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Earth's orbit
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{{Short description|Trajectory of Earth around the Sun}} {{for|objects orbiting Earth|Geocentric orbit}} {{Distinguish|Earth orbit (disambiguation)}} {{Use dmy dates|date=March 2020}} [[File:North season.jpg|thumb|upright=1.5|Earth at seasonal points in its orbit (not to scale)]] [[File:EarthsOrbit en.png|thumb|Earth orbit (yellow) compared to a circle (gray)]] [[Earth]] [[orbit]]s the [[Sun]] at an [[astronomical unit|average distance]] of {{convert|149.60|e6km|e6mi|abbr=unit}}, or 8.317 [[light-second|light-minutes]],<ref>{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Display=Facts&Object=Sun |title=Sun: Facts & Figures |work=Solar System Exploration |publisher=[[National Aeronautics and Space Administration]] |access-date=July 29, 2015 |archive-url=https://web.archive.org/web/20150703111716/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts |archive-date=July 3, 2015 |url-status=dead |df=mdy-all}}</ref> in a [[retrograde and prograde motion|counterclockwise]] direction as viewed from above the [[Northern Hemisphere]]. One complete orbit takes {{gaps|365.256||}} days (1 [[sidereal year]]), during which time Earth has traveled {{convert|940|e6km|e6mi|0|abbr=unit}}.<ref name="AA">[[Jean Meeus]], ''Astronomical Algorithms'' 2nd ed, {{ISBN|0-943396-61-1}} (Richmond, VA: Willmann-Bell, 1998) 238. See [[Ellipse#Circumference]]. The formula by Ramanujan is accurate enough.{{cn|reason=according to whom?|date=November 2021}}</ref> Ignoring the influence of other [[Solar System]] bodies, '''Earth's orbit''', also called '''Earth's revolution''', is an [[ellipse]] with the Earth–Sun [[barycenter]] as one [[focus (geometry)|focus]] with a current [[orbital eccentricity|eccentricity]] of 0.0167. Since this value is close to zero, the center of the orbit is relatively close to the center of the Sun (relative to the size of the orbit). As seen from Earth, the planet's orbital [[prograde motion]] makes the Sun [[diurnal motion|appear to move]] with respect to [[fixed stars|other stars]] at a rate of about 1° eastward per [[solar day]] (or a Sun or Moon diameter every 12 hours).<ref group=nb>Our planet takes about 365 days to orbit the Sun. A full orbit has 360°. That fact demonstrates that each day, Earth travels roughly 1° in its orbit. Thus, the Sun will appear to move across the sky relative to the stars by that same amount.</ref> Earth's [[orbital speed]] averages {{convert|29.78|km/s|mi/s km/h mph|0|abbr=unit}}, which is fast enough to cover the planet's diameter in 7 minutes and [[lunar distance|the distance]] to the [[Moon]] in 4 hours.<ref name="earth_fact_sheet">{{cite web|last=Williams|first=David R.|date=2004-09-01|url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html|title=Earth Fact Sheet|publisher=[[NASA]]|access-date=2007-03-17}}</ref> The point towards which the Earth in its solar orbit is directed at any given instant is known as the "apex of the Earth's way".<ref>{{Cite web|url=https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1893PA......1..373S&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES|title=1893PA......1..373S Page 373|website=articles.adsabs.harvard.edu}}</ref><ref>{{Cite web|url=https://www.tidjma.tn/en/astro/apex/#google_vignette|title=Apex|website=Tidjma.tn}}</ref> From a vantage point above the north pole of either the Sun or Earth, Earth would appear to revolve in a [[clockwise|counterclockwise]] direction around the Sun. From the same vantage point, both the Earth and the Sun would appear to rotate also in a counterclockwise direction. ==History of study== {{Main|Heliocentrism}} [[Image:Heliocentric.jpg|thumb|Heliocentric Solar System]] [[Image:geoz wb en.svg|thumb|Heliocentrism (lower panel) in comparison to the geocentric model (upper panel), not to scale]] [[Heliocentrism]] is the scientific model that first placed the Sun at the center of the [[Solar System]] and put the planets, including Earth, in its orbit. Historically, heliocentrism is opposed to [[geocentrism]], which placed the Earth at the center. [[Aristarchus of Samos]] already proposed a heliocentric model in the third century BC. In the sixteenth