An equinox is the event of the Sun passing over the Earth's equator, in its annual cycle. By extension it also signifies the time at which it happens and the apparent position of the Sun at that moment.
In astronomical terms it refers to the intersection on the celestial sphere between the celestial equator and the plane of the ecliptic. The term is thereby used as a name for a specific junction on a set of spherical coordinates and also, being that these coordinates derive directly from the rotation of the earth and its inclination on its axis as it orbits the sun, a date for the arrival of the earth-sun system on the annual calendar which causes the aforesaid intersection.
The word is also used for the same event happening to other planets, and in setting up a celestial coordinate system—see Equinox (celestial coordinates).
An equinox happens each year at two specific moments in time (not a whole day) when the centre of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 22 each year.
More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e., declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point.
On a day which has an equinox, the centre of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night). In reality, the day is longer than the night at an equinox. Commonly, the day is defined as the period that sunlight reaches the ground in the absence of local obstacles. From Earth, the Sun appears as a disc and not a single point of light; so, when the centre of the Sun is below the horizon, the upper edge is visible. Furthermore, the atmosphere refracts light; so, even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon its centre is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These effects together make the day about 14 minutes longer than the night at the equator, and longer still at sites toward the poles. The real equality of day and night only happens at places far enough from the equator to have at least a seasonal difference in daylength of 7 minutes, and occurs a few days towards the winter side of each equinox.
- Vernal equinox and autumnal equinox. These classical names are direct derivatives of Latin (ver = spring, autumnus = autumn), and as such are more apt to be found in writings of the first half of the 20th century and before.
- March equinox and September equinox. A usage becoming the preferred standard by technical writers choosing to avoid the classical Northern Hemisphere bias implied by the assumption that March is in springtime and September is in autumn; only true for those in the Northern Hemisphere but actually the complete opposite for dwellers in the Southern Hemisphere.
- Northward equinox and southward equinox. Names referring to the apparent motion of the Sun at the times of the equinox.
- Vernal point and autumnal point are the points on the celestial sphere where the Sun is located on the vernal equinox and, respectively, on the autumnal equinox. Again, the seasonal attribution is that of one based in the Northern Hemisphere..
- First point of Aries and first point of Libra. Old names for the above now only used by astrologers (divination / mysticism). Due to the precession of the equinoxes the astrological signs where these equinoxes are located, do not correspond any longer with the actual constellations once ascribed to them.
Heliocentric view of the seasons
The Earth's seasons
are caused by the rotation axis of the Earth not being perpendicular to its orbital plane. The Earth's axis is tilted at an angle of approximately 23.44° from the orbital plane. This tilt is called the axial tilt
. As a consequence, for half a year (from around 20 March
to around 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 when the Sun is directly overhead at 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 day and night are divided equally between the hemispheres.
The table above gives the dates and times of equinoxes and solstices over several years. A few remarks can be made about the equinoxes:
- Because the Sun is a sphere and not a point source of light, the actual crossing of the Sun over the equator takes approximately 33 hours.
- At the equinoxes, the rate of change for the length of daylight and nighttime is the greatest. At the poles, the equinox marks the transition from 24 hours of nighttime to 24 hours of daylight. High in the Arctic Circle, Longyearbyen, Svalbard, Norway has an additional 15 minutes more daylight every day around the time of the Spring equinox. Whereas, in Singapore, which lies virtually on the equator, the amount of daylight each day varies by just seconds.
- 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 occurrence of a leap year. The Gregorian calendar is designed to follow the seasons as accurately as is practical. 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 will slowly shift 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 in many years of the twentieth century the dates of 21 March, 22 June, 23 September and 22 December were much more common, so older books teach, and older people still remember, these dates as the main ones.
- Note that the times are given in UTC, roughly speaking, 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. As seen by an observer on Earth, it may appear to the casual observer that the Sun revolves around the Earth once a year. As such, 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 local time. 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 or more, and even two hours if daylight saving time (summer time) is included. In that case, the Sun can rise for example at 8:00 and set at 20:00, but there would still be 12 hours of daylight.
- Even those people fortunate enough to have their time zone just equal to the local time 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 upper limb is already up for at least one minute before the centre appears, and likewise the upper limb sets one minute later than the centre of the solar disk. Due to 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.
- Height of the horizon on both the sunrise and sunset sides changes the day's length. Going up into the mountains will lengthen the day, while standing in a valley with hilltops on the east and the west can shorten the day significantly. This is why settlements in east-west running valleys are more favourable (daylight-wise) than north-south running valleys.
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.
Celestial coordinate systems
The vernal point (vernal equinox)—the one the Sun passes in March on its way from south to north—is used as the origin of some celestial coordinate systems
Because of the precession of the Earth's axis, the position of the vernal point changes over time and as a consequence both the equatorial and the ecliptic coordinate systems change over time. Therefore, when specifying celestial coordinates for an object, one have to specify at what time the vernal point (and also the celestial equatorial) are taken. That reference time is also called equinox.
