Editing Solar time (section)

==Apparent solar time==
{{Also|Solar day}}
The '''apparent sun''' is the true sun as seen by an observer on Earth.<ref>{{cite web|url=http://www.astro.uvic.ca/~tatum/celmechs/celm6.pdf|title=Celestial Mechanics Chapter 6|first=J.B.|last=Tatum|website=University of Victoria|archive-url=https://web.archive.org/web/20150923175534/http://www.astro.uvic.ca/~tatum/celmechs/celm6.pdf|date=March 27, 2022|archive-date=September 23, 2015|url-status=live}}</ref> '''Apparent solar time''' or '''true solar time'''{{efn|1= 'apparent' is commonly used in English-language sources, but 'true' is used in French astronomical literature and has become nearly as common in English sources. See:
* {{cite book |last1=Vince |first1=Samuel |title=A Complete System Of Astronomy Vol 1 |date=1797 |publisher=Cambridge University Press |page=44 |url=https://archive.org/details/completesystemof025477mbp/page/44/mode/2up|quote=What we call ''apparent'' time the French call ''true''}}
* {{cite web |title=Comprendre - Concepts fondamentaux - Echelles de temps |url=http://www.bdl.fr/fr/ephemerides/astronomie/Promenade/pages3/325.html#tempsvrai |website=Bureau des Longitudes|language=fr |date=November 23, 2009|archive-url=https://web.archive.org/web/20091123072000/http://www.bdl.fr/fr/ephemerides/astronomie/Promenade/pages3/325.html#tempsvrai |archive-date=November 23, 2009 |quote=''temps vrai'' [true time]}} 
* {{cite web|last1=Allison|first1=Michael|last2=Schmunk|first2=Robert|title=Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock|url=http://www.giss.nasa.gov/tools/mars24/help/notes.html|website=[[Goddard Institute for Space Studies]]|publisher=[[National Aeronautics and Space Administration]]|access-date=October 8, 2015|date=June 30, 2015|url-status=live|archive-url=https://web.archive.org/web/20150925020710/http://www.giss.nasa.gov/tools/mars24/help/notes.html|archive-date=September 25, 2015|quote=the solar hour angle or True Solar Time (TST)}}
}} is based on the apparent motion of the actual [[Sun]]. It is based on the '''apparent solar day''', the interval between two successive returns of the Sun to the local [[meridian (astronomy)|meridian]].<ref>{{cite web|url=https://aa.usno.navy.mil/faq/asa_glossary#solar-time,-apparent|work=Glossary, Astronomical Almanac Online|date=2021|publisher=[[Her Majesty's Nautical Almanac Office]] and the [[United States Naval Observatory]]|title=solar time, apparent}}</ref><ref>{{cite web|last1=Yallop|first1=B. D.|last2=Hohenker|first2=C. Y.|date=August 1989|url=https://astro.ukho.gov.uk/nao/aisinfo/ais058.pdf|at=Solar Location Diagram|title=Astronomical Information Sheet No. 58|website=HM Nautical Almanac Office|access-date=June 17, 2022|archive-date=December 23, 2022|archive-url=https://web.archive.org/web/20221223124221/https://astro.ukho.gov.uk/nao/aisinfo/ais058.pdf|url-status=dead}}</ref> Apparent solar time can be crudely measured by a [[sundial]].{{efn|The equivalent on Mars is termed '''Mars local true solar time''' (LTST).<ref>{{cite web|last1=Allison|first1=Michael|last2=Schmunk|first2=Robert|title=Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock|url=http://www.giss.nasa.gov/tools/mars24/help/notes.html|website=[[Goddard Institute for Space Studies]]|publisher=[[National Aeronautics and Space Administration]]|access-date=October 8, 2015|date=June 30, 2015|url-status=live|archive-url=https://web.archive.org/web/20150925020710/http://www.giss.nasa.gov/tools/mars24/help/notes.html|archive-date=September 25, 2015}}</ref><ref>{{cite journal|doi=10.1016/S0032-0633(99)00092-6|url=https://pubs.giss.nasa.gov/abs/al05000n.html|title=A post-Pathfinder evaluation of areocentric solar coordinates with improved timing recipes for Mars seasonal/diurnal climate studies|journal=Planetary and Space Science|volume=48|issue=2–3|pages=215|year=2000|last1=Allison|first1=Michael|last2=McEwen|first2=Megan|bibcode=2000P&SS...48..215A|url-status=live|archive-url=https://web.archive.org/web/20150623105917/http://pubs.giss.nasa.gov/abs/al05000n.html|archive-date=June 23, 2015|hdl=2060/20000097895|s2cid=123014765 |hdl-access=free}}</ref>}}

The length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes. First, due to the eccentricity of [[Earth's orbit]], Earth moves faster when it is nearest the Sun ([[perihelion]]) and slower when it is farthest from the Sun ([[aphelion]]) (see [[Kepler's laws of planetary motion]]). Second, due to Earth's [[axial tilt]] (known as the ''obliquity of the [[ecliptic]]''), the Sun's annual motion is along a [[great circle]] (the [[ecliptic]]) that is tilted to Earth's [[celestial equator]].  When the Sun crosses the equator at both [[equinox]]es, the Sun's daily shift (relative to the background stars) is at an angle to the equator, so the projection of this shift onto the equator is less than its [[mean motion|average]] for the year; when the Sun is farthest from the equator at both [[solstice]]s, the Sun's shift in position from one day to the next is parallel to the equator, so the projection onto the equator of this shift is larger than the average for the year (see [[tropical year]]). In June and December when the sun is farthest from the celestial equator, a given shift along the ecliptic corresponds to a large shift at the equator. Therefore, apparent solar days are shorter in March and September than in June or December.

{| class=wikitable
|+ Length of apparent solar day (1998)<ref name=Meeus>Jean Meeus (1997), ''Mathematical astronomy morsels'' (Richmond, VA: Willmann-Bell) 346. {{ISBN|0-943396-51-4}}.</ref>
|-
!Date
!Duration in mean solar time
|-
| February 11 || 24 hours
|-
| March 26 || 24 hours − 18.1 seconds
|-
| May 14 || 24 hours
|-
| June 19 || 24 hours + 13.1 seconds
|-
| July 25/26 || 24 hours
|-
| September 16 || 24 hours − 21.3 seconds
|-
| November 2/3 || 24 hours
|-
| December 22 || 24 hours + 29.9 seconds
|}

These lengths will change slightly in a few years and significantly in thousands of years.