There are two equinoxes every year – in March and September –
when the Sun shines directly on the equator and the length of night and day are nearly equal.
March Equinox -
Equal Day and Night, Nearly
The Sun Crosses the Equator
The March equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north. This happens on March 19, 20 or 21 every year.
Spring in the North, Fall in the South
Equinox and solstice.
Equinoxes and solstices are opposite on either side of the equator, and the March equinox is also known as the "spring (vernal) equinox" in the northern hemisphere and as the "autumnal (fall) equinox" in the southern hemisphere.
Why is it Called “Equinox”?
On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it's called an “equinox”, derived from Latin, meaning "equal night". However, in reality equinoxes don't have exactly 12 hours of daylight. In reality equinoxes don't have exactly 12 hours of daylight
Latitude Determines Day Length
Even if day and night aren’t exactly equal on the day of the equinox, there are days when day and night are both very close to 12 hours.
Approx date of "Equal Day & Night"
Latitude March September
60° North Mar 18 Sep 25
55° North Mar 17 Sep 25
50° North Mar 17 Sep 25
45° North Mar 17 Sep 25
40° North Mar 17 Sep 26
35° North Mar 16 Sep 26
30° North Mar 16 Sep 27
25° North Mar 15 Sep 27
20° North Mar 14 Sep 28
15° North Mar 12 Sep 30
10° North Mar 8 Oct 4
5° North Feb 24 Oct 17
Equator No equal day and night
5° South Apr 14 Aug 29
10° South Apr 1 Sep 10
15° South Mar 2 Sep 14
20° South Mar 26 Sep 16
25° South Mar 25 Sep 17
30° South Mar 24 Sep 18
35° South Mar 24 Sep 19
40° South Mar 23 Sep 19
45° South Mar 23 Sep 19
50° South Mar 23 Sep 20
55° South Mar 23 Sep 20
60° South Mar 22 Sep 20
However, this date depends on the latitude, and can vary by as much as several weeks from place to place. The table shows approximate dates for when day and night are as similar as possible according to latitude.
More Than 12 Hours' Day
On the equator, the day and night stay approximately the same length all year round, but the day will always appear a little longer than 12 hours, due to the reasons below.
On the equinoxes, the geometric center of the sun is above the horizon for 12 hours, and you might think that the length of the day (hours of daylight) would be 12 hours too.
However, ‘sunrise’ is defined as the moment the upper edge of the sun's disk becomes visible above the horizon – not when the center of the sun is visible. In the same sense, ‘sunset’ refers to the moment the Sun's upper edge, not the center, disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted fromt he night, and therefore the equinox day lasts a little longer than 12 hours.
Refraction: Light Lingers
Another reason why the day is longer than 12 hours on an equinox is that the Earth's atmosphere refracts, sunlight.
This refraction, or bending of the light, causes the Sun’s upper edge to be visible from Earth several minutes before the edge actually reaches the horizon. The same thing happens at sunset, when you can see the sun for several minutes after it has actually dipped under the horizon. This causes every day on Earth – including the days of the equinoxes – to be at least 6 minutes longer than it would have been without this refraction.
The extent of refraction depends on atmospheric pressure and temperature. Our calculations in the Sunrise and Sunset Calculator assumes the standard atmospheric pressure of 101.325 kilopascal and temperature of 15°C or 59°F.
What Happens on the Equinox?
The Earth's axis is always tilted at an angle of about 23.5° in relation to the ecliptic, the imaginary plane created by the Earth's path around the Sun. On any other day of the year, either the southern hemisphere or the northern hemisphere tilts a litte towards the Sun. But on the two equinoxes, the tilt of the Earth's axis is perpendicular to the Sun's rays, like the illustration shows.
Celebrating new Beginnings
The March equinox has long been celebrated as a time of rebirth in the Northern Hemisphere. Many cultures celebrate spring festivals and holidays around the March equinox, like Easter and Passover.
Customs and holidays around the September equinox
The September equinox coincides with many cultural events, religious observances and customs. It's also called the "autumnal (fall) equinox" in the northern hemisphere and the "spring equinox" in the Southern Hemisphere.
The Christian church replaced many early Pagan equinox celebrations with Christianized observances. For example, Michaelmas (also known as the Feast of Michael and All Angels), on September 29, fell near the September equinox.
In many cultures, the September equinox is a sign of fall (autumn) in the northern hemisphere. In Greek mythology fall is associated with when the goddess Persephone returns to the underworld to be with her husband Hades. It was supposedly a good time to enact rituals for protection and security as well as reflect on successes or failures from the previous months.
Aboriginal Australians have, for a long time, had a good knowledge of astronomy and the seasons. Events like the September equinox, which is during the spring in Australia, played a major role in oral traditions in Indigenous Australian culture.
In China the Mid-Autumn Festival, also known as the Moon Festival, is celebrated around the time of the September equinox. It celebrates the abundance of the summer's harvest and one of the main foods is the mooncake filled with lotus, sesame seeds, a duck egg or dried fruit.
Higan, or Higan-e, is a week of Buddhist services observed in Japan during both the September and March equinoxes. Both equinoxes have been national holidays since the Meiji period (1868-1912). Higan means the “other shore” and refers to the spirits of the dead reaching Nirvana. It is a time to remember the dead by visiting, cleaning and decorating their graves.
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