Equinox

An equinox is one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect. They derive their name (equi nox is Latin for equal night) from the fact that when the Sun passes an equinox, the lengths of the day and night everywhere on Earth are equal, that is 12 hours. The term equinox can also be used in a narrower sense of being the instance in time that such a passage happens. The equinoxes then, together with the solstices, are the defining moments of the start of the (astronomical) seasons, except in China, where they mark the center of the respective seasons.

Names

The two equinoxes can be distinguished by different pairs of names, depending on which feature one wants to stress.

Spring equinox and autumn or fall equinox. These names can be used when one wants to explain the cause of the seasons. But as the seasons of the northern hemisphere and southern hemisphere are opposites, the spring equinox of one hemisphere is the autumn equinox of the other, which makes their use ambiguous.
March equinox and September equinox. An alternative to the previous set, but without the ambiguity for which hemisphere they are intended. Still not universal, however, as not all people on Earth use a solar based calendar where the equinoxes occur every year in the same month (Jewish calendar for example), and also not useful for other planets (Mars for example), even though they have seasons.
Vernal equinox and autumnal equinox. These names are direct derivatives of Latin (ver = spring, autumnus = autumn), and as such more apt to be used in literature. Although in principle they are subject to the same problem as the spring/autumn set, their use over the centuries has fixed them to the viewpoint of the northern hemisphere. As such the vernal equinox is the equinox where the Sun passes from south to north, and is a zeropoint in some celestial coordinate systems. In this case the name of the other equinox is seldomly referred to.
First point of Aries and first point of Libra. Alternative names for the previous set, absolutely doing away with any doubt that the vernal equinox may be dependent on a specific hemisphere. Disadvantage is that due to the precession of the equinoxes these astrological signs do not correspond any longer with the actual constellations where these equinoxes are located.
Pisces equinox and Virgo equinox. Names to indicate in which constellations the two equinoxes are currently located. These terms are not widely used.

Heliocentric view of the seasons

The cause of the seasons is that the rotation axis of the Earth is not perpendicular to its orbital plane, but makes an angle of about 23.44°, the obliquity of the ecliptic, and that this axis keeps its orientation in inertial space. By consequence, for half a year (from around 20 March to 22 September) the northern hemisphere tips toward the Sun, with the maximum around 21 June, while for the other half year the southern hemisphere has this honour, with the maximum around 21 December. The two instances that the Sun is overhead on the equator are the equinoxes. Also at that moment both the north pole and south pole of the Earth are just on the terminator, and therefore day and night are equally divided over the whole globe.

The table above gives the instances of equinoxes and solstices over several years. A few remarks can be made.

The actual equinox is a single moment in time — it does not take the whole day. But the crossing of the Sun over the equator is slow enough that the equinox day will have 12 hours of daylight and 12 hours of nighttime, and within an accuracy of a few minutes, the day before and after too.
It is 94 days from the June solstice to the September equinox, but only 89 days from the December solstice to the March equinox. The seasons are not of equal length because of the variable speed the Earth has in its orbit around the Sun.
The instances of the equinoxes are not fixed but fall about six hours later every year, amounting to one full day in four years, but then they are reset by the occurence of a leap year. The Gregorian calendar is designed to follow the seasons as accurately as possible. It is good, but not perfect. Also see: Gregorian calendar#calendar seasonal error.
Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.
Currently the most common equinox and solstice dates are 20 March, 21 June, 22 September and 21 December, the four year average slowly shifting to earlier times in the years to come. This shift is a full day in about 70 years (largely to be compensated by the century leap year rules of the Gregorian calendar). But that also means that as many years ago the dates of 21 March, 22 June, 23 September and 22 December were much more common, as older books teach and older people still remember.
Note that the times are given in UTC, the time at Greenwich (ignoring British Summer Time). People living farther to the east (Asia, Australia) whose local times are in advance, will see the seasons apparently start later, for example in Tonga (UTC+13) an equinox occurred on 24 September 1999; a date which will not happen again until 2103. On the other hand people living far to the west (America) have clocks running behind in time, and may experience an equinox occurring as early as 19 March.

Geocentric view of the seasons

The explanation given in the previous section would be useful for an observer in outer space. Seen from Earth, the explanation remains the same but the orientation changes. Now the Sun revolves in one year around the Earth, moving along a circle in the sky named the ecliptic which is a reflection of the orbit of the Earth around the Sun. The daily motion of the Sun, (day and night), however, takes place parallel to the equator. The equinoxes are now the points where the equator intersects the ecliptic and the solstices the points on the ecliptic farthest away from the equator. Also note, in the drawing, when the Sun appears to be at the vernal equinox as seen from Earth, that seen from the Sun the Earth is 180° away from it, and thus at the autumnal equinox of its orbit. The perihelion of the Earth's orbit, currently located at 101° longitude, therefore occurs at the beginning of January.

