Editing Equinox (section)

==Equinoxes on Earth==
{{main|Sun path}}
{{See also|Equinox (celestial coordinates)}}

=== General ===
Systematically observing the [[sunrise]], people discovered that it occurs between two extreme locations at the [[horizon]] and eventually noted the midpoint between the two. Later it was realized that this happens on a day when the duration of the day and the night are practically equal and the word "equinox" comes from Latin ''aequus'', meaning "equal",  and ''nox'', meaning "night".

In the northern hemisphere, the ''vernal equinox'' (March) conventionally marks the beginning of [[Spring (season)|spring]] in most cultures and is considered the start of the New Year in the [[Assyrian calendar]], Hindu, and the Persian or [[Iranian calendar]]s,{{efn|The year in the [[Iranian calendar]] begins on [[Nowruz]], which means "new day".}} while the ''autumnal equinox'' (September) marks the beginning of autumn.<ref>{{cite web |url=https://www.timeanddate.com/calendar/march-equinox.html |website=Time and Date |title=March Equinox – Equal Day and Night, Nearly |year=2017 |language=en |access-date=22 May 2017}}</ref> Ancient Greek calendars too had the beginning of the year either at the autumnal or vernal equinox and some at solstices. The [[Antikythera mechanism]] predicts the equinoxes and solstices.<ref>Freeth, T., Bitsakis, Y., Moussas, X., Seiradakis, J. H., Tselikas, A., Mangou, H., ... & Allen, M. (2006). Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism. ''Nature'', ''444''(7119), 587-591.</ref>

<gallery widths="200px" heights="160px">
Image:Earth-lighting-equinox_EN.png|Illumination of [[Earth]] by the [[Sun]] at the equinox
Image:Ecliptic path.jpg|The relation between the Earth, Sun, and stars at the March equinox. From Earth's perspective, the Sun appears to move along the [[ecliptic]] (red), which is tilted compared to the [[celestial equator]] (white).
Image:north season.jpg|Diagram of the Earth's [[season]]s as seen from the north. Far right: December solstice.
Image:south season.jpg|Diagram of the Earth's seasons as seen from the south. Far left: June solstice.
</gallery>

The equinoxes are the only times when the [[terminator (solar)|solar terminator]] (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern [[hemispheres of the Earth|hemisphere]]s are equally illuminated.

For the same reason, this is also the time when the Sun rises for an observer at one of Earth's rotational poles and sets at the other. For a brief period lasting approximately four days, both North and South Poles are in daylight.{{efn|This is possible because [[atmospheric refraction]] "lofts" the Sun's apparent disk above its true position in the sky.}} For example, in 2021 sunrise on the North Pole is 18 March 07:09 UTC, and sunset on the South Pole is 22 March 13:08 UTC. Also in 2021, sunrise on the South Pole is 20 September 16:08 UTC, and sunset on the North Pole is 24 September 22:30 UTC.<ref>[https://www.timeanddate.com/sun/@90,0 Sunrise and sunset times in 90°00'N, 0°00'E (North Pole)], timeanddate.com</ref><ref>[https://www.timeanddate.com/sun/@-90,0 Sunrise and sunset times in 90°00'S, 0°00'E (South Pole)], timeanddate.com</ref>

In other words, the equinoxes are the only times when the [[subsolar point]] is on the equator, meaning that the Sun is [[Zenith|exactly overhead]] at a point on the equatorial line. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.

===Date===
When [[Julius Caesar]] established the [[Julian calendar]] in 45&nbsp;BC, he set 25&nbsp;March as the date of the spring equinox;<ref>{{Cite book |last1=Blackburn |first1=Bonnie J. | last2 = Holford-Strevens | first2 = Leofranc |title=The Oxford companion to the year |date=1999 |isbn=0-19-214231-3 |publisher=Oxford University Press | page = 135 }} Reprinted with corrections 2003.</ref> this was already the starting day of the year in the Persian and Indian calendars. Because the Julian year is longer than the [[tropical year]] by about 11.3&nbsp;minutes on average (or 1&nbsp;day in 128&nbsp;years), the calendar "drifted" with respect to the two equinoxes – so that in [[First Council of Nicaea|300&nbsp;AD]] the spring equinox occurred on about 21&nbsp;March, and by the 1580s&nbsp;AD it had drifted backwards to 11&nbsp;March.<ref>{{cite book | last1 = Richards | first1 = E. G. | title = Mapping Time: The Calendar and its History | publisher = Oxford University Press | pages = 250&ndash;251 | date = 1998 | isbn = 978-0192862051}}</ref>

