Easter and leap years

This year, February threw its customary tantrum by taking an extra day for itself, an entitlement it gets through the “leap year” feature of the Julian and Gregorian calendars.

The leap year dates back to around 45 BC, during the time of Julius Caesar as a means to align the calendar year more closely with the astronomical year. Adjustments made in the Gregorian calendar centuries later improved on this, with the changes spurred largely by the desire to more accurately determine when Easter, a moveable feast, should be observed by the Christian world.

Nailing down the date on which Easter falls is in itself contentious. The differences in which astronomical events – equinoxes, lunar phases, type of solar rotation – must be observed to base computations on have led to varying interpretations by many churches, and even empires. Currently, the Roman Catholic Church and the Orthodox Churches are discussing a common date for Easter, expected to be announced in 2025.

The Julian and Gregorian calendars are essentially identical. A year is divided into 12 months, with some having 30 days, others 31 days. The Julian calendar assigned February with 28 days, or 29 days every fourth year. It’s a method meant to approximate both the tropical year and the sidereal year. These refer respectively to the 365 days and around six hours that take the sun to return to the same position in the sky as seen from Earth, and for the Earth to complete a revolution around the sun.

In the Gregorian calendar a leap year has to be divisible exactly by four. An exception is placed on the year ending a century. For it to be counted as a leap year, it must be divisible exactly by four and by 400. This explains why, in the Gregorian calendar, the years 1700, 1800 and 1900 were not leap years, but 1600 and 2000 were. This means 2100 will not be a leap year either.

While the Gregorian calendar could still be off by about 26 seconds per astronomical year, the variance computes merely to a single day lost in around 3,236 years. This ensures Easter celebrations, for over three millennia, are truly close enough to when these should take place.

Let February have its fit.