What is a leap year and why is a day added on to February?
The systematic addition of a Leap Day traces its origins back to ancient Rome and was necessitated by the need to correctly align the calendar.
Just about every four years, a rare calendar occurrence takes place: an extra day is added to February, making it 29 days. This anomaly is due to the fact that it takes Earth 365 days, 5 hours, 48 minutes, and 46 seconds to orbit around the Sun. To compensate for this slight time discrepancy, an additional day is added to the calendar every four years, meaning that what we call a leap year will have 366 days.
This year, 2024, just happens to be a leap year and will have 366 days. The word “leap” comes from the Latin phrase “bis sextus dies ante calendas martii,” meaning repeated on the sixth day before the first day of March.
What does this gap in the calendar correspond to?
February is the shortest month on the calendar, with fewer days than all other months. It can last either 28 or 29 days, depending on the year. This irregularity can be traced back to Ancient Rome and the evolution of the year-recording system. The early Romans followed the Romulus calendar, which had ten months. They left January and February, the two most challenging months to work, and used the remaining time to establish when they would work.
The year used to last for 304 days, but this system did not align with the lunar cycle. To fix this, King Numa Pompilius decided to add two new months at the beginning and end of the calendar, namely, Ianuarius (later renamed January) and Februarius (later renamed February) - the month of purification. The addition of these months pushed the total days in the year to 354.
February lasts 28 days thanks to Julius Caesar
The ancient Roman calendar now had twelve months, but the new months, January and February, had fewer days than the others. The Romans believed that even numbers were unlucky, so starting the year with a month that had an even number of days was not favorable. January and February had 29 and 28 days, respectively.
However, this solution still had a major flaw, the Earth takes just over 365 days to complete an orbit around the Sun. So, as time passed, the seasons became unstable because the length of the year was not accurate. The Romans, like other ancient civilizations, made periodic adjustments to the calendar by adding extra months, known as intercalary or interstitial months. However, the Roman consuls would decide when to add the extra days and it became a political football used for their own benefit and the calendar became completely unpegged from the solar year.
To address this issue, first century BC ruler Julius Caesar proposed that months could have either 30 or 31 days and that the even or odd ending was irrelevant. January was chosen as the first month of the year, and February was reduced to 28 days, making the calendar’s total length 365.25 days.
His Julian calendar was implemented 1 January 45 BC and every four years a Leap Day was added to February. Further tweaks were needed a little over 1,600 years later as the year was just a smidge too long and the calendar was getting out of sync again. So, some of the potential Leap Years were removed.