Article: Why Do We Have Leap Years?

A leap year (or intercalary or bissextile year) is a year containing one additional day (or, in the case of lunisolar calendars, a month) in order to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of days, a calendar that had the same number of days in each year would, over time, drift with respect to the event it was supposed to track. By occasionally inserting (or intercalating) an additional day or month into the year, the drift can be corrected. A year that is not a leap year is called a common year.

A leap year, also known as an intercalary year, is the one containing an extra day, normally February 29th, although this varies in certain calendars. In the Gregorian Calendar, which is used by most of the world, a leap year occurs every four years or 97 years out of every 400. This is done as a way to keep seasons, astronomical events, and time differences in sync. Without leap years, the Gregorian calendar would lose veracity in just over a hundred years, leading to time differences between day and night, and moving the equinox early.

 

The Gregorian method adds a 29th day in all years divisible by 4 except for years that end in -00, like 1900 or 2100, the extra day is not added. However, there is one more exception to the exception: for years ending in -00 that can be divided by 400, the extra day is added. The years 1600 and 2000, for example were leap years, as 2400 will be. This approach makes the Gregorian method the most accurate way to create leap years, allowing it to fall behind only one day every 8,000 years.

Other calendars don’t do such a good job of creating leap years. Examples of that are the Julian and the Coptic calendar, which create leap years by simply adding an extra day every four years, regardless of any other consideration. This method, which was in common use until a few centuries ago, would push the calendar a day forward every 130 years.