Editing Earth's orbit (section)

==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&nbsp;[[astronomical unit|AU]]<ref group=nb name=apsis/>
|-
| [[Apsis|perihelion]]
| {{convert|147.10|e6km|abbr=on}}<br /> 0.98329&nbsp;AU<ref group=nb name=apsis/>
|-
| [[Semi-major axis|semimajor axis]]
| {{convert|149.60|e6km|abbr=on}}<br /> 1.0000010178&nbsp;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}}&nbsp;days<ref>The figure appears in multiple references, and is derived from the VSOP87 elements from section 5.8.3, p.&nbsp;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]]