equinox - Day arcs of the Sun, Coordinate systems, Cultural aspects, Trivia, facts and fables

Either of the two points on the celestial sphere where the ecliptic intersects the celestial equator. Physically these are the points at which the Sun, in its annual motion, appears to cross the celestial equator – the vernal equinox as it crosses from S to N, and the autumnal equinox as it crosses from N to S. The vernal equinox is the zero point in celestial co-ordinate systems. 2 Either of the two instants of time at which the Sun crosses the celestial equator, being about 21 March (vernal) and 23 September (autumnal).

UTC Date and Time of
Solstice and Equinox

year	Equinox Mar		Solstice June		Equinox Sept		Solstice Dec
	day	time	day	time	day	time	day	time
2002	20	19:16	21	13:24	23	04:55	22	01:14
2003	21	01:00	21	19:10	23	10:47	22	07:04
2004	20	06:49	21	00:57	22	16:30	21	12:42
2005	20	12:33	21	06:46	22	22:23	21	18:35
2006	20	18:26	21	12:26	23	04:03	22	00:22
2007	21	00:07	21	18:06	23	09:51	22	06:08
2008	20	05:48	20	23:59	22	15:44	21	12:04
2009	20	11:44	21	05:45	22	21:18	21	17:47
2010	20	17:32	21	11:28	23	03:09	21	23:38
2011	20	23:21	21	17:16	23	09:04	22	05:30
2012	20	05:14	20	23:09	22	14:49	21	11:11
2013	20	11:02	21	05:04	22	20:44	21	17:11
2014	20	16:57	21	10:51	23	02:29	21	23:03

An equinox in astronomy is the event when the Sun can be observed to be directly above the equator. In a wider sense, the equinoxes are the two days each year when the center of the Sun spends an equal amount of time above and below the horizon at every location on Earth.

In practice, at the equinox, the day is longer than the night. The real equality of day and night happens a few days towards the winter side of the equinox. The seasons of the northern hemisphere and southern hemisphere are opposites (the spring equinox of one hemisphere is the autumn equinox of the other) so these names can be ambiguous. 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. One disadvantage is that due to the precession of the equinoxes the astrological signs where these equinoxes are located, do not correspond any longer with the actual constellations. 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 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 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. 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. The daily motion of the Sun, (day and night), however, takes place parallel to 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.

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.

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. The two effects add up to almost seven minutes, making the equinox day 12h 7m long and the night only 11h 53m. 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 will lengthen the day, while standing in a valley with hilltops on the east and the west can shorten the day significantly.

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.

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. 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.

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 March equinox marks the first day of various calendars including the Iranian Calendar and the Bahá'í calendar. The spring equinox marks the Wiccan Sabbat of Ostara (or Eostar), while at the autumn equinox the Wiccan Sabbat of Mabon is celebrated. 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). Earth Day was initially celebrated on 21 March 1970, the equinox day. 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.

Preceded by: Jingzhe (驚蟄)

Solar term (節氣)

Succeeded by: Qingming (清明)

Qiufen (秋分; The important Mid-Autumn Festival is celebrated around the time of the autumn equinox, falling on the full-moon day in the eighth lunar month.

Preceded by: Bailu (白露)

Solar term (節氣)

Succeeded by: Hanlu (寒露)

Trivia, facts and fables

For a Latin word like nox the plural is noctes. Folk tales from various European countries claim 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. Although the word "equinox" implies equal length of day and night, as is noted elsewhere, this simply isn't 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.