Terminology – Leap year 2024 (année bissextile 2024)

Copyright ©Françoise Herrmann

Curious about “leaping”? Here is the scoop.

It takes a bit more than 365 days for the Earth to revolve around the sun. To be precise, it takes 355 days, 5 hours, 48 minutes and 56 seconds or 365.242190 days for the Earth to complete its revolution around the sun. The extra time it takes for the Earth to revolve around the sun amounts to about one-quarter of a day (i.e., about 6 hours) each year, or about one day (i.e., about 24 hours), about every four years. Thus, to re-align (or sync) the calendar year of 365 days, with the real time it takes for the earth to revolve around the sun, one day is added every 4 years, at the end of February, the shortest month of the year.

However, this neat leap year adjustment to the 365-day calendar year, every four years, would still create a calendar year that would eventually become out of sync with the time it takes to revolve around the sun, considering that everything after the decimal point in 365.242190 days does not amount to exactly 6 hours, multiplied by 4. Thus, intercalary (leap) days are added every four years, except those years that are multiples of 100 (i.e., centuries), but not multiples of 400. Thus, for example, the year 2000 was a leap year, but the year 2100 will not be a leap year, for the same reason that the years 1900, 1800 and 1700 were not leap years.

As a result, the term “leap year” really means “sync year”. The French term bissextile (which also exists in English as an archaic term) is derived from the Latin terms (bis)sextus, meaning “twice sixth”. Thus, February 29th is twice sixth day of the month of February because the extra day that is added at the end of February is the second sixth day before March 1rst (calend martius), using the ancient Roman way of designating days, relative to the ones that effectively had a name. In the Roman case, just the calends (first days of the month) or new moon days, and ides (the middle of the month) or full moon days, had names. For example, the nones were the ninth days before the ides of every month, using the same dating system. 

Revolution and rotation of  planet Earth
Credit: NASA/JPL-Caltech

References 

Merriam Webster – Bissextile.

https://www.merriam-webster.com/wordplay/bissextile-#:~:text=The%20extra%20day%20fell%20on,to%20refer%20to%20leap%20day. 

NASA Spaceplace–What is a leap year?

https://spaceplace.nasa.gov/leap-year/en/ 

Gif file – revolution and rotation of the Earth (NASA/JPL-Caltech)

https://spaceplace.nasa.gov/review/leap-year/1day-1year.en.gif