Today is a leap day, which comes round every four years, perhaps especially welcome for anyone whose birthday is on February 29. But why bother with leap days? The simple truth is if they didn’t exist, the calendar would gradually drift out of sync with the seasons and there would be chaos.
This all stems from the usual assumption that a year is 365 days long. A year is the time it takes for the Earth to complete its orbit around the sun, but in fact a year is slightly more than 365 days and that means that a calendar based on exactly 365 drifts out of sync with the seasons.
Ancient civilisations that depended on knowing the timings of the seasons needed a more accurate calendar. Julius Caesar tried improving the calendar based on 365.25 days in a year, created with the help of the Greek astronomer Sosigenes, who was introduced to Caesar by Cleopatra.
In the new Julian calendar, leap days were added every four years to make up the difference with the seasons — but this calendar year was still about 11 minutes shorter than the solar year and so gradually the calendar drifted out of step with the seasons.
In fact, a true solar year is not precisely 365.25 days, it’s 365.2421897 days, and although the difference between 0.25 and 0.242 may seem trifling, over 128 years the solar calendar diverges by an entire day from the Julian calendar.
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That created havoc with the dates of religious festivals, so in 1582 Pope Gregory XIII introduced the Gregorian calendar, still used today. It is somewhat more complicated than the old Julian calendar. The Gregorian calendar made years that are divisible by four leap years, except for those that are divisible by 100, which are not leap years, but with the exception of years divisible by 400, which remain leap years. This Gregorian calendar gives an average year of 365.2425 days, just half a minute longer than the solar year, and so it takes 3,300 years before the Gregorian calendar drifts out by a day from the seasonal cycle.
