A History of the Leap Year

It’s 2016 – that means we have an extra day on February 29th. Have you ever wondered why we have leap years?

A solar year – the length of time it takes the Earth to make one complete orbit around the sun or 365.2425 days long (365 days, 5 hours, 48 minutes and 46 seconds) differs ever so slightly from the calendar system we use.

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Julius Caesar, the Roman dictator, is the “father” of the leap year. To keep the Roman calendar (based on 355 days in a year) in line with the seasons, Roman officials inserted an extra month every so often. But by the time Caesar began to rule Rome, the calendar was seriously out of whack. After consulting with top astronomers, he decided to add one day every four years to make up the discrepancy between the solar and lunar calendars.

The leap year tradition took effect in 45 B.C. after a transition year where three extra months were added to the calendar to make up for the difference that had accumulated over the centuries.

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This didn’t exactly fix the problem though: a solar year is 11 minutes short of 365.25 days. Astronomers in the second century A.D. figured this out but the calendar system didn’t change until the 16th century when it was nearly 10 days off-track. Pope Gregory XIII, concerned that Easter was falling further away from the spring equinox every year, reformed the Julian Calendar in 1582 by restoring the vernal equinox to March 21 from March 11. The Gregorian calendar was born!

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The Gregorian calendar isn’t perfect though. We add a leap day every 4 years, just not exactly every four years. Leap years occur in years divisible by four, EXCEPT for years divisible by 100 and not divisible by 400. Essentially, 97 out of every 400 years are leap years. This all still differs from the solar year by 26 seconds a year. Eventually, the Gregorian calendar will be a full day ahead of the solar year by the year 4909.

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Sources:

History.com