Science Series: Autumn

History of Fall: What is the Autumnal Equinox?

This Sunday

Bill Petro
ILLUMINATION
Published in
2 min readSep 20, 2024

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Equinox. Author’s photo from his iPhone

This time of year represented New Year’s Day, according to the French Republican Calendar. However, since that calendar was only in use from 1793 to 1805, following the fall of the French monarchy in 1792, very few still celebrate this day.

Date of Autumn

Instead, September 22 or 23 marks the beginning of Fall or Autumn, associated with the Equinox. This word is made up of two Latin root words, aequus and nox meaning “equal night,” referring to the fact that daylight and nighttime are equal in duration.

What happens during Equinox?

Astronomical

When the plane of the earth’s equator passes through the center of the Sun, metaphorically speaking, you have evennight, twice a year. This year, the astronomical autumnal equinox (Fall) occurs on September 22 at 12:44 UTC. This means Temps Universel Coordonne (or Coordinated Universal Time) if you speak French, roughly equivalent to Greenwich Mean Time if you’re British, Zulu Time if you’re a pilot. The Vernal Equinox occurs six months later.

Since each equinox occurs at the same time, whether in the Northern Hemisphere or the Southern Hemisphere, though the seasons are…

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Bill Petro
ILLUMINATION

Writer, historian, technologist. Former Silicon Valley tech exec. Author of fascinating articles on history, tech, pop culture, & travel. https://billpetro.com