The autumnal equinox is at ecliptic longitude 180° and at right ascension 12h.
The upper culmination of the vernal point is considered the start of the sidereal day for the observer. The hour angle of the vernal point is, by definition, the observer's sidereal time.
For Western tropical astrology, the same thing holds true; the vernal equinox is the first point (i.e. the start) of the sign of Aries. In this system, it is of no significance that the fixed stars and equinox shift compared to each other due to the precession of the equinoxes.
Cultural aspects of the Equinox
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 Persian new year, Nowruz, is held annually on the vernal equinox, as the beginning of spring.
- The September equinox marks the first day of Mehr or Libra in the Persian calendar. It is one of the Iranian festivals called Jashne Mihragan, or the festival of sharing or love in Zoroastrianism.
- World Citizen Day occurs on the March equinox.
- Sham El Nessim was an ancient Egyptian holiday which can be traced back as far as 2700 B.C. It is still one of the public holidays in Egypt. Sometime during Egypt's Christian period (c.200-639) the date moved to Easter Monday, but before then it coincided with the vernal equinox.
- The Jewish Passover always falls on the full moon following the first new moon after the northern hemisphere vernal equinox.
- The Christian churches calculate Easter as the first Sunday after the first full moon on or after the March equinox. The official church definition for the equinox is March 21. However, as the Eastern Orthodox Churches use the older Julian calendar, while the Western Churches use the Gregorian calendar, both of which designate March 21 as the equinox, the actual date of Easter differs. The earliest possible Easter date in any year is therefore March 22, on each calendar.
- The March equinox marks the first day of various calendars including the Iranian calendar and the Bahá'í calendar. The Persian (Iranian) festival of Nowruz 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 commemorated with festivities for two weeks. These festivities recall the story 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. As well as being a Zoroastrian holiday, it is also a holy day for adherents of the Bahá'í Faith and the Nizari Ismaili Muslims.
- The traditional East Asian calendars divide a year into 24 solar terms (節氣, literally "climatic segments"), and the vernal equinox (Chūnfēn, ) and the autumnal equinox (Qiūfēn, ) mark the middle of the spring and autumn seasons, respectively. In this context, the Chinese character 分 means "division" (within a season).
- 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. Similarly, in September, there is an Autumnal Equinox Day (秋分の日 Shūbun no hi).
- The Mid-Autumn Festival is celebrated on the 15th day of the 8th lunar month, and is an official holiday in many East Asian countries. As the lunar calendar is not synchronous with the Gregorian calendar, this date could be anywhere from mid-September to early October.
- 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 latter in Bangladesh and the East Indian state of West Bengal.
- Earth Day was initially celebrated on March 21 1970, the equinox day. It is currently celebrated in various countries on April 22.
- 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 September 21 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.
- World Storytelling Day is a global celebration of the art of oral storytelling, celebrated every year on the spring equinox in the northern hemisphere, the first day of autumn equinox in the southern.
Myths, fables and facts
- For a Latin word like nox the plural is noctēs. Although this root is retained in English in the adjective: equinoctial — it is not commonly used for the plural, which is equinoxes, rather than equinoctes.
- One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days near the equinox when the sun goes directly behind the satellite relative to Earth (ie, within the beamwidth of the groundstation 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 beamwidth, hence experience shorter duration "Sun outage" windows).
- A modern folk-notion claims that only on the March equinox day (some may add the September equinox day or may explicitly not), one can balance an egg on its point. However one can balance an egg on its point any day of the year if one has the patience.
- Although the word "equinox" implies equal length of day and night, as is noted elsewhere, this is not true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal. Those days are commonly referred to as the "equiluxes" to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart. This way, you can refer to a single date as being the equilux, when, in reality, it spans sunset on one day to sunset the next, or sunrise on one to sunrise the next.
- The equilux counts times when some direct sunlight could be visible, not all hours of usable daylight, which is anytime there is enough natural light to do outdoor activities without artficial light. This is due to twilight, and this part of twilight is officially defined as civil twilight. This amount of twilight can result in more than 12 hours of usable daylight up to a few weeks before the spring equinox, and up to a few weeks after the fall equinox.
- On the contrary, the intensity of light near sunrise and sunset, even with the sun slightly above the horizon, is considerably less than when the sun is high in the sky. The daylight which is useful for illuminating daylit houses and buildings and for producing the full psychological benefit of daylight is shorter than the nominal time between sunrise and sunset, and is present for 12 hours only after the vernal equinox and before the autumnal equinox.
- It is perhaps valuable for people in the Americas and Asia to know that the equinoxes listed as occurring on March 21 that occurred frequently in the twentieth century and that will occur occasionally in the 21st century are presented as such using UTC, which is at least four hours in advance of any clock in the Americas and as much as twelve hours behind Asian clocks. Thus, there will be no spring equinox later than March 20 in the Americas in the coming century.