Ten Thousand Year Calendar




What day of the week was...

@ December 25, 1       - Jesus' Birthday (official)
@ October 15, 1582- Gregorian Calendar Established
@ February 22, 1732- George Washington's Birthday (new style)
@ June 28, 1838- Queen Victoria's Coronation
@ July 20, 1969- Moon Landing
@ December 21, 2012- Mayan Y2K Bug
@ December 31, 9999 - (sigh)


Leap Years

Under the Gregorian Calendar, years divisible by four are leap years... except that century years are not leap years, unless they are divisible by four hundred. (So 1900 wasn't a leap year, and 2100 won't be, but 2000 was.)

Under the "old style" Julian Calendar, all century years were leap years; and by the 1500s the extra leap years had pushed the calendar noticeably out of sync with the Earth's orbit: the vernal equinox was happening on March 11, instead of on March 21 where it had been at the time of the Nicene Council in 325. Pope Gregory XIII addressed the problem by introducing the century exception, and then skipping ahead ten days: what would have been Friday October 5 1582 (old style) became Friday October 15 1582 (new style).

(There seems to be a small logical inconsistency, though: there are only nine extra leap years between 325 and 1582, not the desired ten.)

Britain and its colonies didn't switch to the new style calendar until 1752 (apparently they thought the whole thing was a Catholic trick, totally missing the "bigger picture" involving the Earth and the Sun). By then the two calendars were eleven days out of sync, thanks to the extra leap year of 1700. George Washington was born on February 11 1732 (old style), which became February 22 after converting to new style.



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Prof. William Jefferys explains the method for making calendars like this, and says you can even learn to calculate historic days of the week in your head. (And perhaps you can, if you're an astrophysicist.)

See the Doomsday Algorithm for more.