What is the Julian calendar?

The mess before Caesar

The old Roman year had 355 days, ten short of the sun. To catch up, the pontiffs, Rome’s college of priests, could slip an extra month into February, roughly every other year. They used the power for politics: a longer year kept friends in office, a shorter one cut enemies’ terms. Suetonius says that by Caesar’s day the harvest festivals no longer came in summer, nor the vintage festivals in autumn (Julius 40).

Caesar, as chief pontiff, had the authority to fix it. With Sosigenes, an astronomer from Alexandria, he chose a year based on the sun alone (Pliny, Natural History 18.211). First the seasons had to be put back in place, so 46 BC was stretched to 445 days, with three extra months. Macrobius calls it the last year of confusion. The new calendar began on January 1, 45 BC.

How the leap year works

Caesar added ten days to the year, spread over seven months, which gave the months the lengths they still have. Every fourth year February gained a day. Romans counted days backward from fixed points, so they simply counted the sixth day before the first of March twice: bis sextus, the doubled sixth. That is why a leap year is called bissextile, in Portuguese ano bissexto.

Did Caesar invent the leap year? Not quite. A decree of Egyptian priests at Canopus, in 238 BC, had already ordered an extra day every four years, but it never took hold. Caesar made it stick, barely. After his death the pontiffs, counting inclusively, added the leap day every third year, and Augustus had to skip several leap years to undo the drift (Macrobius, Saturnalia 1.14.13–15).

July and August

Caesar’s birth month, Quintilis, the fifth month of the oldest Roman year, was renamed Iulius in his honor in 44 BC. In 8 BC the Senate renamed Sextilis for Augustus, because his first consulship and the capture of Egypt had both fallen in that month (Macrobius, Saturnalia 1.12.35). September to December still carry their old numbers, seventh to tenth, from the days when the year began in March.

A popular story says Augustus stole a day from February so that his month would not be shorter than Caesar’s. It is a medieval invention, traced to the thirteenth-century scholar Johannes de Sacrobosco. Under Caesar’s reform, Sextilis already had 31 days.

Pope Gregory’s fix

Caesar’s average year of 365¼ days is about 11 minutes longer than the solar year. That adds up to a full day every 128 years or so. By the 1500s the spring equinox, on which the date of Easter depends, had slipped from March 21, where it fell around the time of the Council of Nicaea, to about March 11. In 1582 Pope Gregory XIII, in the bull Inter gravissimas, cut ten days: Thursday, October 4, 1582, was followed by Friday, October 15. He also changed the rule. Century years are leap years only when divisible by 400, so 1600 and 2000 were, and 1700, 1800 and 1900 were not.

Catholic countries switched at once. Britain and its colonies waited until 1752, Russia until 1918, Greece until 1923. The Julian calendar is still alive, though. Several Orthodox churches, among them those of Russia, Serbia and Georgia, keep it for their feasts. It now runs 13 days behind, so their Christmas, December 25, lands on January 7 of the Gregorian calendar. From 2100 the gap will be 14 days.