“Thirty days have September, April, June and November; all the rest have thirty-one, except for February alone, that has 28,” goes the old rhyme. Except for this year, that is. This is a leap year, which means this month has 29 days. But have you ever wondered why some years we have an extra day?
The reason has everything to do with historical attempts to line the calendar year up with the astronomical year. Unlike the calendar, which organizes each year into a neat 365 days, the seasons do not repeat every 365 days, but rather every 365.2422.
Leap years, or intercalary years, as they are also called, date back to the reign of Roman emperor Julius Caesar, in 46 BC. At that time, Caesar in consultation with the astronomer Sosigenes of Alexandria, decreed that a calendar year would be 365 days in length, and contain 12 months. Prior to that, the Romans followed an evolving series of calendars that were roughly based on the Greek lunar calendar, with a total of 354 days, and a “leap month” thrown in every few years to even things out.
Days were added to various months to bring the total number up to 365. Because the seasons didn’t exactly fit the 365-day year, the calendar ended about one-quarter day early, resulting in the calendar becoming a full day off every fourth year. To make up for the error, the Julian calendar, as Caesar’s calendar came to be called, added an extra day to the month of February every fourth year. Any year evenly divisible by four would be a leap year, which made the average length of the calendar 365.25 days.
However, the Julian calendar was still slightly off the mark. Caesar’s correction made the year 11 minutes and 14 seconds too long, which meant that, after 128 years, the calendar would end a full day later than the astronomical year. In 1582, Pope Gregory XII stepped in and ordered yet another correction to the calendar, resulting in the Gregorian calendar, which we use today. According to this reform, century years are not leap years, unless they are evenly divisible by 400. Thus, 1900 was not a leap year, but 2000 was. This made the average length of the calendar 365.244 days and reduced the calendar error to only one day in 3322 years. During the 19th Century, astronomer John Herschel suggested dropping a leap year every 4000 years, to obtain even greater accuracy, however his suggestion never received official support, in part because contemporary astronomers believe the point of the vernal equinox will change by the year 8000, making Herschel’s correction irrelevant.
So why do we call it a “leap” year, anyway?
Common (non-leap) years are composed of exactly 52 weeks, plus one day. This extra day means that if your birthday falls on a Tuesday in one common year, it will fall on a Wednesday the next common year, and so on. However, a leap year changes this scenario. A leap year is comprised of 52 weeks plus two days. So, if your birthday fell on a Wednesday last year, in a leap year it “leaps” over Thursday and lands on Friday. Thus, the name “leap year.”