century, [[Nicolaus Copernicus]]' ''[[De revolutionibus]]'' presented a full discussion of a [[Copernican heliocentrism|heliocentric model]] of the universe <ref>{{Cite book|title = De revolutionibus orbium coelestium|publisher = Johannes Petreius|date = 1543}}</ref> in much the same way as [[Ptolemy]] had presented his geocentric model in the second century. This "[[Copernican Revolution]]" resolved the issue of planetary [[apparent retrograde motion|retrograde motion]] by arguing that such motion was only perceived and apparent. According to historian [[Jerry Brotton]], "Although Copernicus's groundbreaking book ... had been [printed more than] a century earlier, [the Dutch mapmaker] [[Joan Blaeu]] was the first mapmaker to incorporate his revolutionary heliocentric theory into a map of the world."<ref>[[Jerry Brotton]], ''A History of the World in Twelve Maps'', London: Allen Lane, 2012, {{ISBN|9781846140990}} p. 262.</ref> ==Influence on Earth== {{Main|Season}} Because of Earth's [[axial tilt]] (often known as the obliquity of the [[ecliptic]]), the inclination of the Sun's trajectory in the sky (as seen by an observer on Earth's surface) varies over the course of the year. For an observer at a northern latitude, when the north pole is tilted toward the Sun the day lasts longer and the Sun appears higher in the sky. This results in warmer average temperatures, as additional solar radiation reaches the surface. When the north pole is tilted away from the Sun, the reverse is true and the weather is generally cooler. North of the [[Arctic Circle]] and south of the [[Antarctic Circle]], an extreme case is reached in which there is no daylight at all for part of the year, and continuous daylight during the opposite time of year. This is called [[polar night]] and [[midnight sun]], respectively. This variation in the weather (because of the direction of the Earth's axial tilt) results in the [[season]]s.<ref>{{Cite web|url = http://spaceplace.nasa.gov/seasons/en/|title = What causes the seasons? (NASA)|access-date = January 22, 2015}}</ref> ==Events in the orbit== {{See also|Precession (astronomy)|Milankovitch cycles}} By astronomical convention, the four seasons are determined by the [[solstice]]s (the two points in the Earth's orbit of the maximum tilt of the Earth's axis, toward the Sun or away from the Sun) and the [[equinox]]es (the two points in the Earth's orbit where the Earth's tilted axis and an imaginary line drawn from the Earth to the Sun are exactly perpendicular to one another). The solstices and equinoxes divide the year up into four approximately equal parts. In the northern hemisphere [[winter solstice]] occurs on or about December 21; summer solstice is near June 21; spring equinox is around March 20, and autumnal equinox is about September 23.<ref name=":0">{{Cite web|url = http://wwp.greenwichmeantime.com/longest-day/equinox-solstice-2010-2019.htm|title = Date & Time of Solstices & Equinoxes|date = August 28, 2013 |access-date = January 22, 2015 }}</ref> The effect of the Earth's axial tilt in the southern hemisphere is the opposite of that in the northern hemisphere, thus the seasons of the solstices and equinoxes in the southern hemisphere are the reverse of those in the northern hemisphere (e.g. the northern summer solstice is at the same time as the southern winter solstice). In modern times, Earth's [[Perihelion and aphelion|perihelion]] occurs around January 3, and the [[Perihelion and aphelion|aphelion]] around July 4. In other words, the Earth is closer to the Sun in January, and further away in July, which might seem counter-intuitive to those residing in the northern hemisphere, where it is colder when the Earth is closest to the sun and warmer when it is furthest away. The changing Earth-Sun distance results in an increase of about 7% in total solar energy reaching the Earth at perihelion relative to aphelion.<ref>{{cite web|url=https://www.itacanet.org/the-sun-as-a-source-of-energy/part-2-solar-energy-reaching-the-earths-surface/|title=Solar Energy Reaching The Earth's Surface|publisher=ITACA|access-date=2022-01-30|archive-date=30 January 2022|archive-url=https://web.archive.org/web/20220130032443/https://www.itacanet.org/the-sun-as-a-source-of-energy/part-2-solar-energy-reaching-the-earths-surface/|url-status=usurped}}</ref> Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of surface covered by water in the southern hemisphere.