Equinox

===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 |title=NOAA Global Monitoring Laboratory Solar Calculation Details|url=https://gml.noaa.gov/grad/solcalc/calcdetails.html}}</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 predominently 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.
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