Why Do We Have Leap Years? The Calendar Catching Up With the Sun
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The Earth takes 365 days, 5 hours, 48 minutes and 45 seconds to go round the Sun, and a calendar that counted 365 days a year would slide against the seasons by nearly a day every four years, until after a few centuries midsummer fell in what the calendar called autumn. The leap year is the correction, and it is older than the Roman Empire. The rule everyone knows, an extra day every fourth year, is only the first of three, and the second and third are why 1900 was not a leap year and 2000 was.
The problem
A calendar has to serve two masters. The day is the Earth's rotation and the year is its orbit, and the orbit does not take a whole number of rotations; it takes about 365.2422. Any calendar of whole days therefore drifts, and the only choices are to let it drift, as the Islamic lunar calendar does, moving Ramadan through the seasons over a 33-year cycle, or to insert extra time now and then to keep the count in step with the Sun. The extra time is called intercalation, and civilisations have added days, months and even, in the Romans' case, whatever the priests found convenient.
Caesar's fix
By 46 BC the Roman calendar was some three months out of step with the seasons, because the priests who were supposed to insert a leap month had used the power for politics, lengthening years when their friends held office. Julius Caesar, advised by the Alexandrian astronomer Sosigenes, abolished the whole apparatus. He made 46 BC 445 days long to reset the seasons, a year the Romans called the last year of confusion, and from 45 BC the year was 365 days with one day added every fourth year at the end of February, which was then the last month. That gives an average year of 365.25 days, about eleven minutes too long, which nobody minded for a while.
The pope's correction
Eleven minutes a year is a day every 128 years, and by the sixteenth century the Julian calendar had slipped ten days: the spring equinox, which the Church needed for the date of Easter, fell on 11 March instead of 21 March. In 1582 Pope Gregory XIII issued a bull that did two things. It cut ten days from October, so that the day after 4 October 1582 was 15 October, and it changed the leap rule so that the error would not recur. The Gregorian rule:
- •Every year divisible by four is a leap year
- •Except years divisible by 100, which are not
- •Except years divisible by 400, which are
- •So 1600 and 2000 were leap years; 1700, 1800 and 1900 were not; 2100 will not be
Adoption, and the days that vanished
The Gregorian average year is 365.2425 days, which is within 26 seconds of the true figure and will drift by one day in about 3,300 years. Catholic countries adopted it at once; Protestant ones took up to two centuries, suspecting a papal trick. Britain and its colonies switched in 1752, by which time the gap was eleven days, and 2 September was followed by 14 September, and there is a persistent though probably exaggerated story of riots demanding the eleven days back. Russia held out until after the revolution of 1917, which is why the October Revolution happened in November by the rest of the world's reckoning, and Greece changed in 1923. The Orthodox churches still calculate Easter by the Julian calendar, which is why Orthodox and Western Easter fall on different Sundays in most years.
The odd cases
The extra day goes at the end of February because February was the end of the Roman year, and the Romans regarded it as the month for tidying up. Someone born on 29 February has a birthday every four years, and legal systems have had to decide when they come of age; in most, on 1 March in ordinary years. The Sun is not the only thing to catch: the Earth's rotation is slowing very slightly and irregularly, so since 1972 a leap second has been added to the world's atomic clocks when needed, 27 times so far, always at the end of June or December, and always to the annoyance of the computer industry, which has voted to abolish the practice from 2035. And the Iranian calendar, which fixes its new year to the actual moment of the spring equinox observed from Tehran, is more accurate than the Gregorian without any rule at all, at the cost of needing an astronomer every year.
The takeaway
Leap years exist because the Earth's orbit takes about 365.2422 days rather than a whole number, and a calendar of 365 days would drift through the seasons. Caesar added a day every fourth year in 45 BC, which overcorrected by eleven minutes a year, and Pope Gregory XIII cut ten days in 1582 and ruled that century years are not leap years unless divisible by 400, a calendar accurate to a day in 3,300 years that the world took three and a half centuries to adopt.