Here are some leap year facts from The Farmer’s Almanac:
Why Do We Have Leap Years?
• The extra day during some years has to do with our need to keep our modern-day Gregorian Calendar in alignment with the Earth’s revolutions around the sun.
• Unlike the calendar, which organizes each year into a neat 365 days, it actually takes the Earth 365.242199 days — or 365 days, 5 hours, 48 minutes, and 46 seconds — to circle once around the Sun.
When did they come to be?
• 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, after a conversation 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.
• But 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 3,322 years.
Why Do We Call It A “Leap” Year?
• 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.”
Source: The Farmer’s Almanac.