This drift induced [[Pope Gregory XIII]] to establish the modern [[Gregorian calendar]]. The Pope wanted to continue to conform with the edicts of the [[First Council of Nicaea|Council of Nicaea]] in 325&nbsp;AD concerning the [[Easter controversy#Second phase|date of Easter]], which means he wanted to move the vernal equinox to the date on which it fell at that time (21&nbsp;March is the day allocated to it in the Easter table of the Julian calendar), and to maintain it at around that date in the future, which he achieved by reducing the number of leap years from 100 to 97 every 400&nbsp;years. However, there remained a small residual variation in the date and time of the vernal equinox of about ±27&nbsp;hours from its mean position, virtually all because the distribution of 24&nbsp;hour centurial leap-days causes large jumps (see [[:File:Gregoriancalendarleap solstice.svg|Gregorian calendar leap solstice]]).

====Modern dates====
The dates of the equinoxes change progressively during the leap-year cycle, because the Gregorian calendar year is not commensurate with the period of the Earth's revolution about the Sun. It is only after a complete Gregorian leap-year cycle of 400&nbsp;years that the seasons commence at approximately the same time. In the 21st&nbsp;century the earliest March equinox will be 19&nbsp;March 2096, while the latest was 21&nbsp;March 2003. The earliest September equinox will be 21&nbsp;September 2096 while the latest was 23&nbsp;September 2003 ([[Universal Time]]).<ref name="YallopEtAl">{{cite book |last1=Yallop |first1=B.D. |last2=Hohenkerk |first2=C.Y. |last3=Bell |first3=S.A. |chapter=Astronomical Phenomena |editor1-last=Urban |editor1-first=S.E. |editor2-last=Seidelmann |editor2-first=P. K. |year=2013 |title=Explanatory supplement to the astronomical almanac |edition=3rd |location=Mill Valley, CA |publisher=University Science Books |isbn=978-1-891389-85-6 |pages=506–507}}</ref>

