Editing Apsis (section)

==Perihelion and aphelion==
{{Redirect|Perihelion}}
{{Redirect|Aphelion}}
[[File:Perihelion-Aphelion.svg|thumb|Diagram of a body's direct [[orbit]] around the [[Sun]] with its nearest (perihelion) and  farthest (aphelion) points]]

The perihelion (q) and aphelion (Q) are the nearest and farthest points respectively of a body's direct [[orbit]] around the [[Sun]].

Comparing [[osculating elements]] at a specific [[Epoch (astronomy)|epoch]] to those at a different epoch will generate differences. The time-of-perihelion-passage as one of six osculating elements is not an exact prediction (other than for a generic [[Two-body problem|two-body model]]) of the actual minimum distance to the Sun using the [[n-body problem|full dynamical model]]. Precise predictions of perihelion passage require [[numerical integration]].

===Inner planets and outer planets===
The two images below show the orbits, [[orbital node]]s, and positions of perihelion (q) and aphelion (Q) for the planets of the Solar System<ref name=":0">{{cite web|title=the definition of apsis|url=http://dictionary.reference.com/browse/apsis|website=Dictionary.com|access-date=November 28, 2015|archive-date=December 8, 2015|archive-url=https://web.archive.org/web/20151208101127/http://dictionary.reference.com/browse/apsis|url-status=live}}</ref> as seen from above the northern pole of [[ecliptic|Earth's ecliptic plane]], which is [[coplanar]] with [[orbital plane (astronomy)|Earth's orbital plane]]. The planets travel counterclockwise around the Sun and for each planet, the blue part of their orbit travels north of the ecliptic plane, the pink part travels south, and dots mark perihelion (green) and aphelion (orange).

The first image (below-left) features the ''inner'' planets, situated outward from the Sun as Mercury, Venus, Earth, and Mars. The ''reference'' Earth-orbit is colored yellow and represents the [[orbital plane (astronomy)|orbital plane of reference]]. At the time of vernal equinox, the Earth is at the bottom of the figure. The second image (below-right) shows the ''outer'' planets, being Jupiter, Saturn, Uranus, and Neptune.

The orbital nodes are the two end points of the [[orbital node#Node distinction|"line of nodes"]] where a planet's tilted orbit intersects the plane of reference;<ref name="darlinglon">{{cite encyclopedia |url=http://www.daviddarling.info/encyclopedia/L/line_of_nodes.html |title=line of nodes |encyclopedia=The Encyclopedia of Astrobiology, Astronomy, and Spaceflight |first=David |last=Darling |access-date=May 17, 2007 |archive-date=August 23, 2019 |archive-url=https://web.archive.org/web/20190823203510/http://www.daviddarling.info/encyclopedia/L/line_of_nodes.html |url-status=live }}</ref> here they may be 'seen' as the points where the blue section of an orbit meets the pink.

<gallery caption="" widths="300px" heights="300px" class="skin-invert-image">
Image:Inner Planet Orbits 02.svg|The perihelion (green) and aphelion (orange) points of the [[inner planets]] of the Solar System
Image:Outer Planet Orbits 02.svg|The perihelion (green) and aphelion (orange) points of the [[outer planets]] of the Solar System
</gallery>

===Lines of apsides===
The chart shows the extreme range—from the closest approach (perihelion) to farthest point (aphelion)—of several orbiting [[celestial bodies]] of the [[Solar System]]: the planets, the known dwarf planets, including [[ceres (dwarf planet)|Ceres]], and [[Halley's Comet]]. The length of the horizontal bars correspond to the extreme range of the orbit of the indicated body around the Sun. These extreme distances (between perihelion and aphelion) are ''the lines of apsides'' of the orbits of various objects around a host body.

{{Distance from Sun using EasyTimeline}}

===Earth perihelion and aphelion===
Currently, the Earth reaches perihelion in early January, approximately 14 days after the [[December solstice]]. At perihelion, the Earth's center is about {{val|0.98329}} [[astronomical unit]]s (AU) or {{convert|147098070|km|mi|abbr=on}} from the Sun's center. In contrast, 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 {{val|1.01671|u=AU}} or {{convert|152097700|km|mi|abbr=on}}.

