A Date for Easter

This year, Easter falls on April 7. The last time it did that was only 11 years ago, in 1985 and before that, in 1912 and in 1901. That's it for the 20th. Century. See any pattern yet? Not likely, as the grand cycle of Easter dates takes exactly 5 700 000 years to complete.

Easter is always the first Sunday after the full moon which occurs on or next after the March equinox. It sounds like an astronomical event, but it's not. The full moon is reckoned according to an ancient Ecclesiastical calculation which seems to go all the way back to the Council of Nicaea in A.D. 325. Similarly, March 21 is always assumed to be the date of the equinox, when in fact, astronomically, it can occur a day or two earlier.

The earliest possible date for Easter is, therefore, March 22. This happens to be the rarest date, turning up on average only once every 210 years. The last time was in 1818, the next not until 2285. In fact, as many as 1887 years can elapse before a March 22 date repeats.

The most common date for Easter is April 19, occurring precisely 220 400 times in the grand cycle. On average, we have to wait just 11 years until an Easter date recurs, although the minimum interval is 5 years. This brings us back to our current April 7 date. Although it occurs just twice in the next century, in 2075 and 2080, they are only the minimum of 5 years apart.

On the other hand, there are never less than 57 years between Easters on April 25, the latest of the 35 possible dates. The last was in 1943, the next in 2038. Fortunately, we just have to flip open a calendar to know when to dust off the Easter bonnet or buy a new one or when to plan the trip to Grandma's for Easter Sunday dinner.

Rob Roy

Hamilton Amateur Astronomers
Maintained by Rob Roy