As mentioned above, on equinox day the Sun passes through the zenith for observers on the equator and is on the horizon for those on the poles (but see also below). The March equinox marks sunrise at the north pole and sunset at the south pole, while for the September equinox it is just the opposite. For all observers on Earth the altitude of the Sun above the southern horizon at local noon is equal to the complement of the latitude (90° - φ). Example: an observer on 60° northern latitude (φ = +60°) will see the Sun at 30° in the south. An observer on 20° southern latitude (φ = −20°) will see the Sun at 110° in the south. But by then one has overshot the zenith (90° altitude), so that this value corresponds to 70° above the northern horizon.

On the equinox day, the Sun rises in the morning, for every place on Earth (except at the poles), exactly in the east and sets exactly in the west in the evening. (At high latitudes this may be shifted due to atmospheric refraction.) In the half year centred around June it rises and sets more towards the north, which means longer days and shorter nights for the northern hemisphere and shorter days and longer nights for the southern hemisphere. In the half year centred around December the Sun rises and sets more towards the south, and the day and night durations are reversed. Also on the equinox day, the Sun rises, for every place on Earth (except at the poles), at 6:00 in the morning and sets at 18:00 in the evening. But these times are not exact for several reasons.

Most places on Earth use a time zone which is not equal to the local time, differing sometimes up to an hour, and even two hours if summer time is included. In that case, the Sun can rise for example at 8:00 and set at 20:00.
Even those people fortunate enough to have their time zone just equal to the local time, they still will not see sunrise and sunset at 6:00 and 18:00, respectively. This is due to the variable speed of the Earth in its orbit, and is described as the equation of time. It has different values for the March and the September equinox (+8 and −8 minutes respectively).
Sunrise and sunset are commonly defined for the upper limb of the solar disk, and not for its centre. The limb is already up for at least one minute before the centre appears, and likewise sets one minute before the last appearance of the limb sets too.
Due to the atmospheric refraction the Sun, when near the horizon, appears a little more than its own diameter above the position than where it is in reality. This makes sunrise more than another two minutes earlier and sunset the equal amount later. The two effects add up to almost seven minutes, making the equinox day 12h 7m long and the night only 11h 53m. In addition to that, the night includes twilight. When dawn and dusk are added to the daytime instead, the day would be almost 13 hours.
The above numbers are only true for the tropics. For moderate latitudes this discrepancy gets larger (London, for example: 12 minutes), and close to the poles it gets very large. Up to about 100 km from both poles the Sun is up for a full 24 hours on equinox day.
Going up into the mountains away from sea level will also increase the length of the day.

Day arcs of the Sun

Some of the above statements can be made clearer when picturing the day arc: the path the Sun tracks along the celestial dome in its diurnal movement. The pictures show this for every hour on equinox day. In addition, also some 'ghost' suns are indicated below the horizon, up to 18° down. The Sun in this area still causes twilight. The pictures can be used for both the northern and the southern hemisphere. The observer is supposed to sit near the tree on the island in the middle of the ocean. The green arrows give the cardinal directions.

On the northern hemisphere, north is to left, the Sun rises in the east (far arrow), culminates in the south (right arrow) while moving to the right and sets in the west (near arrow).
On the southern hemisphere, south is to the left, the Sun rises in the east (near arrow), culminates in the north (right arrow) while moving to the left and sets in the west (far arrow).

The following special cases are depicted.

The day arc on the equator, passing through the zenith, has almost no shadows at high noon.
The day arc on 20° latitude. The Sun culminates at 70° altitude and also its daily path at sunrise and sunset occurs at a steep 70° angle to the horizon. Twilight is still about one hour.
The day arc on 50° latitude. Twilight is almost two hours now.
The day arc on 70° latitude. The Sun culminates at no more than 20° altitude and its daily path at sunrise and sunset is at a shallow 20° angle to the horizon. Twilight is more than four hours, in fact there is barely any dark night.
The day arc at the pole. If it were not for atmospheric refraction, the Sun would be on the horizon all the time.

Coordinate systems

The vernal equinox, the one the Sun passes in March on its way from south to north, has a special significance in astronomy as it marks the origin of both ecliptic coordinates and equatorial coordinates, and also the start of the sidereal day. The autumnal equinox is at ecliptic longitude 180° and right ascension 12h. For astrology, at least the one derived from the ancient Greeks, the same thing holds true; the vernal equinox is the first point (i.e. the start) of the sign of Aries. In these signs, it is of no significance that the fixed stars and equinox shift compared to each other due to the precession of the equinoxes. The seasons do not shift because of that, only which stars are visible in particular seasons, that changes. Also the coordinates of all stars are affected, but that is for astronomers only one of the many factors they have to take account of anyway, and one of the easier for that matter.

