Understanding Equinoxes and Solstices: A Balance of Light and Darkness

Spring is officially on its way with the arrival of the vernal equinox this Thursday, signaling the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. At this time, the sun will shine directly over the equator at midday. Equinoxes are unique as they are the only occasions when both the North and South Poles are simultaneously illuminated by sunlight.

For centuries, these events have been celebrated worldwide. For instance, Nowruz, the Persian New Year, aligns with the spring equinox. At Chichen Itza, a Mayan archaeological site in Mexico, observers gather to witness a sunlit shadow pattern resembling a serpent slithering down the pyramid known as El Castillo during the equinox.

But what exactly occurs in the sky during these times? Here is a look at how we divide the year based on Earth’s orbit.

So, what is an equinox? As Earth revolves around the sun, it does so at a tilt. Throughout most of the year, Earth’s axis is inclined either toward or away from the sun, causing uneven distribution of sunlight between the northern and southern halves of the planet. However, during the equinox, Earth’s axis is positioned in such a way that both hemispheres receive sunlight evenly.

The term equinox is derived from Latin, meaning “equal night,” because the day and night are nearly equal in length worldwide—though slightly more or less depending on your location.

In the Northern Hemisphere, the spring—or vernal—equinox typically falls between March 19 and 21, while the autumn—or fall—equinox occurs from September 21 to 24.

And what about the solstice? Solstices occur when Earth reaches the peak of its tilt either toward or away from the sun, resulting in highly unequal day and night lengths.

During the summer solstice in the Northern Hemisphere, the region tilts fully toward the sun, bringing the longest day and shortest night of the year, occurring between June 20 and 22. Conversely, during the winter solstice, the Northern Hemisphere leans away from the sun, resulting in the shortest day and longest night, typically happening between December 20 and 23.