Air Date: Week of March 14, 1997

Facts about... calendars, and the earth's revolutions around the sun.

Transcript

CURWOOD: While most scientists will tell you that time travel is impossible, one man proved them wrong. Four hundred and fifteen years ago, Roman Catholics went to sleep on October 4th and woke up the next morning on October 15th. It was all the idea of Pope Gregory XIII, and he did it because the Julian calendar, named after its inventor Julius Caesar, had gotten out of whack. You see, the Earth revolves around the Sun every 365.2422 days. But that's not a whole number. So, to keep the calendar on track, we must adopt a variable year length. Caesar's calendar had a leap year every 4 years, making for 365.25 days, only slightly more than the actual length of the year. But over the centuries the difference mounts up. By the 16th century the spring holiday of Easter was slipping into summer. Pope Gregory reset the clock and made one significant adjustment. There would only be a leap year at the end of a century if the year were divisible by 400. And so, the average year of the Gregorian calendar is exactly 365.2425 days, which gains just 3 days every 10,000 years. And for this week, that's the Living on Earth Almanac.

Links

Timeline of interesting calendar facts