Caesar's Reform, and the Day It Slowly Loses

Julius Caesar introduced the calendar in 45 BCE on the advice of the Alexandrian astronomer Sosigenes, replacing the erratic pre-Julian Roman calendar with a fixed twelve month year averaging three hundred sixty five and a quarter days: three years of three hundred sixty five days followed by a leap year of three hundred sixty six, the extra day inserted after the twenty fourth of February. The reform was a genuine improvement on what preceded it, but the true solar year is slightly shorter than three hundred sixty five and a quarter days, so the calendar drifts against the seasons by about one day every hundred twenty eight years, a gap invisible across a single lifetime but substantial across centuries.

By the sixteenth century, when Pope Gregory XIII's reform corrected it, the Julian calendar's calculated spring equinox had slipped ten days from the astronomical one. The Gregorian reform's fix, dropping the leap day in century years not divisible by four hundred, is the whole difference between the two calendars, and the accumulated gap between them stands at thirteen days today, widening to fourteen after the Gregorian calendar skips its next leap year in 2100 while churches still keeping the Julian count do not.