The dates of perihelion and aphelion change over time due to precession and other orbital factors, which follow cyclical patterns known as [[Milankovitch cycles]]. In the short term, such dates can vary up to 2 days from one year to another.<ref>{{cite web |url=https://www.timeanddate.com/astronomy/perihelion-aphelion-solstice.html |title=Perihelion, Aphelion and the Solstices |publisher=timeanddate.com |access-date=January 10, 2018 |archive-date=January 3, 2018 |archive-url=https://web.archive.org/web/20180103052358/https://www.timeanddate.com/astronomy/perihelion-aphelion-solstice.html |url-status=live }}</ref> This significant variation is due to the presence of the Moon: while the Earth–Moon [[barycenter]] is moving on a stable orbit around the Sun, the position of the Earth's center which is on average about {{convert|4700|km|mi|-2}} from the barycenter, could be shifted in any direction from it—and this affects the timing of the actual closest approach between the Sun's and the Earth's centers (which in turn defines the timing of perihelion in a given year).<ref>{{cite web |url=http://aa.usno.navy.mil/faq/docs/apsides.php |title=Variation in Times of Perihelion and Aphelion |publisher=Astronomical Applications Department of the U.S. Naval Observatory |date=August 11, 2011 |access-date=January 10, 2018 |archive-date=January 11, 2018 |archive-url=https://web.archive.org/web/20180111165154/http://aa.usno.navy.mil/faq/docs/apsides.php |url-status=dead }}</ref>

Because of the increased distance at aphelion, only 93.55% of the radiation from the Sun falls on a given area of Earth's surface as does at perihelion, but this does not account for the [[season]]s, which result instead from the [[Axial tilt|tilt of Earth's axis]] of 23.4° away from perpendicular to the plane of Earth's orbit.<ref>{{cite web|title=Solar System Exploration: Science & Technology: Science Features: Weather, Weather, Everywhere?|url=http://www.nasa.gov/audience/foreducators/postsecondary/features/F_Planet_Seasons.html|publisher=[[NASA]]|access-date=September 19, 2015|archive-date=September 29, 2015|archive-url=https://web.archive.org/web/20150929033150/http://www.nasa.gov/audience/foreducators/postsecondary/features/F_Planet_Seasons.html|url-status=live}}</ref> Indeed, at both perihelion and aphelion it is [[summer]] in one hemisphere while it is [[winter]] in the other one. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of the Earth's distance from the Sun.

In the northern hemisphere, summer occurs at the same time as aphelion, when solar radiation is lowest. Despite this, summers in the northern hemisphere are on average {{convert|2.3|C-change|F-change|0}} warmer than in the southern hemisphere, because the northern hemisphere contains larger land masses, which are easier to heat than the seas.<ref name="Earth at Aphelion">{{cite web |url=http://spaceweather.com/glossary/aphelion.html |title=Earth at Aphelion |publisher=Space Weather |date=July 2008 |access-date=July 7, 2015 |archive-date=July 17, 2015 |archive-url=https://web.archive.org/web/20150717184242/http://spaceweather.com/glossary/aphelion.html |url-status=live }}</ref>

Perihelion and aphelion do however have an indirect effect on the seasons: because Earth's [[orbital speed]] is minimum at aphelion and maximum at perihelion, the planet takes longer to orbit from June solstice to September equinox than it does from December solstice to March equinox. Therefore, summer in the northern hemisphere lasts slightly longer (93 days) than summer in the southern hemisphere (89 days).<ref>{{cite web |last1=Rockport |first1=Steve C. |title=How much does aphelion affect our weather? We're at aphelion in the summer. Would our summers be warmer if we were at perihelion, instead? |url=https://usm.maine.edu/planet/how-much-does-aphelion-affect-our-weather-were-aphelion-summer-would-our-summers-be-warmer-if |website=Planetarium |publisher=[[University of Southern Maine]] |access-date=4 July 2020 |archive-date=July 6, 2020 |archive-url=https://web.archive.org/web/20200706154815/https://usm.maine.edu/planet/how-much-does-aphelion-affect-our-weather-were-aphelion-summer-would-our-summers-be-warmer-if |url-status=live }}</ref>

Astronomers commonly express the timing of perihelion relative to the [[First Point of Aries]] not in terms of days and hours, but rather as an angle of orbital displacement, the so-called [[longitude of the periapsis]] (also called longitude of the pericenter). For the orbit of the Earth, this is called the ''longitude of perihelion'', and in 2000 it was about 282.895°; by 2010, this had advanced by a small fraction of a degree to about 283.067°,<ref>{{cite web|url=http://data.giss.nasa.gov/ar5/srorbpar.html|archive-url=https://web.archive.org/web/20151002065753/http://data.giss.nasa.gov/ar5/srorbpar.html|archive-date=2015-10-02|title=Data.GISS: Earth's Orbital Parameters|website=data.giss.nasa.gov}}</ref> i.e. a mean increase of 62" per year.

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 (Milankovitch cycles).