<ref>{{cite web|last=Williams|first=Jack|date=2005-12-20|url=https://www.usatoday.com/weather/tg/wseason/wseason.htm|title=Earth's tilt creates seasons|publisher=USAToday|access-date=2007-03-17}}</ref> The [[Hill sphere]] ([[gravitational]] sphere of influence) of the Earth is about 1,500,000 [[kilometer]]s (0.01 [[Astronomical unit|AU]]) in radius, or approximately four times the average distance to the Moon.<ref>{{cite web|author=Vázquez, M.|author2=Montañés Rodríguez, P.|author3=Palle, E.|date=2006|url= http://www.iac.es/folleto/research/preprints/files/PP06024.pdf|title=The Earth as an Object of Astrophysical Interest in the Search for Extrasolar Planets|publisher=Instituto de Astrofísica de Canarias|access-date=2007-03-21}}</ref><ref group="nb">For the Earth, the Hill radius is :<math>R_H = a \left(\frac{m}{3M}\right)^{1/3},</math> where ''m'' is the mass of the Earth, ''a'' is an astronomical unit, and ''M'' is the mass of the Sun. So the radius in AU is about <math>\left(\frac{1}{3 \cdot 332\,946}\right)^{1/3} \approx 0.01</math>.{{citation needed|date=January 2010}}</ref> This is the maximal distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects orbiting the Earth must be within this radius, otherwise, they may become unbound by the gravitational perturbation of the Sun. {| class="wikitable" |+ Orbital characteristics |- | [[Epoch (astronomy)|epoch]] | [[J2000.0]]<ref group=nb name=epoch/> |- | [[Apsis|aphelion]] | {{convert|152.10|e6km|abbr=on}}<br /> 1.0167 [[astronomical unit|AU]]<ref group=nb name=apsis/> |- | [[Apsis|perihelion]] | {{convert|147.10|e6km|abbr=on}}<br /> 0.98329 AU<ref group=nb name=apsis/> |- | [[Semi-major axis|semimajor axis]] | {{convert|149.60|e6km|abbr=on}}<br /> 1.0000010178 AU<ref name=VSOP87/> |- | [[Orbital eccentricity|eccentricity]] | 0.0167086<ref name="VSOP87"/> |- | [[inclination]] | 7.155° to [[Sun]]'s [[equator]]<br />1.578690°<ref name=Allen294/> to [[invariable plane]] |- | [[longitude of the ascending node]] | 174.9°<ref name="VSOP87"/> |- | [[longitude of periapsis|longitude of perihelion]] | 102.9°<ref name="VSOP87"/> |- | [[argument of periapsis]] | 288.1°<ref name="VSOP87"/><ref group=nb name=arg_peri/> |- | [[Orbital period|period]] | {{gaps|365.256|363|004}} days<ref>The figure appears in multiple references, and is derived from the VSOP87 elements from section 5.8.3, p. 675 of the following: {{cite journal |title = Numerical expressions for precession formulae and mean elements for the Moon and planets |journal=Astronomy and Astrophysics |volume=282 |issue=2 |pages=663–683 |date=February 1994 |last1=Simon |first1=J. L. |last2=Bretagnon |first2=P. |last3=Chapront |first3=J. |last4=Chapront-Touzé |first4=M. |last5=Francou |first5=G. |last6=Laskar | first6=J. |bibcode=1994A&A...282..663S }}</ref> |- | average [[orbital speed]] | {{convert|29.78|km/s|abbr=on}}<ref name="earth_fact_sheet"/><br />{{convert|107208|km/h|abbr=on}} |- | speed at aphelion | {{convert|29.29|km/s|abbr=on}}<ref name="earth_fact_sheet"/> |- | speed at perihelion | {{convert|30.29|km/s|abbr=on}}<ref name="earth_fact_sheet"/> |} The following diagram illustrates the positions and relationship between the lines of solstices, equinoxes, and [[Apsis|apsides]] of Earth's elliptical orbit. The six Earth images are positions along the orbital ellipse, which are sequentially the perihelion (periapsis—nearest point to the Sun) on anywhere from January 2 to January 5, the point of March equinox on March 19, 20, or 21, the point of June solstice on June 20, 21, or 22, the aphelion (apoapsis—the farthest point from the Sun) on anywhere from July 3 to July 5, the September equinox on September 22, 23, or 24, and the December solstice on December 21, 22, or 23.