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March Equinox - Equal Day and Night, Nearly

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 illustration

Earth's position in relation to the Sun during the equinox (not to scale).

March Equinox in Washington DC, District of Columbia, USA is on
Saturday, March 20, 2021 at 5:37 am EDT (Change city)

March Equinox in Universal Coordinated Time is on
Saturday, March 20, 2021 at 09:37 UTC

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.

2020's March Equinox fell on March 19 in all time zones in the United States, making it the earliest spring equinox in the country since 1896.

Northern Spring, Southern Fall

Illustration showing Earth's position in relation to the Sun at the equinoxes and solstices.

Earth's tilt during the equinoxes and solstices.

timeanddate.com

Equinoxes and solstices are opposite on either side of the equator. The March equinox is the spring (vernal) equinox in the Northern Hemisphere, marking the start of astronomical spring. In the Southern Hemisphere, it is the autumnal (fall) equinox, which marks the beginning of fall.

Why Equinox?

On the equinox, night and day are nearly 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.

What Happens on the Equinox?

Earth's axis is tilted at an angle of about 23.4° in relation to the ecliptic plane, 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 little 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.

Start of the Tropical Year

The March equinox is often used by astronomers to measure a tropical year, the mean time it takes for the Earth to complete a single orbit around the Sun. Also known as a solar year, a tropical year is approximately 365 days, 5 hours, 48 minutes, and 45 seconds long.

The time between one March equinox and the next can vary by only a few minutes or by as many as 30 minutes each year.

 DaysHoursMinutesSeconds
March 2019 – March 20203655514
March 2020 – March 202136554755
March 2021 – March 202236555554
March 2022 – March 202336555055
March 2023 – March 20243655428

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.

The Snake of Sunlight

Illustration image

“The snake of sunlight” at Chichen Itza, Mexico.

©iStockphoto.com/CostinT

One of the most famous ancient Spring equinox celebrations was the Mayan sacrificial ritual by the main pyramid at Chichen Itza, Mexico.

The main pyramid, also known as El Castillo, has four staircases running from the top to the bottom of the pyramid's faces.

The staircases are built at a carefully calculated angle, which makes it look like an enormous snake of sunlight slithers down the stairs on the day of the equinox. The pyramid is notorious for the bloody human sacrifices that used to take place here.

The Mayan calendar was very precise in this respect, but today the Mayan calendar is most famous for ending exactly at 11:11 UTC on the 2012 December Solstice.

Knowledge of the equinoxes and solstices is also crucial in developing dependable calendars, another thing the Mayans clearly had got the hang of.

Topics: Astronomy, Sun, Seasons, Equinox, Earth, March