What Is the Spring Equinox?

Wednesday, March 20, is the spring equinox in 2019. 

And that means what, exactly?

The spring equinox is one of two days during the year when day and night are about the same length. This happens when the earth rotates in such a way that the sun is lined up exactly with the equator. In that 24-hour period, the earth’s axis is tilted neither toward the sun, nor away from it. Everyone on the planet gets the same amount of sunshine, and the same amount of dark.

Equinox = Equal + Night

The word “equinox,” as you might guess, comes from two Latin words: “aequi-,” meaning “equal,” and “noct-,” meaning “night.” 

The root word “aequi-“ can be found in lots of similar words describing things of equal balance, like “equivalent,” “equidistant,” and “equitable.” “Noct-” can be found in several words evoking night: “nocturnal”; “noctambulate,” meaning to walk around at night; and the spooky “noctifer,” an obsolete word that refers to a bringer of night or darkness.  

I’m thinking of the Dementors from “Harry Potter” when I hear that word, or the Nazgūl from “The Lord of the Rings.” Scary.

Chaucer Used the Word 'Equinox'

We can find the word “equinox” used way back in the 1300s, when Chaucer wrote one of the first-ever technical manuals—a user’s guide to the astrolabe, an instrument that’s used to make astronomical measurements. 

Chaucer wrote of a “cercle” around the earth called the Equator. He noted that 

When the sun is at the start of Aries and Libra, the days and nights are of equal length. These signs (Aries and Libra) are called the equinoxes.

Why does Chaucer refer to these zodiac signs? Because as the Earth revolves around the sun, the sun seems to travel through all the thirteen constellations that make up the zodiac. (Really, we’re the ones moving; the sun isn’t “traveling” anywhere.) And on the day of the spring equinox, the sun looks like it’s starting to move through the constellation Aries. For that reason, the spring equinox is also called “the first point of Aries.” The fall equinox, in turn, is known as the “first point of Libra.” 

Don’t Capitalize Terms Like 'Spring Equinox' and 'Fall Equinox'

And the names of all these significant astronomical days are all lowercase. You don’t capitalize terms like “fall equinox” and “summer solstice.” The solstices, by the way, are the...

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