<ref name=":0" /> [[Image:Seasons1.svg|thumb|upright=2.3|Exaggerated illustration of Earth's elliptical orbit around the Sun, marking that the orbital extreme points ([[apoapsis]] and [[periapsis]]) are not the same as the four [[season]]al extreme points ([[equinox]] and [[solstice]])]] [[File:Motion of Sun, Earth and Moon around the Milky Way.jpg|thumb|The orientation of the motion of Earth, Moon and the Sun]] ==Future== {{Main|Stability of the Solar System}} Mathematicians and astronomers (such as [[Pierre-Simon Laplace|Laplace]], [[Joseph-Louis Lagrange|Lagrange]], [[Carl Friedrich Gauss|Gauss]], [[Henri Poincaré|Poincaré]], [[Kolmogorov]], [[Vladimir Arnold]], and [[Jürgen Moser]]) have searched for evidence for the stability of the planetary motions, and this quest led to many mathematical developments and several successive "proofs" of stability for the Solar System.<ref>{{cite encyclopedia|encyclopedia=Encyclopedia of Astronomy and Astropvhysics|editor-last=Murdin|editor-first=Paul|id=article 2198|last=Laskar|first=J.|title=Solar System: Stability|date=2001|publisher=[[Institute of Physics Publishing]]|location=Bristol}}</ref> By most predictions, Earth's orbit will be relatively stable over long periods.<ref>{{cite book|last=Gribbin|first=John|title=Deep simplicity : bringing order to chaos and complexity|date=2004|publisher=[[Random House]]|location=New York|isbn=978-1-4000-6256-0|edition=1st U.S.|url-access=registration|url=https://archive.org/details/deepsimplicitybr00grib}}</ref> In 1989, [[Jacques Laskar]]'s work indicated that Earth's orbit (as well as the orbits of all the inner planets) can become chaotic and that an error as small as 15 meters in measuring the initial position of the Earth today would make it impossible to predict where Earth would be in its orbit in just over 100 million years' time.<ref>{{Cite news|url = http://www.news.com.au/national/earth-venus-smash-up-possible/story-e6frfkp9-1225732704812|title = Earth-Venus smash-up possible|date = June 11, 2009|access-date = Jan 22, 2015|archive-date = 23 January 2015|archive-url = https://web.archive.org/web/20150123024437/http://www.news.com.au/national/earth-venus-smash-up-possible/story-e6frfkp9-1225732704812|url-status = dead}}</ref> Modeling the Solar System is a subject covered by the [[n-body problem]]. ==See also== * [[Earth phase]] * [[Earth's rotation]] * [[Spaceship Earth]] * [[Calendar]] ==Notes== {{reflist|group=nb|refs= <ref name=apsis>aphelion = ''a'' × (1 + ''e''); perihelion = ''a'' × (1 – ''e''), where ''a'' is the semi-major axis and ''e'' is the eccentricity.</ref> <ref name=epoch>All astronomical quantities vary, both [[Secular phenomena|secularly]] and [[Frequency|periodically]]. The quantities given are the values at the instant [[J2000.0]] of the secular variation, ignoring all periodic variations.</ref> <ref name=arg_peri>The reference lists the [[longitude of periapsis|longitude of perihelion]], which is the sum of the longitude of the ascending node and the argument of perihelion. Subtracting from that (102.937°) the node longitude of 174.873° gives −71.936°. Adding 360° gives 288.064°. That addition does not change the angle but expresses it in the usual 0–360° range for longitudes.</ref> }} ==References== {{reflist|refs= <ref name=Allen294>{{cite book | title=Allen's Astrophysical Quantities | author=Allen, Clabon Walter | author2=Cox, Arthur N. | publisher=Springer | date=2000 | isbn=0-387-98746-0 | url=https://books.google.com/books?id=w8PK2XFLLH8C&pg=PA294 | page=294}}</ref> <ref name=VSOP87>{{cite journal |title = Numerical expressions for precession formulae and mean elements for the Moon and planets |journal=Astronomy and Astrophysics |volume=282 |issue=2 |pages=663–683 |date=February 1994 |last1=Simon |first1=J.L. |last2=Bretagnon |first2=P. |last3=Chapront |first3=J. |last4=Chapront-Touzé |first4=M. |last5=Francou |first5=G. |last6=Laskar | first6=J. |bibcode=1994A&A...282..663S }}</ref> }} ==External links== {{commonscat|Orbit of Earth}} * [https://nightsky.jpl.nasa.gov/docs/HowFast.pdf Earth – Speed through space – <!---between 0.8 - 1.9 M mph--->about 1 million miles an hour] – [[NASA]] & ([[Wikipedia:Reference desk/Archives/Science/2019 July 20#How fast are we moving through space?|WP discussion]]) {{Earth}} {{Geodesy navbox}} {{Orbits}} {{Portal bar|Earth|Weather|Astronomy|Stars|Spaceflight|Outer space|Science}} {{DEFAULTSORT:Earth's Orbit}} [[Category:Earth|Orbit]] [[Category: Dynamics of the Solar System]] [[Category:Geodesy]]
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