An equinox is a point on the Earth's orbit is reached when the Sun is exactly at the zenith on the equator.

The equinox also marks the intersection of the ecliptic (apparent path of the trace of the Sun on the celestial sphere) and the equator.

A year has two equinoxes, the first to 20 or 21 March , the second to 22 or 23 September. By extension, the equinoxes designate days of the year in which these passages occur at zenith. The dates of the equinoxes are bound by agreement with those of the early spring and the autumn.

Summary 1 Etymology 2 Astronomy 2.1 Satellite Orbit 2.2 Determination 2.3 Length of day 2.4 Solar Trajectory 3 Dates 4 Cultural aspects 5 Systems of celestial coordinates 6 See also 6.1 Related articles 6.2 External Links 6.3 Notes and references

Etymology

Etymologically, the word equinox comes from Latin quinoctium of Aequus (equal) and nox, noctis (night). This is because at the equinox, day and night are of equal length.

The equinox of spring (or vernal) describes the March equinox in the northern hemisphere and September equinox in the southern hemisphere. The equinox of autumn is September in the northern hemisphere and March in the southern hemisphere.

Astronomy

Earth orbit

The Earth's axis is tilted approximately 23.44 degrees to the plane of its orbit. Accordingly, for about half of the year, its northern hemisphere is tilted toward the Sun, while the orientation is in favor of his southern hemisphere while the other half. During an equinox, the two hemispheres are also facing the Sun and it is located directly overhead the equator. Points North and South are also located in this moment on the terminator , and day and night just divide the two hemispheres.

Conversely, in terms of geocentric , an equinox occurs when the Sun reaches one of two intersections between the ecliptic and celestial equator : his declension is then zero.

The Sun is not a single point of light seen from Earth, it crosses the equator takes about 33 hours.

Determination

The date of the equinox can be determined by observing the sunrise, from the point due east (or due west for sunset): The spring equinox occurs on the day the sun stops rising in the south this point to get up north ( mutatis mutandis for the sunset, and / or to the autumnal equinox). The exact time can be assessed from the solar azimuth at two consecutive sunrises, interpolating when the sun crosses the azimuth 90 (or 270 at sunset).

It is often said that "at the equinox, the sun rises in the east and sets in the West," but this is only approximately correct: this rule neglects the movement of the sun during the day. The Sun can not rise exactly in the East that if he throws at the precise moment of the equinox, which is the case throughout a meridian, but by the time the sun goes down twelve hours later, its variation will be slightly different (one-fifth of a degree), and it does not lie exactly over the West. The difference is not very sensitive to the current observation (third degree in azimuth, for latitudes around 45 ).

The observation of the Sun at sunrise is not very precise astronomical terms, because that's where atmospheric refraction is highest, resulting in uncertainty about the astronomical sunrise and thus its azimuth. An astronomical observatory instead use a meridian circle , to determine (by interpolation between two solar noon consecutive) when the sun passes over the celestial equator , and therefore has a zenith distance equal to the latitude of the observing site.

Length of day

The day of an equinox, the center of the Sun goes around the same time above and below the horizon for all points on the surface of the Earth: 12 hours. But the sun is not seen on Earth as a bright spot but as a sphere, the day is longer than the night because the blade above the Sun can be seen when its center is always located below the horizon. In addition, the Earth's atmosphere refracts sunlight: even if its blade is just below the horizon, its rays can still reach the earth's surface. In practice, the apparent radius of the Sun is about 16 minutes of arc and the atmospheric refraction of 34 minutes of arc. The combination of the two implies that the upper limb of the Sun can be seen, so that its center is located within 50 minutes of arc below the horizon real. En consquence, le jour est plus long de 14 minutes que la nuit l'quateur lors d'un quinoxe. This time increases when moving towards the poles: in London , it reaches 24 minutes, and 100 km at the poles, the Sun remains visible throughout the day.

Some points of the earth's surface sufficiently distant from the equator can experience a day when the length of day and night are nearly identical. His exact date depends on the latitude and longitude, but the days before the vernal equinox (or after the autumnal equinox) have one day more than 12 hours. Consider the twilight further reduces the duration of the night.

At the equinoxes, the daily variation of the length of day and night is greater. At the poles, the equinox marks the transition between six months and six months day and night. Located in Svalbard far beyond the Arctic Circle , the town of Norway to Longyearbyen experiencing 15 minutes of daylight every day around the spring equinox. In Singapore (about 1 north ), this variation is only a few seconds.

