The autumnal equinox, which this year occurs before sunup on Friday, is one of two times during the year when the sun crosses the equator and day and night are of roughly equal length, everywhere in the world. For the French revolutionaries of 1793, it celebrated equalities of another sort. Out with the class system! Down with the authority of the Catholic Church! And goodbye to the Gregorian calendar — the one with which we still mark our days. A new calendar was instituted, retroactive to the founding of the First Republic on Sept. 22 of the previous year.

The French Republican Calendar, as it was called, was as radical as the overthrow of a king. In the spirit of the Enlightenment, it aspired to a more rational, nature-based way of organizing the year. The autumnal equinox was designated as the year’s first day, and the 12 months that followed were renamed to correspond, more or less, to each month’s weather or agricultural pursuits. Fall consisted of Vendemiaire, Brumaire and Frimaire (grape harvest, fog and frost), winter of Nivose, Pluviose and Ventose (snow, rain and wind), spring of Germinal, Floreal and Prairial (seed, flower and meadow), and summer of Messidor, Thermidor and Fructidor (harvest, heat and fruit).

In addition, each day of the year was assigned the name of a plant, a farm tool or a domesticated animal, replacing the saints’ days hitherto observed. Henceforth, for example, Saint Eustachius the Martyr would be replaced by the pistachio, Saint Elfleda of Whitby by the billhook.

The new system ran up against cosmological technicalities (equinoxes do not fall on the same date each year) and it disrupted many old customs, most notably the seven-day week. The months, each 30 days long, were now divided into periods of 10 days each. This was part of the far more successful and lasting institution of the metric system, perceived as a more natural way of counting, weighing and measuring. We do have 10 fingers, after all. But, as any union man could have predicted, France’s workers were not happy about waiting nine days for their day of rest. For various reasons, the calendar was abolished by Napoleon.

We can chuckle at such incompetent idealism, as did the British wag who dubbed the new French months “Wheezy, Sneezy, Freezy, Slippy, Drippy, Nippy, Showery, Flowery, Bowery, Wheaty, Heaty and Sweety.” But there is something to be said for linking our intervals of time to the physical realities of the year, and to our outdoor labors. Today we pay too little attention to the sun’s milestones and the phases of the moon. Much of the time we live indoors in a weatherless world and don’t notice that each day is its own tiny season.

This Friday, may the sun shine on your fall harvest, even as darkness inches ahead of the light.

Damrosch is a freelance writer and the author of “The Garden Primer.”