General Knowledge
Why do we have leap years?
- A year is about a quarter of a day longer than 365
- The Moon adds a day
- To keep months even
The answer is A year is about a quarter of a day longer than 365. Earth takes 365.2422 days, so the quarters are collected up.
Because the Earth takes about 365.2422 days to go round the Sun, not 365. Ignore the quarter-day and the calendar slides against the seasons by about a day every four years, which is a month every century and eventually has midsummer in the snow.
Adding a day every four years overcorrects slightly, because the extra is a little under a quarter. So the Gregorian rule skips the leap year in century years unless they divide by 400 - 1900 was not a leap year, 2000 was - which brings the error down to about one day in 3,000 years.
The correction was needed because the older Julian calendar had drifted by ten days, putting Easter badly out of place. When the change was made in 1582, ten days were simply removed from October. Protestant countries refused a papal calendar for a long time: Britain and its colonies held out until 1752 and had to delete eleven days, and Russia until 1918, which is why the October Revolution happened in November.