Science Puzzle
Why Was 2000 a Leap Year but 1900 Wasn’t?
Most of us learn that a leap year comes every four years, adding 29 February to the calendar. 1896 and 1904 were leap years. Yet 1900, which can be divided by four, was not. And 2000, another century year, was.
Why the exceptions?
The Answer
The year is a little shorter than a leap day every four years assumes, so three leap days are dropped every 400 years.
The seasons repeat every 365 days, 5 hours, 48 minutes and 45 seconds or so. A leap day every four years would be perfect for a year of exactly 365 days and 6 hours. The real year is about 11 minutes shorter than that, so a calendar with a leap day every four years drifts out of step with the seasons by one whole day roughly every 128 years.
That was the old Julian calendar, which Julius Caesar brought in from 45 BC. By the 1500s it had drifted about ten days out of step with the seasons, which mattered because it set the date of Easter. In 1582 Pope Gregory XIII brought in a fix. Ten days were cut from the calendar at once: Thursday 4 October 1582 was followed by Friday 15 October. From then on, century years would only be leap years if they divide by 400. So 1700, 1800 and 1900 were not leap years; 2000 was; 2100 won't be.
Britain held out until 1752, by which time it had to skip 11 days: Wednesday 2 September was followed by Thursday 14 September. The Gregorian calendar we use now is so close to the real year that it will drift by only about a day in 3,000 years.
The principle: Calendars follow Earth's orbit. Earth doesn't go round the Sun in a whole number of days, so calendars add and skip leap days to stay in step with the seasons.