Origin and Explanation of the Leap Year
Why does February 29th exist?
A leap year has 366 days instead of the 365 days of a common year. This difference is due to the solar year (the time it takes for the Earth to complete one orbit around the Sun) being approximately 365 days, 5 hours, 48 minutes, and 45.25 seconds. This means that each year, there is a small amount of extra time (approximately 6 hours). After four years, these accumulated hours (4 x 6 = 24 hours) form an additional day.
The purpose of this adjustment is to keep the calendar synchronized with the astronomical and seasonal year. The seasons, as well as other astronomical events, do not occur on a fixed number of days. Without this adjustment, the calendar would gradually drift out of sync with the natural phenomena it is meant to mark.
To correct this desynchronization, an extra day is added to the month of February, resulting in February 29th. This way, the calendar aligns with the seasons and the tropical year.
Origin of the Term "Leap Year"
The term leap year comes from the Latin bisextus, which originally referred to February 24th, known as "twice sixth." This was because, in the Roman calendar, the 6th day of each month was a reference point, and during a leap year, this day was counted twice before the calends of March.
In Italian, bisesto refers to the extra day added to the calendar, while bisestil (from the Latin bisextilis) refers to the year in which this extra day is added.
The Importance of the Adjustment
The extra day in a leap year is crucial for correcting the discrepancy with the exact duration of the tropical year. Without this adjustment, the calendar would gradually drift out of sync, and over time, the seasons would shift away from the dates they are traditionally assigned.
To prevent this accumulated error, the Gregorian calendar establishes that a year must be a leap year if it is divisible by 4, but not all years divisible by 100 are leap years unless they are also divisible by 400. This system ensures a precise correction of the discrepancy that occurs over time.
