Why does Easter seem late this year? It has to do with the moon

Kathryn Sucich

Easter falls on April 24 this year, which seems exceptionally late for Christians to be celebrating their most important liturgical day of the calendar.

So why the late date? And why does the Easter date seem to jump around every year?

According to the U.S. Naval Observatory Astronomical Applications Department, Roman Catholic and Protestant churches use the Gregorian calendar to determine the date of Easter, while Orthodox churches use the Julian calendar. The dates in the calendars are slightly different — that’s why the Easter date can vary some years between the Western and Eastern churches. (This year, Easter is the same date for the churches; next year, they differ by one week.)

For Western churches, Easter can fall anywhere between March 22 and April 25.

While many people believe that Easter occurs the first Sunday after the first full moon following the vernal equinox (or first day of spring), that’s not entirely true. Easter is calculated by an ecclesiastical moon — which is not always the same as an astronomical full moon. It is determined by tables that are centuries old, and can vary from the astronomical full moon by up to two days.

The precise rules:

• Easter falls on the first Sunday after the first ecclesiastical full moon, which happens on or after the vernal equinox (fixed as March 21 in the ecclesiastical calendar, although this can be March 20 astronomically).

• The ecclesiastical full moon happens the 14th day of a new moon.

What does that mean for 2011? According to the Gregorian calendar, the latest an ecclesiastical full moon can occur after March 21 is April 18, and this year it is on April 17. Since April 17 falls on a Sunday, Easter is the first Sunday after that, or April 24, hence the exceptionally late date this year.

The last time Easter was so late? In 1943, when it was on April 25. It won’t be this late again until 2038, when it will fall on April 25. Easter next year will be on April 8, and in 2013 it will be relatively early: March 31.