Wikipedia

Equinox

This article is about an astronomical event. For the celestial coordinates, seeEquinox (celestial coordinates).For other uses, seeEquinox (disambiguation).

UTdate and time of
equinoxesandsolsticeson Earth[1][2]
event equinox solstice equinox solstice
month March[3] June[4] September[5] December[6]
year day time day time day time day time
2019 20 21:58 21 15:54 23 07:50 22 04:19
2020 20 03:50 20 21:43 22 13:31 21 10:03
2021 20 09:37 21 03:32 22 19:21 21 15:59
2022 20 15:33 21 09:14 23 01:04 21 21:48
2023 20 21:25 21 14:58 23 06:50 22 03:28
2024 20 03:07 20 20:51 22 12:44 21 09:20
2025 20 09:02 21 02:42 22 18:20 21 15:03
2026 20 14:46 21 08:25 23 00:06 21 20:50
2027 20 20:25 21 14:11 23 06:02 22 02:43
2028 20 02:17 20 20:02 22 11:45 21 08:20
2029 20 08:01 21 01:48 22 17:37 21 14:14

A solarequinoxis a moment in time when theSuncrosses the Earth'sequator,which is to say, appearsdirectly abovethe equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and set "due west". This occurs twice each year, around20 Marchand23 September.[a]

More precisely, an equinox is traditionally defined as the time whenthe planeofEarth'sequatorpasses through the geometric center of theSun's disk.[7][8]Equivalently, this is the moment whenEarth's rotation axisis directly perpendicular to the Sun-Earth line, tilting neither toward nor away from the Sun. In modern times[when?],since the Moon (and to a lesser extent the planets) causesEarth's orbittovary slightlyfrom aperfect ellipse,the equinox is officially defined by the Sun's more regularecliptic longituderather than by itsdeclination.The instants of the equinoxes are currently defined to be when the apparent geocentric longitude of the Sun is 0° and 180°.[9]

The word is derived from theLatinaequinoctium,fromaequus(equal) andnox(night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. Contrary to popular belief,[10][11]they are not exactly equal because of theangular sizeof the Sun,atmospheric refraction,and the rapidly changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality, primitive equatorial cultures noted the day when the Sun rises dueeastand sets duewest,and indeed this happens on the day closest to the astronomically defined event. As a consequence, according to a properly constructed and alignedsundial,the daytime duration is 12 hours.

In theNorthern Hemisphere,theMarch equinoxis called the vernal or spring equinox while theSeptember equinoxis called the autumnal or fall equinox. In theSouthern Hemisphere,the reverse is true. During the year, equinoxes alternate withsolstices.Leap yearsand other factors cause the dates of both events to vary slightly.[12]

Hemisphere-neutral names arenorthward equinoxfor theMarch equinox,indicating that at that moment the solar declination is crossing the celestial equator in a northward direction, andsouthward equinoxfor theSeptember equinox,indicating that at that moment the solar declination is crossing the celestial equator in a southward direction.

Daytimeis increasing at the fastest at the vernal equinox and decreasing at the fastest at the autumnal equinox.

Equinoxes on Earth

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Main article:Sun path
See also:Equinox (celestial coordinates)

General

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Systematically observing thesunrise,people discovered that it occurs between two extreme locations at thehorizonand 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 Latinaequus,meaning "equal", andnox,meaning "night".

In the northern hemisphere, thevernal equinox(March) conventionally marks the beginning ofspringin most cultures and is considered the start of the New Year in theAssyrian calendar,Hindu, and the Persian orIranian calendars,[b]while theautumnal equinox(September) marks the beginning of autumn.[13]Ancient Greek calendars too had the beginning of the year either at the autumnal or vernal equinox and some at solstices. TheAntikythera mechanismpredicts the equinoxes and solstices.[14]

  • Illumination ofEarthby theSunat the equinox
  • The relation between the Earth, Sun, and stars at the March equinox. From Earth's perspective, the Sun appears to move along theecliptic(red), which is tilted compared to thecelestial equator(white).
  • Diagram of the Earth'sseasonsas seen from the north. Far right: December solstice.
  • Diagram of the Earth's seasons as seen from the south. Far left: June solstice.

