Equinox#Celestial coordinate systems

{{main|Sun path}}

{{See also|Equinox (celestial coordinates)}}

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 in most cultures and is considered the start of the New Year in the Assyrian calendar, Hindu, and the Persian or Iranian calendars,{{efn|The year in the Iranian calendar begins on Nowruz, which means "new day".}} while the autumnal equinox (September) marks the beginning of autumn.{{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}} 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.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.

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 seasons 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.

The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern hemispheres 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.[https://www.timeanddate.com/sun/@90,0 Sunrise and sunset times in 90°00'N, 0°00'E (North Pole)], timeanddate.com[https://www.timeanddate.com/sun/@-90,0 Sunrise and sunset times in 90°00'S, 0°00'E (South Pole)], timeanddate.com

In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is 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.

When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox;{{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. 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 minutes on average (or 1 day in 128 years), the calendar "drifted" with respect to the two equinoxes – so that in 300 AD the spring equinox occurred on about 21 March, and by the 1580s AD it had drifted backwards to 11 March.{{cite book | last1 = Richards | first1 = E. G. | title = Mapping Time: The Calendar and its History | publisher = Oxford University Press | pages = 250–251 | date = 1998 | isbn = 978-0192862051}}

This drift induced Pope Gregory XIII to establish the modern Gregorian calendar. The Pope wanted to continue to conform with the edicts of the Council of Nicaea in 325 AD concerning the date of Easter, which means he wanted to move the vernal equinox to the date on which it fell at that time (21 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 years. However, there remained a small residual variation in the date and time of the vernal equinox of about ±27 hours from its mean position, virtually all because the distribution of 24 hour centurial leap-days causes large jumps (see Gregorian calendar leap solstice).

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 years that the seasons commence at approximately the same time. In the 21st century the earliest March equinox will be 19 March 2096, while the latest was 21 March 2003. The earliest September equinox will be 21 September 2096 while the latest was 23 September 2003 (Universal Time).{{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}}

• 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.{{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}}{{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}}{{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}} It has become increasingly common for people to refer to the September equinox in the southern hemisphere as the Vernal equinox.{{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}}{{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}}
• 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).{{cite book |first=Paula |last=La 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}} Although the terms have become very common in the 21st century, they were sometimes used at least as long ago as the mid-20th century.{{cite book |title=Popular Astronomy |url=https://books.google.com/books?id=CcEzAQAAIAAJ |year=1945}}
• 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 years ago.{{cite book |title=Notes and Queries |url=https://archive.org/details/notesandqueries06whitgoog |year=1895 |publisher=Oxford University Press}}
• First point of Aries and first point of Libra: names referring to the astrological signs the Sun is entering. However, the precession of the equinoxes has shifted these points into the constellations Pisces and Virgo, respectively.{{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}}

File:Hours of daylight vs latitude vs day of year with tropical and polar circles.svg

File:GOES 16 September Equinox 2022.jpg

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.{{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 }} With this definition, the day is longer than the night at the equinoxes:

1. 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 eastern horizon. At that instant, the disk's centre is still below the horizon.
2. The Earth's atmosphere 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 arcminutes, and the assumed semidiameter (apparent radius) of the Sun is 16 arcminutes. (The apparent radius varies slightly depending on time of year, slightly larger at 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 arcminutes below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer.{{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}}

These effects make the day about 14 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 minutes,{{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}} 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.{{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}}

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 arcminutes for a viewer standing on the sea shore to 160 arcminutes for a mountaineer on Everest.{{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}} 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 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 isophot.{{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}} The newer meaning of "equilux" is modern (c. 1985 to 1986), and not usually intended: Technical references since the beginning of the 20th century (c. 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).{{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}} The earliest confirmed use of the modern meaning was in a post on the Usenet group net.astro,{{cite web |date=14 March 1986 |url=https://groups.google.com/forum/#!original/net.astro/u1ufhWfdA00/eGRinwb18n0J |website=net.astro |title=Spring Equilux Approaches}} 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 years until its broader adoption as a neologism (c. 2006), and then its subsequent use by more mainstream organisations (c. 2012).{{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}}}} 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.{{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}}

Mirror-image conjugate auroras have been observed during the equinoxes.{{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 }}

{{Main|March equinox#Culture|September equinox#Culture}}

The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditional harvest festivals 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.{{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}}

Catholic churches, since the recommendations of Charles Borromeo, have often chosen the equinox as their reference point for the orientation of churches.{{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}}

