It’s leap day: A look at how it all started

It’s leap day: A look at how it all started


Leap days keep our modern-day Gregorian calendar in alignment with Earth’s revolutions around the sun. It takes Earth 365.242189 days, or 365 days, five hours, 48 minutes and 45 seconds, to circle the sun. This is called a tropical year, and it starts on the spring equinox in March.

The modern Gregorian calendar added leap day because its precursor, the Julian calendar, did not precisely calculate the Earth’s rotation around the sun.

This Julian system was instituted in the Roman Empire by Julius Caesar around 46 B.C. But this calendar was 0.0078 days (11 minutes and 14 seconds) longer than the tropical year. Between 46 B.C. and 1582, this accumulated error amounted to 12.7 days.

In 1582, Pope Gregory XIII reformed the calendar by specifying the leap rules:

The year must be evenly divisible by four.

If the year can also be evenly divided by 100, it is not a leap year unless …

The year is also evenly divisible by 400. Then it is a leap year.

According to these rules, the years 2000 and 2400 are leap years, while 1800, 1900, 2100, 2200, 2300 and 2500 are not.

Ten days of birthdays lost!

The Gregorian calendar came into use in Roman Catholic countries in October 1582, when Thursday, Oct. 4, was followed by Friday, Oct. 15.

America gets on time

Britain and its colonies did not introduce the Gregorian calendar until September 1752, by which time an additional one-day correction was required.

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Born on leap day

A baby has about a 1 in 1,500 chance of being born on Feb. 29. Each state has different laws about how Feb. 29 birthdays are handled when it comes to getting a driver’s license. In Michigan, leap day babies have to wait until March 1 to go to the DMV, but in some places (including California) the date is Feb. 28. This law doesn’t affect too many people in the United States – it’s estimated that fewer than 200,000 leap day babies are Americans.