Solar Trajectory

At the equinoxes, the Sun rises exactly in the east and sets exactly in the West. The North Pole to South Pole , all points on the Earth are on the same meridian , while simultaneously receive sunlight during the day.

In the northern hemisphere , the Sun rises in the south, in the southern hemisphere , it culminates in the north to the equator, it rises to the zenith.

The following diagrams describe schematically the apparent path of the sun at an equinox day for different latitudes.

• 

  0 ( equator ) the sun rises overhead.

• 

  20 : the sun rises to an altitude of 70 and disappears below the horizon at a trajectory angle of 70 with respect thereto. Twilight lasts about an hour.

• 

  50 : the twilight lasts nearly two hours.

• 

  70 : the sun only peaked at 20 altitude and disappears below the horizon at an angle very small. Twilight lasts four hours, in fact, the dark night is almost nonexistent.

• 

  90 ( pole ) if the atmospheric refraction did not come into the center of the sun on the horizon would stay all day.

Dates

Date and Time ( UTC ) of the solstices and equinoxes at the beginning of XXI century
Year	Equinox March		Solstice June		Equinox from Sept.		Solstice dec.
	day	hour	day	hour	day	hour	day	hour
2001	20	1:31 p.m.	21	7:37	22	11:04 p.m.	21	7:21 p.m.
2002	20	7:16 p.m.	21	1:25 p.m.	23	4:55	22	1:15
2003	21	1:00	21	7:10 p.m.	23	10:46	22	07:04
2004	20	6:48	21	0:57	22	4:30 p.m.	21	12:41
2005	20	12:33	21	6:46	22	22:23	21	6:35 p.m.
2006	20	6:25 p.m.	21	12:25	23	4:03	22	0:21
2007	21	0:07	21	6:06 p.m.	23	9:50	22	6:07
2008	20	5:48	20	11:59 p.m.	22	3:44 p.m.	21	12:03
2009	20	11:43	21	5:45	22	9:18 p.m.	21	5:46 p.m.
2010	20	5:31 p.m.	21	11:29	23	3:09	21	11:38 p.m.
2011	20	11:20 p.m.	21	5:16 p.m.	23	9:04	22	5:29
2012	20	5:14	20	11:08 p.m.	22	2:48 p.m.	21	11:11
2013	20	11:01	21	5:04	22	8:44 p.m.	21	5:11 p.m.
2014	20	4:56 p.m.	21	10:51	23	2:28	21	11:02 p.m.
2015	20	10:45 p.m.	21	4:38 p.m.	23	8:20	22	4:48
2016	20	4:40	20	10:34 p.m.	22	2:21 p.m.	21	10:44
2017	20	10:28	21	4:23	22	8:01 p.m.	21	4:27 p.m.
Reference: IMCCE Institute of Celestial Mechanics ephemeris calculation.

In the Gregorian calendar , the dates of equinoxes vary among years (table at right summarizes them for years near). The following facts are taken into account:

• The Earth orbit is not perfectly circular and its speed depends on its position. Consequently, the seasons are unequal length:
  • Spring North (Fall Southern ), the March equinox to the solstice of June: 92.7 days;
  • Summer northern (southern winter), the June solstice to the equinox of September: 93.7 days;
  • Fall northern (southern spring), the September equinox to the solstice of December: 89.9 days;
  • Winter North (austral summer), the December solstice to March equinox: 89.0 days.
• The calendar year is standard as of 365 days, the tropical year is approximately 365.2422 days. The equinoxes occur therefore almost six hours later from one year to another. The leap year can shift the dates of equinoxes one day every four years.
• This offset compensates for leap lightly over the difference between the calendar year and the tropical year. After 70 years he led the equinoxes to occur one day earlier (which is the problem of the Julian calendar ). This point is partially offset by the absence of leap years for years divisible by 100 (but not 400).

The March equinox occurs then, in UTC on 19 , 20 or March 21. In the ^nineteenth and ^twentieth centuries, the March equinox is always fallen on 20 or March 21. He fell on March 19 to 15 times in the second half of the ^seventeenth century and 5 times in the late ^eighteenth century (the last case in 1796 ). It will again fall on March 19 in 2044. At ^twenty-first century , it is dropped on March 21 in 2003 and 2007. It was not until 2102 , so that it falls on March 21.

The September equinox may occur in UTC on 21 , 22 , 23 or 24 September. It will fall on September 21 in 2092 for the first time since the introduction of the Gregorian calendar in 1582. It will happen again in 2096 and again in 2464. It fell one September 24th 2 times in the early ^nineteenth century and eight times in the early ^twentieth century , he falls for the last time on this date in 2303.