Vernal Equinox: Science Behind the First Day of Spring 2017

March 20, 2017

The vernal equinox, officially bringing the arrival of spring, happens Monday morning.

During the equinox, nearly equal amounts of daylight and darkness are experienced throughout the world.

Twice a year, around March 20 or 21 and September 22 or 23, the sun's most direct rays shine on the equator. These two days are known as the vernal (spring) equinox and the autumnal (fall) equinox, respectively.

This year, the vernal equinox takes place on Monday, March 20 at 6:29 a.m. EDT. At this time, the sun's most direct rays cross over from the southern hemisphere into the northern hemisphere. During this process, the sun is shining directly over the earth's equator, bathing the earth's northern and southern hemispheres in nearly an equal amount of sunlight.

(MORE: How Weather Impacts Your Spring Allergies)

Instead of a tilt away from or toward the sun, the Earth's axis of rotation is perpendicular to the line connecting the centers of the Earth and the sun during an equinox. During the equinox, both day and night are balanced to nearly 12 hours each all over the world.

Good news for those in the northern hemisphere: Daylight continues to grow longer until the summer solstice, which occurs on Wednesday, June 21. The opposite occurs in the southern hemisphere, where daylight continues to grow shorter toward their winter solstice on the same day.

MORE: Scenes from the First Day of Spring


The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.