Alexandre Cherman

The Leap Year

The train was back on its tracks, thanks to the Year of Confusion. But how to make sure it would not derail again? Julius Caesar got his recipe from Egypt.

Due to its geographical location, the Egyptian Civilization practiced extreme care in keeping track of time. They divided the year in 13 parts, 12 parts with 30 days each, and a thirteenth part with 5 days. These five days were called “celestial”.

This division resulted in a 365-day long year, and was based on accurate observations of Sirius, the bright star from Canis Major. I already wrote about that here… And I ended that post saying that the astronomical observations showed them that every four years they needed to add an extra day to the year, and they did that creating an extra celestial day. But I did not say exactly why they needed to do that…

The year reflects the duration of time Earth takes to complete its path around the Sun. This time period, unfortunately, can not be expressed, in days, using an integer number. The “astronomical year” is roughly 365 days, 5 hours and 48 minutes long. Respecting the Egyptian knowledge of that time, we can say 365 days and 6 hours.

Thus the Egyptians started intercalating a day in their calendar every four years: they simply ignored the extra six hours for three years in a row, fabricating a deficit of 18 hours total between the civil and the astronomical calendars. On the fourth year, they will add the six hours of that year to the 18 hours they owed and they would pay their debt in full, creating an extra day with 24 hours.

And that was what Julius Caesar did, following the lead of Sosigenes of Alexandria, an Egyptian astronomer: he brought to Rome the intercalation of one extra day every four years!

Julius Caesar brought the Leap Year to our calendar! ■