{(1-e^2)\mu a}</math>
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| * [[specific orbital energy]] <math>\epsilon=-\frac{\mu}{2a}</math>
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| where: | |
| * <math>a\!\,</math> is the [[semi-major axis]], equal to <math>\frac{r_\mathrm{per}+r_\mathrm{ap}}{2}</math>
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| * <math>\mu\!\,</math> is the [[standard gravitational parameter]]
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| * <math>e\!\,</math> is the [[Orbital eccentricity|eccentricity]], defined as <math>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}</math>
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| 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.
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| The [[arithmetic mean]] of the two limiting distances is the length of the [[semi-major axis]] <math>a</math>.
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| The [[geometric mean]] of the two distances is the length of the [[semi-minor axis]] <math>b</math>.
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| The geometric mean of the two limiting speeds is <math>\sqrt{-2\epsilon}=\sqrt{\mu/a}</math> which is the speed of a body in a circular orbit whose radius is <math>a</math>.
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| ==Terminology==
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| The words "pericenter" and "apocenter" are occasionally seen, although periapsis/apoapsis are preferred in technical usage.
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| Various related terms are used for other [[Celestial body|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.<ref>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}}.</ref>
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| {| class="wikitable" style="margin-left:auto; margin-right:auto;"
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| ! Body !! Closest approach !! Farthest approach
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| |-
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| | General || Periapsis/Pericenter || Apoapsis/Apocenter
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| |-
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| | [[Galaxy]] || Perigalacticon<ref name="alchemy">{{Cite book
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| | last1 = Croswell
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| | first1 = Ken
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| | authorlink1 = Ken Croswell
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| | year = 1995
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| | title = [[The Alchemy of the Heavens]]
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| | isbn = 0-385-47214-5
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| | publisher = [[Anchor Books]]
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| }}</ref>
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| || Apogalacticon
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| |-
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| | [[Star]] || Periastron || Apastron
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| |-
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| | [[Black hole]] || Perimelasma/Peribothra/Perinigricon || Apomelasma/Apobothra/Aponigricon
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| |-
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| | [[Sun]] || Perihelion || Aphelion
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| |-
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| | [[Mercury (planet)|Mercury]] || Perihermion || Aphermion
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| |-
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| | [[Venus]] || Pericytherion/Pericytherean/Perikrition || Apocytherion/Apocytherean/Apokrition
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| |-
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| | [[Earth]] || Perigee || Apogee
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| |-
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| | [[Moon]] || Periselene/Pericynthion/Perilune || Aposelene/Apocynthion/Apolune
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| |-
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| | [[Mars]] || Periareion || Apoareion
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| |-
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| | [[Jupiter]] || Perizene/Perijove || Apozene/Apojove
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| |-
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| | [[Saturn]] || Perikrone/Perisaturnium || Apokrone/Aposaturnium
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| |-
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| | [[Uranus]] || Periuranion || Apouranion
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| |-
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| | [[Neptune]] || Periposeidion || Apoposeidion
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| |-this is true
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| |}
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| Because "peri" and "apo" are Greek, it is considered by some purists<ref>{{cite web| url=http://www.nso.edu/press/glossary.html#apsis| publisher=National Solar Observatory| work= Glossary of Terms| title=Apsis| accessdate=2006-09-30|date=2005-02-21| archiveurl= http://web.archive.org/web/20061014184651/http://www.nso.edu/press/glossary.html| archivedate= 14 October 2006 <!--DASHBot-->| deadurl= no}}</ref> 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 suffix for every orbitable body in the Solar System (and beyond) is the main reason that the generic '-apsis' has become almost universal, with the exception, of course, being the Sun and Earth.
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| * 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 "[[Artemis#Epithets|Cynthia]]")<ref>[http://www.merriam-webster.com/dictionary/pericynthion Merriam–Webster "pericynthion"]</ref> 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.
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| * 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.
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| * 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)).
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| ==Perihelion and aphelion of the Earth==
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| 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 [[orbital eccentricity|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]]''.
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| 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 unit]]s (AU) or 147,098,070 kilometers (about 91,402,500 miles) from the Sun's center.
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| 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 {{convert|152,097,700|km|mi|sp=us}}.
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| 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 [[axial precession (astronomy)|precession of the axis]].)