===Names===
* Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin (''ver'' = spring, and ''autumnus'' = autumn). These are the historically universal and still most widely used terms for the equinoxes, but are potentially confusing because in the southern hemisphere the vernal equinox does not occur in spring and the autumnal equinox does not occur in autumn. The equivalent common language English terms ''spring equinox'' and ''autumn (or fall) equinox'' are even more ambiguous.<ref name="Skye2007">{{cite book |first=Michelle |last=Skye |title=Goddess Alive!: Inviting Celtic & Norse Goddesses Into Your Life |url=https://books.google.com/books?id=s1x2ATL66UcC&pg=PT69 |year=2007 |publisher=Llewellyn Worldwide |isbn=978-0-7387-1080-8 |pages=69ff}}</ref><ref name="Curtis2013">{{cite book |first=Howard D. |last=Curtis |title=Orbital Mechanics for Engineering Students |url=https://books.google.com/books?id=2U9Z8k0TlTYC&pg=PA188 |year=2013 |publisher=Butterworth-Heinemann |isbn=978-0-08-097748-5 |pages=188ff}}</ref><ref name="GrewalWeill2007">{{cite book |first1=Mohinder S. |last1=Grewal |first2=Lawrence R. |last2=Weill |first3=Angus P. |last3=Andrews |title=Global Positioning Systems, Inertial Navigation, and Integration |url=https://books.google.com/books?id=6P7UNphJ1z8C&pg=PA459 |year=2007 |publisher=John Wiley & Sons |isbn=978-0-470-09971-1 |pages=459ff}}</ref> It has become increasingly common for people to refer to the September equinox in the southern hemisphere as the Vernal equinox.<ref>{{cite book |last=Bowditch |first=Nathaniel |department=National Imagery and Mapping Agency |title=The American practical navigator: An epitome of navigation |url=https://books.google.com/books?id=pXjHDnIE_ygC&pg=PA229 |year=2002 |publisher=Paradise Cay Publications |isbn=978-0-939837-54-0 |pages=229ff}}</ref><ref>{{cite book |title=Exploring the Earth | year=2016 |url=https://books.google.com/books?id=hs-PBSZTCBMC&pg=PT31 |publisher=Allied Publishers |isbn=978-81-8424-408-3 |pages=31ff}}</ref>
* [[March equinox]] and [[September equinox]]: names referring to the months of the year in which they occur, with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all cultures use a solar-based calendar where the equinoxes occur every year in the same month (as they do not in the [[Islamic calendar]] and [[Hebrew calendar]], for example).<ref name="LaRocque2007">{{cite book |first=Paula |last=La&nbsp;Rocque |title=On Words: Insights into how our words work – and don't |url=https://books.google.com/books?id=7VPSb8py5jUC&pg=PA89 |year=2007 |publisher=Marion Street Press |isbn=978-1-933338-20-0 |pages=89ff}}</ref>  Although the terms have become very common in the 21st&nbsp;century, they were sometimes used at least as long ago as the mid-20th&nbsp;century.<ref>{{cite book |title=Popular Astronomy |url=https://books.google.com/books?id=CcEzAQAAIAAJ |year=1945}}</ref>
* [[Northward equinox]] and [[southward equinox]]: names referring to the apparent direction of motion of the Sun. The northward equinox occurs in March when the Sun crosses the equator from south to north, and the southward equinox occurs in September when the Sun crosses the equator from north to south. These terms can be used unambiguously for other planets. They are rarely seen, although were first proposed over 100&nbsp;years ago.<ref>{{cite book |title=Notes and Queries |url=https://archive.org/details/notesandqueries06whitgoog |year=1895 |publisher=Oxford University Press}}</ref>
* [[First point of Aries]] and first point of [[Libra (astrology)|Libra]]: names referring to the [[zodiac|astrological signs]] the Sun is entering. However, the [[precession of the equinoxes]] has shifted these points into the [[constellation]]s [[Pisces (constellation)|Pisces]] and [[Virgo (constellation)|Virgo]], respectively.<ref>{{cite book |title=Spherical Astronomy |url=https://books.google.com/books?id=9KFRhcsn8-UC&pg=PA233 |publisher=Krishna Prakashan Media |pages=233ff |id=GGKEY:RDRHQ35FBX7}}</ref>

===Length of equinoctial day and night===
[[File:Hours of daylight vs latitude vs day of year with tropical and polar circles.svg|thumb|300px|Contour plot of the hours of daylight as a function of latitude and day of the year, showing approximately 12&nbsp;hours of daylight at all latitudes during the equinoxes]]
[[File:GOES 16 September Equinox 2022.jpg|thumb|Earth at the September&nbsp;2022 equinox]]

On the date of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, so night and day{{efn|Here, "day" refers to when the Sun is above the horizon.}} are about the same length.  Sunrise and sunset can be defined in several ways, but a widespread definition is the time that the top limb of the Sun is level with the horizon.<ref>{{cite journal |doi=10.1016/0304-3800(94)00034-F |url=https://www.ikhebeenvraag.be/mediastorage/FSDocument/171/Forsythe+-+A+model+comparison+for+daylength+as+a+function+of+latitude+and+day+of+year+-+1995.pdf |title=A model comparison for day length as a function of latitude and day of year|journal=Ecological Modelling |volume=80 |pages=87–95 |year=1995 |last1=Forsythe| first1=William C. |last2=Rykiel |first2=Edward J. |last3=Stahl |first3=Randal S. |last4=Wu |first4=Hsin-i |last5=Schoolfield |first5=Robert M.|issue=1 |bibcode=1995EcMod..80...87F }}</ref> With this definition, the day is longer than the night at the equinoxes:<ref name="USNO FAQ" />
# From the Earth, the Sun appears as a disc rather than a point of light, so when the centre of the Sun is below the horizon, its upper edge may be visible. [[Sunrise]], which begins daytime, occurs when the top of the Sun's disk appears above the [[Horizon|eastern horizon]]. At that instant, the disk's centre is still below the horizon.
# The Earth's atmosphere [[refraction|refracts]] sunlight. As a result, an observer sees daylight before the top of the Sun's disk appears above the horizon.

