Why do we have leap years?

Most years, February has 28 days. But every four years it gets an extra one, the 29th. A year like that is called a leap year.

A day is one turn

The Earth spins like a top. From noon to noon it turns round once relative to the Sun, and that is one day. The calendar only counts whole days.

A year is one lap

At the same time, the Earth travels round the Sun. One lap takes a year. In that time, spring, summer, autumn and winter each come round once.

The two do not fit

From one spring to the next, the days do not come out even. It takes 365 days, 5 hours and 49 minutes. So after 365 days, the Earth still has almost a quarter of a day to go.

A calendar that slips

If every year had only 365 days, spring would start almost six hours later every year. After 4 years that is a day, after 120 years a month. In about 600 years, Christmas would fall in the middle of summer.

Caesar adds a day

In Julius Caesar’s time, the Roman calendar was a mess. Priests slipped in whole extra months whenever it suited them, and it was about three months out of step with the seasons. So Caesar brought in a new calendar, the Julian calendar: a year has 365 days, with one extra day every fourth year, because four missing quarters make a whole day. To bring the calendar back in line, the year 46 BC lasted 445 days.

A quarter is a bit too much

Caesar’s calendar made the problem much smaller, but did not get rid of it. A year is not a full 6 hours longer than 365 days, only 5 hours and 49 minutes. So the leap days add 11 minutes a year too many, and spring keeps arriving a little earlier in the calendar, just far more slowly than before. Nobody would notice in a lifetime, but after 128 years it adds up to a whole day, and after 1,000 years to almost eight.

Ten days that never happened

So spring came earlier and earlier in the calendar. By the 16th century it was starting on 11 March instead of 21 March, where it had fallen in 325, when the Church settled the rules for Easter. This bothered the Church most: the date of Easter is worked out from the start of spring, so as the calendar slipped, Easter drifted away from it. So in 1582 Pope Gregory XIII had ten days skipped: Thursday 4 October was followed straight by Friday 15 October.

The rule for centuries

This gave us today’s Gregorian calendar, named after Pope Gregory. In it, a year divisible by 4 is a leap year. But if it is divisible by 100, it is not, and if it is divisible by 400, it is again. That is why 2000 was a leap year, but 2100 will not be. Over 400 years, Caesar’s calendar adds about three days too many, and the new rule leaves out exactly those three.

Still not quite right

Even the Gregorian calendar is not perfect. Its year lasts 365 days, 5 hours, 49 minutes and 12 seconds on average, while a real year is, more precisely, 365 days, 5 hours, 48 minutes and 45 seconds. That leaves 27 seconds a year, so the calendar still drifts – but only by a whole day in about 3,200 years.

So why do we have leap years?

Because the Earth goes round the Sun in 365 days and almost six hours, and the calendar only knows whole days. Thanks to leap years, spring still starts around 21 March. With the rule for centuries, the calendar drifts by only about a day in 3,200 years.