The equinox happens twice a year, around March 20 and September 22, when day and night are nearly equal in length all over the world. The word itself comes from the Latin aequus (equal) and nox (night). On these days, the sun shines directly over the equator, marking the official start of spring in March (the vernal equinox) and fall in September (the autumnal equinox) for the Northern Hemisphere.

The equinox occurs because of Earth’s tilted axis. Our planet is tilted about 23.5 degrees as it orbits the sun. Most of the year, this tilt causes one hemisphere to lean toward the sun (longer days, more direct sunlight, and summer) while the other leans away (shorter days, less sunlight, and winter). But during the equinox, the tilt is “sideways” relative to the sun, so both hemispheres receive sunlight almost evenly.

While the equinox doesn’t always give us a perfect 12 hours of daylight everywhere, it’s the closest we get. Factors like atmospheric refraction and your location on Earth add a few minutes of daylight. Still, the equinox is a powerful reminder of how Earth’s tilt and orbit create the rhythm of the seasons we experience year after year.

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Jennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.