This week, as they so often do, my Sunday School class of bright 12- and 13-year-olds posed a tough question: why did Easter fall on Evan's birthday last year, but it's falling on Abby's birthday this year? Though I couldn't answer on the spot, I knew I had a secret weapon back at the office—saved for just such an occasion: a short article by Farrell Brown, a retired chemistry professor with an interest in the historical interactions between science and religion. Here, as a public service for those still scratching their heads over the calendrical wandering of Easter, is Dr. Brown's answer to my Sunday School kids' question—and thrown in for free, the story of why Easter dates still differ in different parts of the world:

The date of Easter Sunday, a so-called movable feast day in the Christian Church year, may seem mysterious to many who celebrate it. There are 35 possible dates in the spring season (northern hemisphere) for celebrating a one-time event. Why this wandering? The answer comes from decisions made several centuries after Christianity's inception.

And why do most Eastern Orthodox Christian Churches observe Easter occasionally on the same Sunday as the rest of Christendom and at other times as much as five weeks later? This answer lies primarily in how different people reacted to a centuries-old papal decree.

Our first stop on this tour of the wandering Easter is a quick study of how calendars were used in the Biblical lands around 30 A.D. Although the Julian or solar-based calendar of the Roman Empire had been in place since 45 B.C., it did not supplant the lunar calendar that was the chart and compass of 2,000 years of Jewish history. (A lunar year is 12 lunar cycles of 29.53 days each or 354.36 days while a Julian ...