A solarequinox is a moment in time when theSun appearsdirectly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, around20 March and23 September.[a]
An equinox is equivalently defined as the time whenthe plane of Earth's equator passes through the geometric center of theSun's disk.[7][8] This is also the moment whenEarth's rotation axis is 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 orbit tovary slightly from aperfect ellipse, the equinox is officially defined by the Sun's more regularecliptic longitude rather 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 size of 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, equatorial cultures noted the day when the Sun rises dueeast and 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.
Hemisphere-neutral names arenorthward equinox for the March equinox, indicating that at that moment the solar declination is crossing the celestial equator in a northward direction, andsouthward equinox for the September equinox, indicating that at that moment the solar declination is crossing the celestial equator in a southward direction.
Daytime is increasing at the fastest at the vernal equinox and decreasing at the fastest at the autumnal equinox.
AtEarth's poles the Sun appears at the horizon only and all day around equinox, marking the change between the half year longpolar night andpolar day. The picture shows theSouth Pole right before March equinox, with the Sun appearing throughrefraction despite being still below the horizon.
Systematically observing thesunrise, people discovered that it occurs between two extreme locations at thehorizon 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 Latinaequus, meaning "equal", andnox, meaning "night".
In the northern hemisphere, thevernal equinox (March) conventionally marks the beginning ofspring in 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 mechanism predicts the equinoxes and solstices.[14]
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 southernhemispheres 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.[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 point is on the equator, meaning that the Sun isexactly 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.
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'sseasons as seen from the north. Far right: December solstice.
Diagram of the Earth's seasons as seen from the south. Far left: June solstice.
WhenJulius Caesar established theJulian calendar in 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 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 in300 AD the spring equinox occurred on about 21 March, and by the 1580s AD it had drifted backwards to 11 March.[18]
This drift inducedPope Gregory XIII to establish the modernGregorian calendar. The Pope wanted to continue to conform with the edicts of theCouncil of Nicaea in 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).
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]
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 equinox andautumn (or fall) equinox are 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 equinox andSeptember 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 calendar andHebrew 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 equinox andsouthward 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]
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 equinoxesEarth 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]
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.
The Earth's atmosphererefracts sunlight. As a result, an observer sees daylight before the top of the Sun's disk appears above the horizon.
In sunrise/sunset tables, theatmospheric refraction is assumed to be 34 arcminutes, and the assumed semidiameter (apparentradius) of the Sun is 16 arcminutes. (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 (longitude andlatitude), 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 length of 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]
The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditionalharvest festivals are celebrated on the date of the equinoxes.
Religious architecture is often determined by the equinox; theAngkor Wat Equinox during which the sun rises in a perfect alignment overAngkor Wat inCambodia is one such example.[39]
One effect of equinoctial periods is the temporary disruption ofcommunications satellites. For allgeostationary satellites, there are a few days around the equinox when the Sun goesdirectly 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.)[41]
Satellites ingeostationary orbit also experience difficulties maintaining power during the equinox because they have to travel throughEarth'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.[42]
WhenSaturn is at equinox itsrings reflect little sunlight, as seen in this image byCassini in 2009.
Equinoxes are defined on any planet with a tilted rotational axis. A dramatic example is Saturn, where the equinox places itsring system edge-on facing the Sun. When seen from above – a view seen during an equinox for the first time from theCassini space probe in 2009 – they receive very littlesunshine; indeed, they receive moreplanetshine than 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 6 May 2025.[44]
Mars's most recent equinoxes were on 12 January 2024 (northern autumn), and on 26 December 2022 (northern spring).[45]
^In this article, 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.
^This is possible becauseatmospheric refraction "lofts" the Sun's apparent disk above its true position in the sky.
^Here, "day" refers to when the Sun is above the horizon.
^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 theUsenet group net.astro,[35] which refers to "discussion last year exploring the reasons why equilux and equinox are not coincident". Use of this particular pseudo-Latinprotologism 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 aneologism (c. 2006), and then its subsequent use by more mainstream organisations (c. 2012).[36]
^ab"Equinoxes".Astronomical Information Center.United States Naval Observatory. 14 June 2019. Archived fromthe original on 21 August 2019. Retrieved9 July 2019.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 asemidiameter of 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
^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.
^Freeth, T.; Bitsakis, Y.; Moussas, X.; Seiradakis, J. H.; Tselikas, A.; Mangou, H.; Zafeiropoulou, M.; Hadland, R.; Bate, D.; Ramsey, A.; Allen, M.; Crawley, A.; Hockley, P.; Malzbender, T.; Gelb, D.; Ambrisco, W.; Edmunds, M. G. (2006). "Decoding the ancient Greek astronomical calculator known as the Antikythera Mechanism".Nature.444 (7119):587–591.Bibcode:2006Natur.444..587F.doi:10.1038/nature05357.PMID17136087.
^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.
^Richards, E. G. (1998).Mapping Time: The Calendar and its History. Oxford University Press. pp. 250–251.ISBN978-0192862051.
^Seidelman, P. Kenneth, ed. (1992).Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 32.ISBN0-935702-68-7.
