This Saturday, March 20, we’ll experience the first of two annual equinoxes. For the Northern Hemisphere, it’ll be the vernal equinox—marking the first day of spring—while the Southern Hemisphere will welcome fall with the autumnal equinox.

Equinox isn’t just the technical term for a season change. The word derives from the Latin for equal night, which offers a clue about what actually happens during an equinox: daylight hours are roughly equal to darkness hours. Since the Earth rotates on an axis tilted at about 23.5 degrees, the North Pole is usually angled either slightly toward or slightly away from the sun. If it’s angled toward the sun, the Northern Hemisphere experiences longer days than nights; if it’s angled away from the sun, nights outlast days. In the Southern Hemisphere, the opposite occurs. But twice a year, neither hemisphere is tilted toward or away from the sun, so they both experience about 12 hours of night and 12 hours of daylight. At this point—the equinox—the sun appears directly overhead along the Equator.

What an equinox looks like from the outside.Przemyslaw "Blueshade" Idzkiewicz, Wikimedia Commons // CC BY-SA 2.0

Solstices, on the other hand, happen when the poles are at their farthest and nearest points from the sun. During the winter solstice in late December, the North Pole is angled farthest away, and the Northern Hemisphere experiences the longest night of the year. The sun will be right above the Tropic of Capricorn, the southernmost circle of latitude that the sun hits head-on. As Britannica explains, the process is flipped for the summer solstice in late June. The sun is directly above its northernmost circle of latitude—the Tropic of Cancer—because the North Pole is tilted toward the sun, so the Northern Hemisphere gets more daylight hours than any other time of the year. The Southern Hemisphere, meanwhile, is sleeping through its longest night.

Earth during the equinoxes and solstices of 2004.Tom Ruen, Full Sky Observatory, Wikimedia Commons // Public Domain

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