In Hindu astrology on the other hand, their 'vernal equinox' was fixed to the stars about 17 centuries ago, and has been drifting away from the seasons since then, now amounting to 22 days.

Cultural aspects

In the list below the terms March and September equinoxes are used when the celebration is fixed in time, while the terms spring and autumn equinoxes refer to those which are different in the two hemispheres.

The calculation of Easter in the Christian church (first Sunday after the first full moon on or after the March equinox), uses its own definition for the equinox — it always falls on 21 March. The earliest Easter date is therefore 22 March.
The March equinox marks the first day of various calendars including the Iranian Calendar and the Bahá'í calendar.Bahia calendar The Persian (Iranian) festival of Norouz is celebrated then. According to the ancient Persian mythology Jamshid, the mythological king of Persia, ascended to the throne on this day and each year this is comemorated with festivities for two weeks. These festivities recall the myth of creation and the ancient cosmology of Iranian and Persian people. It is also a holiday for Azerbaijan, Afghanistan, India, Turkey, Zanzibar, Albania, and various countries of Central Asia, as well as among the Kurds.Norooz As well as being a Zoroastrian holiday, it is also a holy day for adherents of the Bahá'í Faith.
The spring equinox marks the Wiccan Sabbat of Ostara (or Eostar), while at the autumn equinox the Wiccan Sabbat of Mabon is celebrated. The vernal equinox is the most significant day in the pagan calendar, and kemetic pagans spend the day praying to the goddess of cats, Bast.
In Japan, (March) Vernal Equinox Day (春分の日 Shunbun no hi) is an official national holiday, and is spent visiting family graves and holding family reunions. Likewise is (September) Autumnal equinox Day (秋分の日 Shūbun no hi).
Tamil and Bengali New Years follow the Hindu zodiac and are celebrated according to the sidereal vernal equinox (14 April). The former is celebrated in the South Indian state of Tamil Nadu, and the later in Bangladesh and the East Indian state of West Bengal.

Earth Day was initially celebrated on 21 March 1970, the equinox day. It is currently celebrated in America on 22 April.
In many Arab countries, Mother's Day is celebrated on the March equinox.

The September equinox was "New Year's Day" in the French Republican Calendar, which was in use from 1793 to 1805. The French First Republic was proclaimed and the French monarchy was abolished on 21 September 1792, making the following day the equinox day that year, the first day of the "Republican Era" in France. The start of every year was to be determined by astronomical calculation, (that is: following the real Sun and not the mean Sun as all other calendars).
The harvest festival in the United Kingdom is celebrated on the Sunday of the full moon closest to the September equinox.

Solar terms in Chinese astronomy

Chunfen (Traditional Chinese: 春分; Simplified Chinese: 春分; pinyin: chūn fēn; Japanese: 春分; Korean: 춘분) is a solar term or period of time when the Sun lies between the celestial longitudes of 0° and 15°. It often refers in particular to the day when Sun is exactly at a celestial longitude of 0°. It usually begins around March 20 and ends around April 5.

Qiufen (秋分) is a solar term which begins when Sun lies between the celestial longitude of 180° and 195°. It often refers in particular to the day when Sun is exactly at the celestial longitude of 180°. It usually begins around September 23, and ends around October 8.

Trivia, facts and fables

For a Latin word like nox the plural is noctes. Although this form is retained in English in the genitive: equinoctial — it is not commonly used for the plural, which is equinoxes, rather than equinoctes.
One of the effects of equinoctial periods is their temporary disruptive effect on communications satellites. For most geostationary satellites, there is almost always a point when the sun is directly behind the satellite relative to Earth. 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 an hour to a few minutes.
Folk tales from various European countries claim that only on the March equinox day (some may add the September equinox day or may explicitely not), one can balance an egg on its point.

References

External links

Details about the Length of Day and Night at the Equinoxes
Calculation of Length of Day (Formulas and Graphs)
Equinoctial Points - The Nuttall Encyclopaedia
Java applet showing parts of the Earth in night and day
Table of times for Equinoxes, Solstices, Perihelion, and Aphelion in 1992-2020
Dates and Times of Equinoxes and Solstices has an online calculator.
Calculate the Time of Equinoxes in Excel, CAD or your other programs. The Sun API is free and extremely accurate. For Windows Computers.
Earth's Seasons—Equinoxes, Solstices, Perihelion, and Aphelion—1992–2020

Astrodynamics | Astrological factors | Celestial mechanics | Spherical astronomy | Solar terms

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