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Equinox: A celestial phenomenon

An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location (the subsolar point) on the Earth's equator, where the center of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.

The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long. But in reality, the day is longer than the night at an equinox.

An equinox occurs when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator.

The length of day and night changes throughout the year because of the way Earth moves around the sun. In winter, days are short and nights are long. In summer, days are long and nights are short. In March, as the Sun is moving northwards along the ecliptic, this is called the vernal (spring) equinox and in September as the Sun is moving southwards we refer to it as the autumnal equinox. The equinoxes are also the points on the celestial sphere where the ecliptic and equator cross and the vernal equinox is used as the zero point in measuring star co-ordinates.

March and September Equinox explained

The March equinox is the movement when the sun crosses the true celestial equator – or the line in the sky above the earth’s equator – from south to north, around March 20 (or March 21) of each year. At that time, day and night are balanced to nearly 12 hours each all over the world and the earth’s axis of rotation is perpendicular to the line connecting the centers of the earth and the sun.

During the September equinox, the sun crosses the celestial equator and moves southward in the northern hemisphere.

The location on the earth where the sun is directly overhead at solar noon is known as the subsolar point. The subsolar point occurs on the equator during the September equinox and March equinox. At that time, the earth’s axis of rotation is perpendicular to the line connecting the centers of the earth and the sun.

Historical Evidence

Hipparchus, a Greek astrologer, astronomer, geographer, and mathematician of the Hellenistic period, is credited for discovering the precession of the equinoxes. He made observations of the equinox and solstice.

According to Claudius Ptolemaeus, a mathematician, astronomer, geographer, astrologer and a poet of a single epigram in the Greek Anthology, Hipparchus measured the longitude of Spica and Regulus and other bright stars. Comparing his measurements with data from his predecessors, Timocharis and Aristillus, he concluded that Spica had moved 2° relative to the autumnal equinox.

He also compared the lengths of the tropical year (the time it takes the Sun to return to an equinox) and the sidereal year (the time it takes the Sun to return to a fixed star), and found a slight discrepancy.

Hipparchus concluded that the equinoxes were moving ("precessing") through the zodiac, and that the rate of precession was not less than 1° in a century.

Equinoxes differ each year

The Earth takes approximately 365.25 days to go around the Sun. This is the reason we have a leap year every 4 years, to add another day to our calendar so that there is not a gradual drift of date through the seasons. For the same reason the precise time of the equinoxes are not the same each year, and generally will occur about 6 hours later each year, with a jump of a day (backwards) on leap years.

The table below shows the dates and times of both the vernal and autumnal equinoxes for a period of ten years: 

Year Vernal Equinox
Autumnal Equinox
Leap year
2000 20 March, 07.35
22 September, 17.27 Yes
2001 20 March, 13.31 22 September, 23.04
20 March, 19.16 23 September, 04.55

2003 21 March, 01.00 23 September, 10.47

2004 20 March, 06.49 22 September, 16.30
2005 20 March, 12.33 22 September, 22.23

2006 20 March, 18.25
23 September, 04.03

2007 21 March, 00.07
23 September, 09.51

20 March, 05.48
22 September, 15.44
20 March, 11.43
22 September, 21.18

20 March, 17.32
23 September, 03.09

[* All times are UTC (GMT) ].

National Maritime Museum, UK


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