The equinoxes are the only times when thesolar terminator(the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southernhemispheresare 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.[c]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.[15][16]

In other words, the equinoxes are the only times when thesubsolar pointis on the equator, meaning that the Sun isexactly overheadat 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

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WhenJulius Caesarestablished theJulian calendarin 45 BC, he set 25 March as the date of the spring equinox;[17]this was already the starting day of the year in the Persian and Indian calendars. Because the Julian year is longer than thetropical yearby about 11.3 minutes on average (or 1 day in 128 years), the calendar "drifted" with respect to the two equinoxes – so that in300 ADthe spring equinox occurred on about 21 March, and by the 1580s AD it had drifted backwards to 11 March.[18]

This drift inducedPope Gregory XIIIto establish the modernGregorian calendar.The Pope wanted to continue to conform with the edicts of theCouncil of Nicaeain 325 AD concerning thedate 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 (seeGregorian calendar leap solstice).

Modern dates

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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).[12]

Names

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  • Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin (ver= spring, andautumnus= 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 termsspring equinoxandautumn (or fall) equinoxare even more ambiguous.[19][20][21]It has become increasingly common for people to refer to the September equinox in the southern hemisphere as the Vernal equinox.[22][23]
  • March equinoxandSeptember 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 theIslamic calendarandHebrew calendar,for example).[24]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.[25]
  • Northward equinoxandsouthward 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.[26]
  • First point of Ariesand first point ofLibra:names referring to theastrological signsthe Sun is entering. However, theprecession of the equinoxeshas shifted these points into theconstellationsPiscesandVirgo,respectively.[27]

Length of equinoctial day and night

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Contour plot of the hours of daylight as a function of latitude and day of the year, showing approximately 12 hours of daylight at all latitudes during the equinoxes
Earth at the September 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[d]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.[28]With this definition, the day is longer than the night at the equinoxes:[7]

  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 theeastern horizon.At that instant, the disk's centre is still below the horizon.
  2. The Earth's atmosphererefractssunlight. As a result, an observer sees daylight before the top of the Sun's disk appears above the horizon.

In sunrise/sunset tables, theatmospheric refractionis assumed to be 34 arcminutes, and the assumed semidiameter (apparentradius) of the Sun is 16arcminutes.(The apparent radius varies slightly depending on time of year, slightly larger atperihelion 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.[29]

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,[30]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.[31]

The times of sunset and sunrise vary with the observer's location (longitudeandlatitude), 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.[32]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 anequilux;theneologism,believed to have been coined in the 1980s, achieved more widespread recognition in the 21st century.[e]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.[37]

Auroras

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Mirror-imageconjugate aurorashave been observed during the equinoxes.[38]

Cultural aspects

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Main articles:March equinox § Culture,andSeptember equinox § Culture

The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditionalharvest festivalsare celebrated on the date of the equinoxes.

People in countries including Iran, Afghanistan, Tajikistan celebrateNowruzwhich is spring equinox in northern hemisphere. This day marks the new year inSolar Hijri calendar.

Religious architecture is often determined by the equinox; theAngkor Wat Equinoxduring which the sun rises in a perfect alignment overAngkor WatinCambodiais one such example.[39]

Catholic churches,since the recommendations ofCharles Borromeo,have often chosen the equinox as their reference point for theorientation of churches.[40]

In India

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vishu

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Vishu,from SanskritViṣuvam,literally means 'equal', and it connoted to the celebration ofspring equinoxin the past. The spring equinox however occurs 24 days before the day of Vishu, on 21 March/Meenam7, due toprecession of equinoxes.

Effects on satellites

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One effect of equinoctial periods is the temporary disruption ofcommunications satellites.For allgeostationarysatellites, there are a few days around the equinox when the Sun goesdirectly behindthe 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.)[41]

Satellites ingeostationary orbitalso experience difficulties maintaining power during the equinox because they have to travel throughEarth's shadowand 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.[42]

Equinoxes on other planets

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WhenSaturnis at equinox itsringsreflect little sunlight, as seen in this image byCassiniin 2009.