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| 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.<ref>[http://aom.giss.nasa.gov/srorbpar.html NASA.gov]</ref>
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| The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:<ref name=solex>{{cite web
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| |title=Solex by Aldo Vitagliano
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| |url=http://chemistry.unina.it/~alvitagl/solex/
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| |accessdate=2012-07-09}} (calculated by Solex 11)</ref>
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| {| class="wikitable" style="margin-left:auto; margin-right:auto;"
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| ! rowspan=2 width=50 | Year
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| ! colspan=2 | Perihelion
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| ! colspan=2 | Aphelion
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| |-
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| | style="background:#f2f2f2; text-align:center;" width=81 | Date || style="background:#f2f2f2; text-align:center;" width=70 | Time ([[Coordinated Universal Time|UT]])
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| | style="background:#f2f2f2; text-align:center;" width=81 | Date || style="background:#f2f2f2; text-align:center;" width=70 | Time ([[Coordinated Universal Time|UT]])
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| |-
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| ! 2007
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| |January 3 || 19:43
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| | July 6 || 23:53
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| |-
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| ! 2008
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| |January 2 || 23:51
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| | July 4 || 07:41
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| |-
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| ! 2009
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| |January 4 || 15:30
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| | July 4 || 01:40
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| |-
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| ! 2010
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| |January 3 || 00:09
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| | July 6 || 11:30
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| |-
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| ! 2011
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| |January 3 || 18:32
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| | July 4 || 14:54
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| |-
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| ! 2012
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| |January 5 || 00:32
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| | July 5 || 03:32
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2013|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2013|gold}}" | 2013
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| |January 2 || 04:38
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| | July 5 || 14:44
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2014|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2014|gold}}" | 2014
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| |January 4 || 11:59
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| | July 4 || 00:13
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2015|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2015|gold}}" | 2015
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| |January 4 || 06:36
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| | July 6 || 19:40
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2016|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2016|gold}}" | 2016
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| |January 2 || 22:49
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| | July 4 || 16:24
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2017|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2017|gold}}" | 2017
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| |January 4 || 14:18
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| | July 3 || 20:11
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2018|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2018|gold}}" | 2018
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| |January 3 || 05:35
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| | July 6 || 16:47
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2019|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2019|gold}}" | 2019
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| |January 3 || 05:20
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| | July 4 || 22:11
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| |- style="background-color: {{#ifeq:{{CURRENTYEAR}}|2020|gold}}" |
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| !style="background-color: {{#ifeq:{{CURRENTYEAR}}|2020|gold}}" | 2020
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| |January 5 || 07:48
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| | July 4 || 11:35
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| |}
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| ==Planetary perihelion and aphelion==
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| The following table shows the distances of the [[planet]]s and [[dwarf planet]]s from the Sun at their perihelion and aphelion.<ref>NASA planetary comparison chart http://solarsystem.nasa.gov/planets/compchart.cfm</ref>
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| <!-- It's surprising that values are precise to the km, but the data is from NASA... -->
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| {| class="wikitable" style="margin-left:auto; margin-right:auto;"
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| ! Type of body !! Body !! Distance from Sun at perihelion !! Distance from Sun at aphelion
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| | rowspan="8" | Planet
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| | Mercury || {{convert|46001009|km|mi|abbr=on}} || {{convert|69817445|km|mi|abbr=on}}
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| |-
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| | Venus || {{convert|107476170|km|mi|abbr=on}} || {{convert|108942780|km|mi|abbr=on}}
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| |-
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| | Earth || {{convert|147098291|km|mi|abbr=on}} || {{convert|152098233|km|mi|abbr=on}}
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| |-
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| | Mars || {{convert|206655215|km|mi|abbr=on}} || {{convert|249232432|km|mi|abbr=on}}
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| |-
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| | Jupiter || {{convert|740679835|km|mi|abbr=on}} || {{convert|816001807|km|mi|abbr=on}}
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| |-
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| | Saturn || {{convert|1349823615|km|mi|abbr=on}} || {{convert|1503509229|km|mi|abbr=on}}
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| |-
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| | Uranus || {{convert|2734998229|km|mi|abbr=on}} || {{convert|3006318143|km|mi|abbr=on}}
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| |-
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| | Neptune || {{convert|4459753056|km|mi|abbr=on}} || {{convert|4537039826|km|mi|abbr=on}}
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| |-
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| | rowspan="5" | Dwarf planet
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| | Ceres || {{convert|380951528|km|mi|abbr=on}} || {{convert|446428973|km|mi|abbr=on}}
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| |-
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| | Pluto || {{convert|4436756954|km|mi|abbr=on}} || {{convert|7376124302|km|mi|abbr=on}}
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| |-
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| | Makemake || {{convert|5671928586|km|mi|abbr=on}} || {{convert|7894762625|km|mi|abbr=on}}
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| |-
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| | Haumea || {{convert|5157623774|km|mi|abbr=on}} || {{convert|7706399149|km|mi|abbr=on}}
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| |-
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| | Eris || {{convert|5765732799|km|mi|abbr=on}} || {{convert|14594512904|km|mi|abbr=on}}
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| |}
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| The following chart shows the range of distances of the planets, dwarf planets and [[Halley's Comet]] from the Sun.
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| {{Distance from Sun using EasyTimeline}}
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| The images below show the perihelion (green dot) and aphelion (red dot) points of the inner and outer planets.
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| <gallery caption="Perihelion and aphelion points" widths="300px" heights="300px"> | |
| Image:Inner Planet Orbits.jpg|The perihelion and aphelion points of the [[inner planets]] of the Solar System
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| Image:Outer Planet Orbits.jpg|The perihelion and aphelion points of the [[outer planets]] of the Solar System
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| </gallery>
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| ==See also==
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| * [[Apsidal precession]]
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| * [[Eccentric anomaly]]
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| * [[Elliptic orbit]]
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| * [[Perifocal coordinate system]]
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| * [[Solstice]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| {{Wiktionary|apsis}}
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| * [http://www.perseus.gr/Astro-Lunar-Scenes-Apo-Perigee.htm Apogee - Perigee] Photographic Size Comparison, perseus.gr
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| * [http://www.perseus.gr/Astro-Solar-Scenes-Aph-Perihelion.htm Aphelion - Perihelion] Photographic Size Comparison, perseus.gr
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| * [http://www.usno.navy.mil/USNO/astronomical-applications/data-services/earth-seasons Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000-2020], usno.navy.mil
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| {{orbits}}
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| <!-- Please respect alphabetical order -->
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| [[Category:Orbits]]
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| [[Category:Earth]]
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