One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days around the equinox when the Sun goes directly behind the satellite relative to Earth (i.e. within the beam-width of the ground-station antenna) for a short period each day. The Sun's immense power and broad radiation spectrum overload the Earth station's reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from a few minutes to an hour. (For a given frequency band, a larger antenna has a narrower beam-width and hence experiences shorter duration "Sun outage" windows.){{cite web |url=http://www.intelsat.com/tools-resources/library/satellite-101/satellite-sun-interference/ |title=Satellite Sun Interference |website=Intelsat |language=en-US |access-date=20 March 2019}}

Satellites in geostationary orbit also experience difficulties maintaining power during the equinox because they have to travel through Earth's shadow and rely only on battery power. Usually, a satellite travels either north or south of the Earth's shadow because Earth's axis is not directly perpendicular to a line from the Earth to the Sun at other times. During the equinox, since geostationary satellites are situated above the Equator, they are in Earth's shadow for the longest duration all year.{{cite news |url=https://news.viasat.com/blog/scn/how-satellites-are-affected-by-the-spring-and-autumn-equinoxes/ |title=How satellites are affected by the spring and autumn equinoxes |last=Abrahamian |first=David |date=17 April 2018 |website=Viasat, Inc |language=en-US |access-date=20 March 2019}}

File:Saturn, its rings, and a few of its moons.jpg is at equinox its rings reflect little sunlight, as seen in this image by Cassini in 2009.]]

Equinoxes are defined on any planet with a tilted rotational axis. A dramatic example is Saturn, where the equinox places its ring system edge-on facing the Sun. When seen from above – a view seen during an equinox for the first time from the Cassini space probe in 2009 – they receive very little sunshine; indeed, they receive more planetshine than light from the Sun.{{cite web |url=http://photojournal.jpl.nasa.gov/catalog/PIA11667 |title=PIA11667: The Rite of Spring |publisher=Jet Propulsion Laboratory, California Institute of Technology |access-date=21 March 2014}} This phenomenon occurs once every 14.7 years on average, and can last a few weeks before and after the exact equinox. Saturn's most recent equinox was on 11 August 2009, and its next will take place on 6 May 2025.{{cite web |url=http://www.planetary.org/blogs/emily-lakdawalla/2016/06031044-oppositions-conjunctions-rpx.html |title=Oppositions, conjunctions, seasons, and ring plane crossings of the giant planets |last=Lakdawalla |first=Emily |author-link=Emily Lakdawalla |date=7 July 2016 |website=The Planetary Society |access-date=31 January 2017}}

Mars's most recent equinoxes were on 12 January 2024 (northern autumn), and on 26 December 2022 (northern spring).{{cite web |url=http://www.planetary.org/explore/space-topics/mars/mars-calendar.html |title=Mars Calendar |publisher=The Planetary Society}}

{{Clear}}

{{div col|colwidth=20em}}

• Analemma
• Anjana (Cantabrian mythology) – fairies believed to appear on the spring equinox
• Angkor Wat Equinox
• Aphelion – occurs around 5 July (see formula)
• Geocentric view of the seasons
• Iranian calendars
• Kōreisai – days of worship in Japan that began in 1878
• Lady Day
• Nowruz
• Orientation of churches
• Perihelion and aphelion
• Solstice
• Songkran
• Sun outage – a satellite phenomenon that occurs around the time of an equinox
• Tekufah
• Wheel of the Year
• Zoroastrian calendar

{{div col end}}

{{notelist|1}}

{{Reflist}}

{{Commons category|Equinox}}

{{Wiktionary}}

{{Wikiquote}}

• {{cite web|url=http://www.timeanddate.com/worldclock/sunearth.html?iso=20150923T0820|title=Day and Night World Map (night and day map on equinox)}}
• {{cite web|url=http://www.gandraxa.com/length_of_day.xml|title=Calculation of Length of Day (Formulas and Graphs)}}
• {{cite web|url=http://www.gutenberg.org/dirs/1/2/3/4/12342/12342-h/12342-h.htm#E|title=Equinoctial Points|website=The Nuttall Encyclopædia}}
• {{cite web|url=http://nshdpi.ca/is/equinox/eqindex.html|title=Table of times of spring Equinox for a thousand years: 1452–2547}}
• {{cite web|last=Gray|first=Meghan|title=Solstice and Equinox|url=http://www.sixtysymbols.com/videos/solstice.htm|website=Sixty Symbols|editor-link=Brady Haran|editor=Haran, Brady|publisher=University of Nottingham|author2=Merrifield, Michael}}

{{Time measurement and standards}}

{{Wheel of the Year}}

{{Portal bar|Astronomy|Physics|Stars|Outer space|Holidays}}

{{Authority control}}

Category:Dynamics of the Solar System

Category:March observances

Category:Technical factors of astrology

Category:September observances

Category:Time in astronomy