When the month of February has an extra day (the 29th), the year is said to be a leap year. But how to explain this strangeness of the calendar? To understand it, you have to look at the movement of the Earth.
An extra day had slipped into the calendar in 2020: Saturday, February 29. This will also be the case in 2024. These years are called leap years: a year then lasts 366 days and not 365, the month of February having 29 days instead of 28. But where does this addition come from? What is it used for ?
” We try to match three unrelated elements: the rotation of the Earth (the day), the revolution of the Moon (the month) and the revolution of the Earth (the year) “Explains to Numerama Florent Deleflie, astronomer at the Paris observatory.
These three realities are unrelated, but our calendar tries to give them consistency with years, months and days. Since the year is not an exact number of days, it is rounded to 365 days. To make up for this discrepancy, a day is added to the calendar every 4 years (with a few rare exceptions): there were 365 days each year in 2017, 2018 and 2019.
What would happen without leap years?
Removing 5 hours, 48 minutes and a few seconds from a year may not seem like much. However, if all the years were only 365 days long, this would eventually cause a significant lag. An animation posted on Twitter by planetary scientist James O’Donoghue of Jaxa (the Japanese space agency) illustrates this very well. The video shows that if there was never a leap year, the seasons would shift throughout the year. In a few hundred years, the month of July (which for us is a summer month) would take place during the winter, as NASA explains.
Why February 29?
The idea of integrating a leap day to make up for this discrepancy goes back to Antiquity. Before the calendar reform imposed by Julius Caesar in 45 BC. J.-C., it is the Roman calendar which was used. This 355-day calendar was organized into 12 months (of 29 and 31 days). The year began in “Martius” and “Februarius” was the last month of the year. In order to catch up with the solar cycle, an intercalary month was then added at the end of the year, every 3 years: “Mercedonius”.
With the reform of Julius Caesar, the calendar becomes solar: the length of the months is changed so that the year lasts 365 days. An extra day is added every 4 years between Februarus 24 and 25. This day is called “bis sextus ante calendas Martis”, a term which will then give that of leap year.
This calendar is used in most of the Western world until 1582. It is then gradually replaced by the Gregorian calendar, which uses the principle of the leap day during leap years in February (even if it is no longer the last months of the year).
Are we also talking about leap years for the other planets?
The principle of the leap year can also apply to other planets than the Earth, as NASA confirms: this happens when the rotation of a planet and its revolution do not correspond.
The space agency explains that Mars has more leap years than non-leap years. A Martian year lasts approximately 668 sols (the name given to days on the planet). But it takes a little longer for Mars to make its revolution around the Sun: 668.6 sols. It is therefore necessary, over a period of 10 Martian years, that 4 years have 668 sols, and that 6 years are leap years with 669 sols.