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Why is Today the First Day of Spring? The Science Behind the Seasons

The vernal (or spring) equinox occurs just after 7 a.m. EDT on Wednesday, March 20. If you live in the United States or other places in earth’s Northern Hemisphere, spring began for you at that time.

The Latin word equinox means “equal night.” On the day of an equinox, daytime and nighttime are almost the exact same length. This will happen again on the autumnal equinox in exactly one half year.

Why doesn’t every day have equal parts day and night?

This constantly changing variation in the length of daytime is actually caused by the same thing that gives us changing seasons: the tilt in the earth’s axis.

The earth spins on an axis that has an angle which is 23.5 degrees different from the angle of the path the earth makes around the sun.

Here’s a longer explanation:

Night and day are caused by the rotation of the earth on its axis. Sometimes your part of the earth is facing the sun (day), and sometimes your part is facing away (night). One rotation takes about twenty-four hours.

Meanwhile, the earth is traveling around the sun in a path called an orbit. One full orbit takes one full year.

If the angle of the earth’s orbit and the angle of the earth’s equator were the same, the sun’s rays would hit all of the places of the earth the same way all year long. In addition, every place on earth would have one never-ending season with temperatures that got colder as you moved further from the equator.

However, our orbit and our equator don’t line up. The earth is tilted on its axis. For half a year’s time, the bottom half (Southern Hemisphere) of the earth gets more sun than the top half (Northern Hemisphere). During the next six months, the top half gets more. The vernal equinox begins six months of longer and warmer days for the Northern Hemisphere.

Learn more about the seasons of the year from NASA.