By Sarah Gray
March 20, 2018

For the Northern Hemisphere, Tuesday, March 20, marks the vernal, or spring, equinox — the first day of spring.

The National Weather Service shared this image from space of the Earth just before “the sun crossed the equator,” or just before the start of spring. For those living north of the equator — the imaginary latitude line that marks the middle of the Earth — the days will begin to get longer.

What does that mean?

Equinoxes happen twice per year (the vernal and autumnal equinox) when the sun crosses over the equator. During an equinox the length of day and night of the Northern and Southern Hemispheres are nearly the same. (The word equinox means “equal night”.)

Why does this happen?

“Earth’s axis is an imaginary pole going right through the center of Earth from ‘top’ to ‘bottom,'” NASA explains. “Earth spins around this pole, making one complete turn each day. That is why we have day and night, and why every part of Earth’s surface gets some of each.”

We have seasons because Earth’s axis is on a tilt, and that tilt “always points in the same direction,” according to NASA. So as Earth rotates around the sun, during part of the year, the North Pole tilts towards the sun, and for half of the year the South Pole tilts towards the sun. Thus, causing our seasons (summer in the Northern Hemisphere when the North Pole is tilting towards the sun, and summer for the Southern Hemisphere when the South Pole is tilting towards the sun).

On an equinox, “the tilt of the Earth’s axis is perpendicular to the Sun’s rays,” according to Time and Date. After this vernal equinox, the Northern Hemisphere will begin to experience the sun’s rays more directly, hence spring and summer!

Though, for some in the Northern Hemisphere who are preparing for Winter Storm Toby, it may not feel like spring just yet.

Contact us at editors@time.com.

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