March 21 marks the spring (vernal) equinox in the Northern Hemisphere is also known as the March equinox. It’s called the “autumnal (fall) equinox” in the Southern Hemisphere.
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.At this moment, Earth’s axis is tilted neither away from nor towards the Sun, but is rather perpendicular to the Sun’s rays. On equinox everybody on Earth gets to experience a day and night of equal lengths. At the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. The word equinox comes from the Latin aequus (equal) and nox (night).
The dates of the equinoxes and solstices aren’t fixed due to the Earth’s elliptical orbit of the sun. The Earth’s orbit around the sun means that in early January, the sun is closest (known as perihelion) and in early July it is most distant (aphelion).
On Equinox, the day and night will be of almost equal duration at most time zones in the world. The second equinox, the September Equinox, takes place on or around September 22 every year.
But, since Earth never stops moving around the sun, these days of equal sunlight and night will change quickly. At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator.