Tropical year
The time for the seasons to repeat, about 365.2422 days; calendars need leap years to keep up with it.
A tropical year is the time the Sun takes to return to the same position in the cycle of the seasons, for example from one spring equinox to the next. It lasts about 365.24219 days, or 365 days, 5 hours, 48 minutes and 45 seconds. A calendar made of whole days has to keep catching up with that awkward fraction.
The Julian calendar, introduced under Julius Caesar, added a leap day every four years, making the average year 365.25 days. That is about 11 minutes too long. It sounds trivial, but it adds up to roughly one day every 128 years. By 1582 the spring equinox had drifted about 10 days earlier in the calendar than in AD 325, upsetting the calculation of the date of Easter.
The Gregorian calendar of 1582 fixed this in two steps. First it dropped 10 days to put the equinox back in place. Second it changed the leap-year rule: century years are leap years only if they divide by 400, so 1600 and 2000 were leap years but 1700, 1800 and 1900 were not. That gives 97 leap years in every 400, an average of 365.2425 days, which drifts by only about a day in 3,000 years.
Countries adopted the reform at different times. Britain and its colonies waited until 1752, by which point they had to skip 11 days.
See it in action
Coming up in future stories
- The Poet's Calendar That Beat the Gregorian 1079 · Isfahan, Seljuk Iran
- The Ten Days That Never Happened 1582 · Rome and Catholic Europe