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, called the [[apsidal precession]]. (This is closely related to the [[Axial precession (astronomy)|precession of the axes]].) The dates and times of the perihelions and aphelions for several past and future years are listed in the following table:<ref>{{Cite web|url=http://astropixels.com/ephemeris/perap2001.html|title=Earth at Perihelion and Aphelion: 2001 to 2100|last=Espenak|first=Fred|website=astropixels|access-date=June 24, 2021|archive-date=July 13, 2021|archive-url=https://web.archive.org/web/20210713131143/http://astropixels.com/ephemeris/perap2001.html|url-status=live}}</ref>

{| class="wikitable" style="margin-left:auto; margin-right:auto;"
! rowspan=2 width=50 | Year
! colspan=2 | Perihelion
! colspan=2 | Aphelion
|-
! width=95| Date !! width=80 | Time ([[Coordinated Universal Time|UT]])
! width=95| Date !! width=80 | Time ([[Coordinated Universal Time|UT]])
|-
! 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
|-
!2021
|January 2
|13:51
|July 5
|22:27
|-
!2022
|January 4
|06:55
|July 4
|07:11
|-
!2023
|January 4
|16:17
|July 6
|20:07
|-
!2024
|January 3
|00:39
|July 5
|05:06
|-
!2025
|January 4
|13:28
|July 3
|19:55
|-
!2026
|January 3
|17:16
|July 6
|17:31
|-
!2027
|January 3
|02:33
|July 5
|05:06
|-
!2028
|January 5
|12:28
|July 3
|22:18
|-
!2029
|January 2
|18:13
|July 6
|05:12
|}

===Other planets===
The following table shows the distances of the [[planet]]s and [[dwarf planet]]s from the Sun at their perihelion and aphelion.<ref>{{Cite web |url=http://solarsystem.nasa.gov/planets/compare |title=NASA planetary comparison chart |access-date=August 4, 2016 |archive-url=https://web.archive.org/web/20160804162808/http://solarsystem.nasa.gov/planets/compare |archive-date=August 4, 2016 }}</ref>
<!-- It's surprising that values are precise to the km, but the data is from NASA... -->
{| class="wikitable sortable" style="margin-left:auto; margin-right:auto;"
!class="unsortable"|Type of body !!Body !!class="unsortable"|Distance from Sun at perihelion !!class="unsortable"|Distance from Sun at aphelion !!difference (%) !![[insolation]]<br/>difference (%)
|-
|rowspan=8|Planet
|{{sort|1|[[Mercury (planet)|Mercury]]}}||{{convert|46001009|km|mi|abbr=on}}||{{convert|69817445|km|mi|abbr=on}} || 34% || 57%
|-
|{{sort|2|[[Venus]]}}||{{convert|107476170|km|mi|abbr=on}}||{{convert|108942780|km|mi|abbr=on}} || 1.3% || 2.8%
|-
|{{sort|3|[[Earth]]}} || {{convert|147098291|km|mi|abbr=on}}||{{convert|152098233|km|mi|abbr=on}} || 3.3% || 6.5%
|-
|{{sort|4|[[Mars]]}} || {{convert|206655215|km|mi|abbr=on}}||{{convert|249232432|km|mi|abbr=on}} || 17% || 31%
|-
|{{sort|5|[[Jupiter]]}}||{{convert|740679835|km|mi|abbr=on}}||{{convert|816001807|km|mi|abbr=on}} || 9.2% || 18%
|-
|{{sort|6|[[Saturn]]}}||{{convert|1349823615|km|mi|abbr=on}}||{{convert|1503509229|km|mi|abbr=on}} || 10% || 19%
|-
|{{sort|7|[[Uranus]]}}||{{convert|2734998229|km|mi|abbr=on}}||{{convert|3006318143|km|mi|abbr=on}} || 9.0% || 17%
|-
|{{sort|8|[[Neptune]]}}||{{convert|4459753056|km|mi|abbr=on}}||{{convert|4537039826|km|mi|abbr=on}} || 1.7% || 3.4%
|-
|rowspan=5|Dwarf planet
|{{sort|9|[[Ceres (dwarf planet)|Ceres]]}}||{{convert|380951528|km|mi|abbr=on}}||{{convert|446428973|km|mi|abbr=on}} || 15% || 27%
|-
|{{sort|10|[[Pluto]]}}||{{convert|4436756954|km|mi|abbr=on}}||{{convert|7376124302|km|mi|abbr=on}} || 40% || 64%
|-
|{{sort|11|[[Haumea (dwarf planet)|Haumea]]}}||{{convert|5157623774|km|mi|abbr=on}}||{{convert|7706399149|km|mi|abbr=on}} || 33% || 55%
|-
|{{sort|12|[[Makemake (dwarf planet)|Makemake]]}}||{{convert|5671928586|km|mi|abbr=on}}||{{convert|7894762625|km|mi|abbr=on}} || 28% || 48%
|-
|{{sort|13|[[Eris (dwarf planet)|Eris]]}}||{{convert|5765732799|km|mi|abbr=on}}||{{convert|14594512904|km|mi|abbr=on}} || 60% || 84%
|}