Equinoxes are defined on any planet with a tilted rotational axis. A dramatic example is Saturn, where the equinox places itsring systemedge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth. When seen from above – a view seen during an equinox for the first time from theCassinispace probe in 2009 – they receive very littlesunshine;indeed, they receive moreplanetshinethan light from the Sun.[43]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.[44]

Mars's most recent equinoxes were on 12 January 2024 (northern autumn), and on 26 December 2022 (northern spring).[45]

See also

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Footnotes

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  1. ^This article follows the customary Wikipedia style detailed atManual of Style/Dates and numbers#Julian and Gregorian calendars;dates before 15 October 1582 are given in the Julian calendar while more recent dates are given in the Gregorian calendar. Dates before 1 March 8 AD are given in the Julian calendar as observed in Rome; there is an uncertainty of a few days when these early dates are converted to theproleptic Julian calendar.
  2. ^The year in theIranian calendarbegins onNowruz,which means "new day".
  3. ^This is possible becauseatmospheric refraction"lofts" the Sun's apparent disk above its true position in the sky.
  4. ^Here, "day" refers to when the Sun is above the horizon.
  5. ^Prior to the 1980s there was no generally accepted term for the phenomenon, and the word "equilux" was more commonly used as a synonym forisophot.[33]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).[34]The earliest confirmed use of the modern meaning was in a post on theUsenetgroup net.astro,[35]which refers to "discussion last year exploring the reasons why equilux and equinox are not coincident". Use of this particular pseudo-latinprotologismcan 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 years until its broader adoption as aneologism(c. 2006), and then its subsequent use by more mainstream organisations (c. 2012).[36]

