Why We Have Leap Years
Vernal equinoxes are used to calculate leap years. Pope Gregory first recognised seasonal drift and devised leap year rules and the Gregorian Calender.
The term “leap year” refers to years where an extra day is inserted into February, normally the shortest month of the year at 28 days. During a leap year, however February has 29 days.
The rules for leap years are as follows: a year is a leap year only if it is divisible by 4. However, if it’s also divisible by 100 then it cannot be a leap year unless it’s divisible by 400. Going by this rule, the year 2000 is not only the beginning of a new millennium, but is also a leap year. This “century” rule hasn’t happened since 1600!
Why the need for leap years? So that our calendars are aligned with the earth’s rotation round the sun. The time between two vernal equinoxes is about 365.2422 days. If calendars used 365 days each successive year, after 100 years they would be approximately 25 days ahead of the seasons.
Julius Caesar introduced the concept of leap years into the Roman Calendar using 365.25 days. However, this calculation caused a drift of 1 day every 128 years. By the 16th century there was a very noticeable drift in the seasons. Pope Gregory instituted the Gregorian Calendar after realizing that if calculations weren’t changed, Easter would fall not in the spring but around Christmas. Consequently 10 days were stricken from October in the year 1582 which brought the seasons back into proper phase. The year lengths were set at 365.2425 days, a calculation that would result in a drift of only 1 day every 3236 years.
Oddly enough, Britain did not adopt the Gregorian Calendar until 1752, when 12 days had to be cancelled from September to regain correct seasonal timing.
Unfortunately our calendars cannot be perfect. Drift by seconds, minutes, hours and days is inevitable. Eventually new leap year rules will have to be introduced. However, we don’t need to worry about tinkering with Pope Gregory’s leap year rules any time soon.