(NEWSnet/AP) – The spring equinox, also known as the vernal equinox, happens at 11:09 p.m. Eastern time Tuesday.

The equinox is one of the quarterly astronomical measurements of the Earth’s orbit, marking the start of spring for the Northern Hemisphere and start of fall for the Southern Hemisphere. It happens around March 19 to March 21 on the calendar year; with an earlier synchronicity this year because of the leap year calculations.

For most of the year, the Earth’s axis is tilted either toward or away from the sun. That means the sun’s warmth and light fall unequally on the northern and southern halves of the planet.

During the equinox, the Earth’s axis and its orbit line up so that both hemispheres get an equal amount of sunlight.

The summer and winter quarterly orbit steps are called solstices, when the Earth is at its most extreme tilt toward or away from the sun. This means the northern and southern hemispheres get very different amounts of sunlight — and days and nights are at their most unequal.

The astronomical seasons fall close to, but are not the same as, meteorological seasons that break up a year by the calendar months. This is why spring on the meteorological calendar starts March 1, while summer starts June 1.

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