References

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  1. ^Astronomical Applications Department ofUSNO."Earth's Seasons - Equinoxes, Solstices, Perihelion, and Aphelion".Retrieved1 August2022.
  2. ^"Solstices and Equinoxes: 2001 to 2100".AstroPixels.20 February 2018.Retrieved21 December2018.
  3. ^Équinoxe de printemps entre 1583 et 2999
  4. ^Solstice d’été de 1583 à 2999
  5. ^Équinoxe d’automne de 1583 à 2999
  6. ^Solstice d’hiver
  7. ^ab"Equinoxes".Astronomical Information Center.United States Naval Observatory.14 June 2019.Archivedfrom the original on 21 August 2019.Retrieved9 July2019.On the day of an equinox, the geometric center of the Sun's disk crosses the equator, and this point is above the horizon for 12 hours everywhere on the Earth. However, the Sun is not simply a geometric point. Sunrise is defined as the instant when the leading edge of the Sun's disk becomes visible on the horizon, whereas sunset is the instant when the trailing edge of the disk disappears below the horizon. These are the moments of first and last direct sunlight. At these times the center of the disk is below the horizon. Furthermore, atmospheric refraction causes the Sun's disk to appear higher in the sky than it would if the Earth had no atmosphere. Thus, in the morning the upper edge of the disk is visible for several minutes before the geometric edge of the disk reaches the horizon. Similarly, in the evening the upper edge of the disk disappears several minutes after the geometric disk has passed below the horizon. The times of sunrise and sunset in almanacs are calculated for the normal atmospheric refraction of 34 minutes of arc and asemidiameterof 16 minutes of arc for the disk. Therefore, at the tabulated time the geometric center of the Sun is actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level region
  8. ^"ESRL Global Monitoring Division - Global Radiation Group".NOAA.esrl.noaa.gov.U.S. Department of Commerce.Retrieved9 July2019.
  9. ^Astronomical Almanac.United States Naval Observatory.2008. Glossary.
  10. ^Grieser, Justin (22 September 2014)."Autumn arrives: The fall equinox explained in six images".The Washington Post.Archivedfrom the original on 8 June 2021.Retrieved29 June2024.
  11. ^Plait, Phil (22 September 2023)."The Equinox Is Not What You Think It Is".Scientific American.Retrieved29 June2024.
  12. ^abYallop, B.D.; Hohenkerk, C.Y.; Bell, S.A. (2013). "Astronomical Phenomena". In Urban, S.E.; Seidelmann, P. K. (eds.).Explanatory supplement to the astronomical almanac(3rd ed.). Mill Valley, CA: University Science Books. pp. 506–507.ISBN978-1-891389-85-6.
  13. ^"March Equinox – Equal Day and Night, Nearly".Time and Date.2017.Retrieved22 May2017.
  14. ^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.
  15. ^Sunrise and sunset times in 90°00'N, 0°00'E (North Pole),timeanddate
  16. ^Sunrise and sunset times in 90°00'S, 0°00'E (South Pole),timeanddate
  17. ^Blackburn, Bonnie J.; Holford-Strevens, Leofranc (1999).The Oxford companion to the year.Oxford University Press. p. 135.ISBN0-19-214231-3.Reprinted with corrections 2003.
  18. ^Richards, E. G. (1998).Mapping Time: The Calendar and its History.Oxford University Press. pp. 250–251.ISBN978-0192862051.
  19. ^Skye, Michelle (2007).Goddess Alive!: Inviting Celtic & Norse Goddesses Into Your Life.Llewellyn Worldwide. pp. 69ff.ISBN978-0-7387-1080-8.
  20. ^Curtis, Howard D. (2013).Orbital Mechanics for Engineering Students.Butterworth-Heinemann. pp. 188ff.ISBN978-0-08-097748-5.
  21. ^Grewal, Mohinder S.; Weill, Lawrence R.; Andrews, Angus P. (2007).Global Positioning Systems, Inertial Navigation, and Integration.John Wiley & Sons. pp. 459ff.ISBN978-0-470-09971-1.
  22. ^Bowditch, Nathaniel (2002).The American practical navigator: An epitome of navigation.National Imagery and Mapping Agency. Paradise Cay Publications. pp. 229ff.ISBN978-0-939837-54-0.
  23. ^Exploring the Earth.Allied Publishers. 2016. pp. 31ff.ISBN978-81-8424-408-3.
  24. ^La Rocque, Paula (2007).On Words: Insights into how our words work – and don't.Marion Street Press. pp. 89ff.ISBN978-1-933338-20-0.
  25. ^Popular Astronomy.1945.
  26. ^Notes and Queries.Oxford University Press. 1895.
  27. ^Spherical Astronomy.Krishna Prakashan Media. pp. 233ff. GGKEY:RDRHQ35FBX7.
  28. ^Forsythe, William C.; Rykiel, Edward J.; Stahl, Randal S.; Wu, Hsin-i; Schoolfield, Robert M. (1995)."A model comparison for day length as a function of latitude and day of year"(PDF).Ecological Modelling.80(1): 87–95.Bibcode:1995EcMod..80...87F.doi:10.1016/0304-3800(94)00034-F.
  29. ^Seidelman, P. Kenneth, ed. (1992).Explanatory Supplement to the Astronomical Almanac.Mill Valley, CA: University Science Books. p. 32.ISBN0-935702-68-7.
  30. ^"Sunrise and Sunset".21 October 2002.Retrieved22 September2017.
  31. ^"NOAA Global Monitoring Laboratory Solar Calculation Details".
  32. ^Biegert, Mark (21 October 2015)."Correcting Sextant Measurements for Dip".Math Encounters (blog).Retrieved22 September2017.
  33. ^Owens, Steve (20 March 2010)."Equinox, Equilux, and Twilight Times".Dark Sky Diary (blog).Retrieved31 December2010.
  34. ^Walsh, John William Tudor (1947).Textbook of Illuminating Engineering (Intermediate Grade).I. Pitman.
  35. ^"Spring Equilux Approaches".net.astro.14 March 1986.
  36. ^"The Equinox and Solstice".U.K. Meteorological Office.
  37. ^"On the equinox, are day and night equal?".earthsky.org.19 March 2024.Retrieved23 June2024.
  38. ^Davis, Neil (1992).The Aurora Watcher's Handbook.University of Alaska Press. pp. 117–124.ISBN0-912006-60-9.
  39. ^DiBiasio, Jame (15 July 2013).The Story of Angkor.Silkworm Books.ISBN978-1-63102-259-3.
  40. ^Johnson, Walter (18 November 2011).Byways in British Archaeology.Cambridge University Press.ISBN978-0-521-22877-0.
  41. ^"Satellite Sun Interference".Intelsat.Retrieved20 March2019.
  42. ^Abrahamian, David (17 April 2018)."How satellites are affected by the spring and autumn equinoxes".Viasat, Inc.Retrieved20 March2019.
  43. ^"PIA11667: The Rite of Spring".Jet Propulsion Laboratory, California Institute of Technology.Retrieved21 March2014.
  44. ^Lakdawalla, Emily(7 July 2016)."Oppositions, conjunctions, seasons, and ring plane crossings of the giant planets".The Planetary Society.Retrieved31 January2017.
  45. ^"Mars Calendar".The Planetary Society.
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