So, when is Easter, anyway?

Most of the holidays and observances we celebrate here in America are pretty easy to remember and can be classified, somewhat arbitrarily, into two groups.

First, there are the events that occur on the same date of the same month, but on different days of the week, every year. Examples are New Year's Day, Valentine's Day, Independence Day, Halloween, Veterans Day and Christmas.

There's another set of holidays that fall on the same day of the week of a particular month, but the dates are different from year to year. You are familiar with them; Mother's Day, Memorial Day, Father's Day, Labor Day and Thanksgiving.

But there's one observance that isn't so easily pinned down. Easter seems to be somewhat of an odd duck (or bewildered bunny?) in this regard.

Established to commemorate the resurrection of Jesus Christ from the dead after his crucifixion, Easter is probably the most important annual religious feast to practicing Christians. The rule for when to celebrate Easter was established way back in 325 A.D. at the First Council of Nicaea. Convened by Roman emperor Constantine, this was the first ecumenical (referring to worldwide Christian unity) meeting of church bishops whose task was to develop a uniform Christian

doctrine.

The rule states that Easter shall be celebrated on the first Sunday that occurs after the first full moon on or after the vernal equinox. To gain a clearer understanding of how this works, let's examine each of these conditions in reverse order:

The vernal equinox occurs when the sun is directly overhead at the equator on its annual journey from Earth's southern to its northern hemisphere, thus marking the beginning of spring. The date of this event, however, is somewhat variable, occurring anywhere from March 19 through March 22.

There are two main reasons for this. One is the way our solar-based Gregorian calendar handles leap years, by occasionally adding an extra day in February. The other has to do with the fact that Earth's axis of rotation isn't fixed in space but wobbles a bit, causing the equinoxes to drift. To avoid having to deal with this "jumpiness" to the start of spring, overseers of the Easter rule decided that the vernal equinox would always occur on March 21.

There's also a small difference in how lunar phases are referenced. In astronomical terms, there are 29-1/2 days that separate like phases of the moon (e.g. from one full moon to the next full moon.) This means that, on average, the new moon and full moon are separated by 14.75 days.

But the Easter rule uses the ecclesiastical (church) lunar calendar to reference moon phases. In this system, the new moon is always the first day of the month, and there are 14.5 days between new and full moons. While reconciling these two methods of timekeeping is a rather complex exercise, the upshot is that the ecclesiastical and astronomical full moons usually match up pretty well but can be up to two days different from one another.

To determine the earliest and latest possible dates for Easter, let's apply the rule. When the full moon falls on Saturday, March 21, then the next day, March 22, is Easter. On the other hand, when the moon is full on Saturday, March 20, we need to add the number of days to the next full moon (29) plus 7 more days to get to the next Sunday. This works out to April 25.

So, let's see now...the 2010 calendar shows that the full moon in March falls on day 29, which is after March 21. So far, so good. That must mean that Easter is the following Sunday, namely April 4. And sure enough, there it is.