Here in the Northern Hemisphere, today is the first full day of Spring, with the earth’s tilt sideways to the sun, and day and night equal everywhere.
Humanity has, for thousands of years, used calendars to keep track of the Past, Present, and Future. The basics of the world’s most popular calendar were laid down by Julius Caesar in 46 BC (as that year has since come to be called).
But the Julian calendar had a bug. It assumed the earth rotated around the sun in exactly 365 and one-quarter days, accounting for the quarter day by adding a day to February every fourth (leap) year. However the earth’s rotation around the sun is actually 365.2422 days, or 11 minutes and 14 seconds less than 365 and a quarter days.
Most bugs take only hours or days to notice. The Julian calendar bug took centuries. Every year since 46 BC, the calendar has been a tiny bit slow. By the year 1582, it was almost 11 days slow, with the vernal equinox (the start of Spring) occurring on March 11. The slow calendar was affecting not just planting season, but Easter.
So in 1582 Pope Gregory XIII decided to fix the calendar. Two fixes were needed: the calendar had to catch up to where it was supposed to be, and the definition of a year had to be fixed so that the bug wouldn’t happen again.
The first fix resulted in 10 calendar days never existing: October 4 was followed by October 15 in 1582. But only in Roman Catholic countries. The British empire (including its American colonies) didn’t adopt the fix until 1752, by which time 11 days had to be skipped (September 3-13, 1752). Other countries skipped other days.
The second fix, to prevent the calendar bug from recurring, was even more complicated:
- Continue to add a day to February every four years
- Except do not add a day to February every 100 years
- Except do add a day to February every 400 years
Every four years would have the normal leap year exception, every 100 the exception to the exception, and every 400 the exception to the exception to the exception! The years 1600 and 2000 were leap years due to this third exception*.
The new Gregorian calendar is much better, but still not perfect. It is off by 27 seconds per year, or one day every 3236 years. Give or take a second or two.
*The year 1600 was actually a leap year in Britain and elsewhere due to the lack of both the second and third exceptions. The year 1700 was a leap year in Britain but not many other places. Talk about confusing!