Alexandre Cherman


Back to our regular programming (our series of entries about the history of our calendar) and to the time period I’ve been dreading to address…

There was Numa Pompilius and then there was Julius Caesar. But from “point A” to “point B” we must go through over 600 hundred years of Roman history. And not all of these years treated the calendar well. (Probably none!)

I’m not a trained historian, so I’ll excuse myself and talk about the calendar and the calendar alone. The key word to our journey is a mouthful: incommensurability. (If you like to play hangman with the kids, consider this your “weapon of mass destruction”…)

Incommensurability is the property of two quantities that have no common measure between them. For our purposes, we can focus on the duration of Earth’s rotation and the duration of Earth’s revolution.

Our planet spins around itself, completing a turn every 23 hours, 56 minutes and 4 seconds. It also moves around the Sun, with a period of 365 days, 5 hours, 48 minutes and 47.5 seconds. With such odd numbers, it is impossible to fit a regular amount of rotations into a standard revolution. There is no simple way to create a year (revolution) based on days (rotations). These quantities are incommensurable.

But simple is a word that can easily be associated with Numa’s calendar. Its year had 365 days, and just ignored the few hours after that. Evidently, it was a calendar doomed to fail, since it would rapidly stop representing the reality of the seasons. And failed it did!

After Rome became a republic, senators addressed this matter by creating, every now and then (and definitely not on a regular basis), extra months, trying top ut the calendar back on the seasonal track. But senators would be senators, and this intercalation rapidly degenerated into a political weapon to create a longer year for your allies to rule (or shorter years to end the opposition’s term of office).

All in all, after over 600 years of unscientific calendar temper, the seasons where completely out of sync with the civilian calendar. This had to be addressed. And it was! We’ll get into it next week… ■