In sunrise/sunset tables, the [[atmospheric refraction]] is assumed to be 34&nbsp;arcminutes, and the assumed semidiameter (apparent [[radius]]) of the Sun is 16&nbsp;[[Minute and second of arc|arcminutes]]. (The apparent radius varies slightly depending on time of year, slightly larger at [[perihelion and aphelion|perihelion in January than aphelion in July]], but the difference is comparatively small.) Their combination means that when the upper limb of the Sun is on the visible horizon, its centre is 50&nbsp;arcminutes below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer.<ref>{{cite book |editor-last=Seidelman |editor-first=P. Kenneth |title=Explanatory Supplement to the Astronomical Almanac |year=1992 |publisher=University Science Books |location=Mill Valley, CA |isbn=0-935702-68-7 |page=32}}</ref>

These effects make the day about 14&nbsp;minutes longer than the night at the equator and longer still towards the poles. The real equality of day and night only happens in places far enough from the equator to have a seasonal difference in day length of at least 7&nbsp;minutes,<ref>{{cite web |title=Sunrise and Sunset |date=21 October 2002 |url=http://www.cso.caltech.edu/outreach/log/NIGHT_DAY/sunrise.htm |access-date=22 September 2017}}</ref> actually occurring a few days towards the winter side of each equinox. One result of this is that, at latitudes below ±2.0 degrees, all the days of the year are longer than the nights.<ref>{{Cite web|url=https://gml.noaa.gov/grad/solcalc/calcdetails.html|title=Solar Calculator - NOAA Global Monitoring Laboratory|first=GML Web|last=Team|website=gml.noaa.gov}}</ref> 

The times of sunset and sunrise vary with the observer's location ([[longitude]] and [[latitude]]), so the dates when day and night are equal also depend upon the observer's location.

A third correction for the visual observation of a sunrise (or sunset) is the angle between the apparent horizon as seen by an observer and the geometric (or sensible) horizon. This is known as the dip of the horizon and varies from 3&nbsp;arcminutes for a viewer standing on the sea shore to 160&nbsp;arcminutes for a mountaineer on Everest.<ref>{{cite web |first=Mark |last=Biegert |title=Correcting Sextant Measurements for Dip |date=21 October 2015 |work=Math Encounters (blog) |url=http://mathscinotes.com/2015/10/correcting-sextant-measurements-for-dip/ |access-date=22 September 2017}}</ref> The effect of a larger dip on taller objects (reaching over 2½° of arc on Everest) accounts for the phenomenon of snow on a mountain peak turning gold in the sunlight long before the lower slopes are illuminated.

