The March equinox is almost upon us, which will usher in astronomical spring in the Northern Hemisphere and astronomical fall in the Southern Hemisphere.
This year, the vernal equinox, or spring equinox, occurs at 10:46 a.m. EDT (14:46 UTC) on March 20 in the Northern Hemisphere, according to Time and Date. Because equinoxes are global events governed by Earth’s tilted axis relative to the sun, the March equinox occurs at the same moment across the globe.
Why are there seasons?
Earth experiences seasons because its axis is tilted by an average of 23.5 degrees relative to the planet’s orbit around the sun; this tilt causes different places on Earth to receive varying amounts of sunlight over the year. On the equinox, neither hemisphere is tilted toward or away from the sun, so the sun shines nearly directly over the equator, producing almost equal hours of day and night.
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Equinoxes happen twice each year, with approximately 12 hours of daylight and 12 hours of darkness in both the Southern and Northern hemispheres. The word comes from the Latin words “aequus” (equal) and “nox” (night). After the equinox, nights will shorten in the Northern Hemisphere and lengthen in the Southern Hemisphere until the solstice on Sunday, June 21.
Equinoxes are among the most obvious waymarks of the solar calendar, because the sun rises and sets due east from nearly all locations on Earth (except at the poles), just as it does on the September equinox.
Written in stone and stars
Equinoxes have been celebrated by cultures throughout history. At Chichén Itzá, a Maya complex on Mexico’s Yucatán Peninsula, light from the sunrise on the equinox makes it appear as though a serpent were moving down the Kukulcán Pyramid. It’s thought to mark the beginning of the planting season.
Numerous other temples are believed to be aligned with sunrise on the equinox, including the Great Sphinx of Giza in Egypt and Angkor Wat in Cambodia. Other celebrations linked to the March equinox include Nowruz (Persian New Year), which is thought to date back 3,000 years.
If you can’t witness the sunrise on the equinox in a place with known alignments, try looking just after sunset on March 20, when a 5%-lit waxing crescent moon will shine just above Venus in the west-southwest.
There will also be signs of spring in the stars: The Big Dipper will ride high in the northeast for skywatchers at mid-northern latitudes. (You can remember its position with the mnemonic “spring up, fall down.”) Meanwhile, in the southern night sky for mid-northern latitudes will be three prominent stars of spring: Arcturus (in the constellation Boötes), Spica (in Virgo) and Denebola (in Leo), which together make the Spring Triangle asterism.
