4/8/2015 PerihelionandaphelionWikipedia,thefreeencyclopedia
http://en.wikipedia.org/wiki/Perihelion_and_aphelion 1/2
1.Planetataphelion2.Planetatperihelion3.Sun.Drawingnottoscale.
PerihelionandaphelionFromWikipedia,thefreeencyclopedia
Theperihelionisthepointintheorbitofaplanet,asteroid,cometorotherSunorbitingbodywhereitisnearesttotheSun.Itistheoppositeofaphelion,whichisthepointintheorbitwheretheobjectisfarthestfromtheSun.
ThewordperihelionstemsfromtheGreekwords"peri",meaningnear,and"helios",meaningtheSun.Aphelionderivesfromtheprepositionapo,meaningaway,off,apart.(Thesimilarwordsperigeeandapogeerefertothenearestandfurthestpointsinsomeobjects'orbitaroundtheEarth.)
Allplanets,cometsandasteroidsinoursolarsystemhaveapproximatelyelliptical(akindofnoncircular)orbits.[1](AnysinglerevolutionofabodyaroundtheSunisonlyapproximatelyelliptical,becausethephenomenonknownasprecessionoftheperihelionpreventstheorbitfrombeingasimpleclosedcurvesuchasanellipse.)Thus,theyallhaveaclosestandafarthestpointfromtheSun:aperihelionandanaphelion,knowncollectivelyasapsides.Orbitaleccentricitymeasurestheflatness(departurefromaperfectcircle)oftheorbit.
EarthcomesclosesttotheSuneveryyeararoundJanuary3.ItisfarthestfromtheSuneveryyeararoundJuly4.(Foratableofthesedatesforvariousyears,seeApsis.)
ThedifferenceindistancebetweenEarth'snearestpointtotheSuninJanuaryandfarthestpointfromtheSuninJulyisabout5millionkilometers(3.1millionmiles).Earthisabout147.1millionkilometers(91.4millionmiles)fromtheSunatperihelioninearlyJanuary,incontrasttoabout152.1millionkilometers(94.5millionmiles)ataphelioninearlyJuly.Becauseoftheincreaseddistanceataphelion,only93.55%ofthesolarradiationfromtheSunfallsonagivensquareareaoflandthanatperihelion.Aswinteralsofallsinthesouthernhemisphereatthesametimeasaphelion,thisdecreaseinsolarradiationduetotheaphelionplusshorterperiodsofdaylightcauses,ingeneral,lessheatfromtheSuntohitthesouthernhemisphereinwinterthansolarradiationhittingthenorthernhemisphereduringitswinteratperihelionsixmonthslater.
WhenEarthisclosesttotheSun,itiswinterinthenorthernhemisphereandsummerinthesouthernhemisphere.ThusEarth'sdistancefromtheSundoesnotaffectwhatseasonoccurs.Instead,Earth'sseasonscomeandgobecauseEarthdoesnotrotatewithitsaxisexactlyuprightwithrespecttotheplaneofitsorbitaroundtheSun.Earth'saxialtiltis23.4degrees.ThisputstheSunfarthersouthinDecemberandJanuary,sothenorthhaswinterandthesouthhassummer.Thuswinterfallsonthatpartoftheglobewheresunlightstrikesleastdirectly,andsummerfallswheresunlightstrikesmostdirectly,regardlessoftheEarth'sdistancefromtheSun.
Seealso
Solstice
References
4/8/2015 PerihelionandaphelionWikipedia,thefreeencyclopedia
http://en.wikipedia.org/wiki/Perihelion_and_aphelion 2/2
1.
"IntroductoryAstronomy:Elipses"(http://astro.wsu.edu/worthey/astro/html/lecellipse.html).WashingtonStateUniversity.
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Categories: Orbits Earth
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