The date on which the day and night are exactly the same is known as an ''equilux''; the [[neologism]], believed to have been coined in the 1980s, achieved more widespread recognition in the 21st&nbsp;century.{{efn|Prior to the 1980s there was no generally accepted term for the phenomenon, and the word "equilux" was more commonly used as a synonym for [[wiktionary:isophot|isophot]].<ref>{{cite web |first=Steve |last=Owens |title=Equinox, Equilux, and Twilight Times |date=20 March 2010 |work=Dark Sky Diary (blog) |url=http://darkskydiary.wordpress.com/2010/03/20/equinox-equilux-and-twilight-times/ |access-date=31 December 2010}}</ref> The newer meaning of "equilux" is modern (c.&nbsp;1985 to 1986), and not usually intended: Technical references since the beginning of the 20th&nbsp;century (c.&nbsp;1910) have used the terms "equilux" and "isophot" interchangeably to mean "of equal illumination" in the context of curves showing how intensely lighting equipment will illuminate a surface. See for instance Walsh (1947).<ref>{{cite book |first=John William Tudor |last=Walsh |url=https://books.google.com/books?id=iC46AAAAMAAJ |title=Textbook of Illuminating Engineering (Intermediate Grade) |publisher=I. Pitman |year=1947}}</ref> The earliest confirmed use of the modern meaning was in a post on the [[Usenet]] group net.astro,<ref>{{cite web |date=14 March 1986 |url=https://groups.google.com/forum/#!original/net.astro/u1ufhWfdA00/eGRinwb18n0J |website=net.astro |title=Spring Equilux Approaches}}</ref> which refers to "discussion last year exploring the reasons why equilux and equinox are not coincident". Use of this particular pseudo-Latin [[protologism]] can only be traced to an extremely small (less than six) number of predominantly U.S. American people in such online media for the next 20&nbsp;years until its broader adoption as a [[neologism]] (c.&nbsp;2006),<!-- board.chrisisaak.com/index.php?showtopic=707 2006 September 22 --> and then its subsequent use by more mainstream organisations (c.&nbsp;2012).<ref>{{cite web |url=https://www.metoffice.gov.uk/weather/learn-about/weather/seasons/equinox-and-solstice |title=The Equinox and Solstice |publisher=U.K. Meteorological Office}}</ref>}} At the most precise measurements, a true equilux is rare, because the lengths of day and night change more rapidly than any other time of the year around the equinoxes. In the mid-latitudes, daylight increases or decreases by about three minutes per day at the equinoxes, and thus adjacent days and nights only reach within one minute of each other. The date of the closest approximation of the equilux varies slightly by latitude; in the mid-latitudes, it occurs a few days before the spring equinox and after the fall equinox in each respective hemisphere.<ref>{{Cite web |date=2024-03-19 |title=On the equinox, are day and night equal? |url=https://earthsky.org/astronomy-essentials/equal-day-and-night-on-the-equinox-march/ |access-date=2024-06-23 |website=earthsky.org |language=en-US}}</ref>

===Auroras===
Mirror-image [[Aurora#Conjugate auroras|conjugate auroras]] have been observed during the equinoxes.<ref>{{cite book |title=The Aurora Watcher's Handbook |pages=117–124 |first=Neil |last=Davis |publisher=University of Alaska Press |date=1992 |isbn=0-912006-60-9 }}</ref>

===Cultural aspects===
{{Main|March equinox#Culture|September equinox#Culture}}
The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditional [[harvest festival]]s  are celebrated on the date of the equinoxes.

People in countries including Iran, Afghanistan, Tajikistan celebrate [[Nowruz]] which is spring equinox in northern hemisphere.  This day marks the new year in [[Solar Hijri calendar]]. 

Religious architecture is often determined by the equinox; the [[Angkor Wat Equinox]] during which the sun rises in a perfect alignment over [[Angkor Wat]] in [[Cambodia]] is one such example.<ref>{{Cite book |last=DiBiasio |first=Jame |url=https://books.google.com/books?id=fg4LBAAAQBAJ&dq=angkor+equinox&pg=PT37 |title=The Story of Angkor |date=2013-07-15 |publisher=Silkworm Books |isbn=978-1-63102-259-3 |language=en}}</ref>

[[Catholic churches]], since the recommendations of [[Charles Borromeo]], have often chosen the equinox as their reference point for the [[orientation of churches]].<ref>{{Cite book |last=Johnson |first=Walter |url=https://books.google.com/books?id=MZQeHSDPe0MC&dq=equinox+as+their+reference+point+for+the+orientation+of+churches.&pg=PA229 |title=Byways in British Archaeology |date=2011-11-18 |publisher=Cambridge University Press |isbn=978-0-521-22877-0 |language=en}}</ref>