One Easter With Two Different Dates

Often times the Orthodox and Roman Catholic celebration of Easter falls on different days. Why does this happen?

Easter Sunday is the Sunday immediately following the Paschal Full Moon, or the first full moon of spring. Don’t let that term fool you, though. Technically it is the first ecclesiastical full moon. Only in the early days of Christianity was actual astronomical data gathered to do this calculation. For the last several hundred of years an official table is consulted instead of the sky. Here’s where the potential difference comes into play.

Europe used the Julian calendar from 326 A.D. to 1582 A.D. This system of dating used a periodic intercalary month to line up with the tropical year of 355 days. Most Julian years lasted 355 days, but every few years a 377 or 378 day year was added. With this system the Julian calendar averaged 366 ¼ days a year over a four year period. This is a relatively reliable system, but ultimately error prone because it was difficult to dependably disseminate the proper information for the intercalary months in the ancient world. One had to keep obscenely close track of the Julian system or it would become untrustworthy.

In October 1582 The Roman Catholic Church under Pope Gregory XIII adopted the Gregorian calendar (wonder how it got that name?). This is the modern calendar most of the western world uses. Computation of leap years in this new system is practically fool-proof, but it did shift the calculation of Easter because the Gregorian calendar removed 13 days from the Julian one.