Feb 29, 2008
Why do we have a leap year?
[Chris Turney, a professor of geography at the University of Exeter, is the author of “Bones, Rocks and Stars: The Science of When Things Happened.”]
... Many of the earliest calendars were based on the phases of the moon. Each 29.5-day cycle amounted to one month, and the first versions counted only 10 months in a year. That turned out to be too few months, but even when two more were added, the problem remained: the calendar could not keep up with the seasons.
A group of Roman priests was charged with the task of adding days through the year, but they were easily corrupted. They’d frequently add or delay the extra days either for personal financial gain or to see their preferred candidates hold offices of power for as long as possible.
By Julius Caesar’s time, the calendar was running 90 days behind. Acting on the advice of an astronomer, he created a calendar based on the time it takes the Earth to circle the Sun. During the well-named “year of confusion,” in 46 B.C., Caesar lengthened several of the months and added a couple of temporary ones as a correction. The jubilant Roman public believed Caesar had extended their lives by the extra 90 days (you just can’t buy publicity like that). And by 45 B.C., the calendar was back in phase with the seasons.
The Earth’s trip around the Sun does not take exactly 365 days, however. It lasts an extra 5 hours and about 49 minutes. By adding an extra day every four years, Caesar could roughly make up for the discrepancy. Even then his scheme ended up being 11 minutes a year too long. This may not sound like much; you wouldn’t notice the difference during your lifetime. But by the mid-16th century, the calendar had moved ahead 10 days.
This shift had serious implications for the question of when to celebrate Easter. In 1582, a task force called by Pope Gregory XIII proposed that 10 days should be removed from October that year. And to make sure the calendar would then be self-correcting, leap years were subtracted from the last year of most centuries. Only those divisible by 400 would get the extra day. (That means 1600 was a leap year, but not 1700, 1800 and 1900.) This way, the calendar would gain only half a minute a year, and it would take 2,880 years before another day would need to be added. The trusty Gregorian calendar had arrived....
... Many of the earliest calendars were based on the phases of the moon. Each 29.5-day cycle amounted to one month, and the first versions counted only 10 months in a year. That turned out to be too few months, but even when two more were added, the problem remained: the calendar could not keep up with the seasons.
A group of Roman priests was charged with the task of adding days through the year, but they were easily corrupted. They’d frequently add or delay the extra days either for personal financial gain or to see their preferred candidates hold offices of power for as long as possible.
By Julius Caesar’s time, the calendar was running 90 days behind. Acting on the advice of an astronomer, he created a calendar based on the time it takes the Earth to circle the Sun. During the well-named “year of confusion,” in 46 B.C., Caesar lengthened several of the months and added a couple of temporary ones as a correction. The jubilant Roman public believed Caesar had extended their lives by the extra 90 days (you just can’t buy publicity like that). And by 45 B.C., the calendar was back in phase with the seasons.
The Earth’s trip around the Sun does not take exactly 365 days, however. It lasts an extra 5 hours and about 49 minutes. By adding an extra day every four years, Caesar could roughly make up for the discrepancy. Even then his scheme ended up being 11 minutes a year too long. This may not sound like much; you wouldn’t notice the difference during your lifetime. But by the mid-16th century, the calendar had moved ahead 10 days.
This shift had serious implications for the question of when to celebrate Easter. In 1582, a task force called by Pope Gregory XIII proposed that 10 days should be removed from October that year. And to make sure the calendar would then be self-correcting, leap years were subtracted from the last year of most centuries. Only those divisible by 400 would get the extra day. (That means 1600 was a leap year, but not 1700, 1800 and 1900.) This way, the calendar would gain only half a minute a year, and it would take 2,880 years before another day would need to be added. The trusty Gregorian calendar had arrived....