If you lived in Rome, Madrid or Lisbon in 1582, you went to bed on Thursday, 4 October and woke up on Friday, 15 October. Nobody slept for eleven days. Ten dates were simply deleted, by order of the Pope, to fix an error of about 11 minutes a year that had been piling up for more than twelve centuries.
The calendar most of Europe used in 1582 was Julius Caesar's, introduced in 45 BC on the advice of the astronomer Sosigenes of Alexandria. Its rule was elegant: 365 days, plus a leap day every fourth year. That makes the average calendar year exactly 365.25 days long.
The trouble is that the seasons don't run on 365.25 days. The time from one spring equinox to the next, the tropical year, is about 365.2422 days. The Julian year is roughly 0.0078 of a day too long, which is about 11 minutes. Each year the calendar overshoots the sun by those 11 minutes, so the equinox arrives a little earlier by the calendar than it did the year before.
Eleven minutes sounds like nothing. But it adds up to a whole day every 128 years or so. Try it yourself: pick a rule, then drag through the centuries and watch where the spring equinox lands.
For farmers, a few days' drift hardly mattered. For the Church it mattered a lot. Easter is set by the spring equinox: it falls on the first Sunday after the first full moon on or after the equinox. By the time of the Council of Nicaea in AD 325, the equinox fell around 21 March, and the church's Easter tables came to treat 21 March as fixed.
But the real sun kept moving. From 325 to 1582 is 1,257 years; at one day per 128 years, that is almost ten days. By the 1500s the astronomical equinox was landing around 11 March, while the tables still said the 21st. Easter was drifting away from the season it was meant to mark, and scholars had been complaining about it for centuries.
The reform that Pope Gregory XIII announced in the bull Inter gravissimas on 24 February 1582 was based largely on the work of the Italian physician Aloysius Lilius, and was championed by the Jesuit astronomer Christopher Clavius. It did two things.
The bold part was a one-off jump. To put the equinox back on 21 March, ten days were removed from the calendar. October was chosen, reportedly because it had few major church feasts to disrupt. The days of the week carried on normally: Thursday was followed by Friday. Only the numbers skipped.
| MON | TUE | WED | THU | FRI | SAT | SUN |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 15 | 16 | 17 |
| 18 | 19 | 20 | 21 | 22 | 23 | 24 |
| 25 | 26 | 27 | 28 | 29 | 30 | 31 |
| 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| 12 | 13 | 14 |
The clever part stopped the drift from coming back. Keep a leap year every four years, but skip it in century years, unless the year divides by 400. So 1700, 1800 and 1900 were not leap years, but 1600 and 2000 were. That removes 3 leap days every 400 years, leaving 97 leap years per 400, and an average year of 365.2425 days.
That is still a tiny bit too long, by about 27 seconds a year. The error only reaches a full day after roughly three thousand years. Switch the simulator to Gregorian and drag all the way to 6000 to see how slowly it creeps.
A papal bull only carried weight where the Pope did. Catholic countries switched almost immediately; Protestant and Orthodox ones took their time, and the longer they waited, the bigger the gap grew, because the Julian calendar kept adding leap days in 1700, 1800 and 1900 that the Gregorian one skipped.
| 1582 | Italy, Spain, Portugal and Poland: Thursday 4 October is followed by Friday 15 October. |
| 1752 | Great Britain and its colonies: Wednesday 2 September is followed by Thursday 14 September, eleven days gone. |
| 1918 | Soviet Russia: 31 January is followed by 14 February, thirteen days gone. That is why the "October Revolution" of 1917 is commemorated in November. |
| 1923 | Greece, the last country in Europe to switch for civil use. |
Stories of English crowds rioting and chanting "give us our eleven days" are popular, but historians have found little evidence that such riots really happened. Many Orthodox churches still keep the Julian calendar for their feasts, which is why their Christmas, 25 December Julian, now falls on 7 January. The gap is 13 days, and it grows to 14 in 2100.
The underlying problem has no perfect solution. A year is not a whole number of days, so any calendar has to approximate 365.2422 with whole leap days sprinkled in. Caesar's approximation was 365 + 1/4. Gregory's was 365 + 97/400. Both are fractions standing in for a number that doesn't fit neatly into either.
The Gregorian rule works well enough that the next correction is someone else's problem, thousands of years from now. And meanwhile, the Earth's rotation is slowly changing too, so the length of a day in seconds is not quite fixed either. Somewhere far in the future, a calendar committee will need to hold another meeting.