FIGURING THE DATE OF EASTER REQUIRES MATH FORMULA

Few people stop to question the date of Easter each year, even though it always changes. Last year Easter fell in March; this year it is in April.

Because the date changes each year, Easter poses some interesting mathematical questions. Can it fall in March two years in a row? How often does the pattern repeat itself?

The answers come from mathematical formulas. There is a pattern to the date on which Easter occurs, although it is an erratic one. Sometimes the holiday falls in March and other times in April.

Regardless of the month in which it occurs, Easter always falls on a Sunday, said Richard L. Francis, math professor at Southeast Missouri State University.

Francis, like other mathematicians and astronomers in centuries past, realized that the date of Easter poses a complicated math question involving lunar and solar cycles.

The date of Easter was fixed in A.D. 325 by the Council of Nicea. The council declared that Easter would be celebrated on the first Sunday after the first full moon on or after the spring equinox.

Because the date is dependent upon both lunar and solar cycles, some involved and complicated mathematical questions arise, Francis said. The full moon determination must be made in Rome for the Nicean decree to be valid; otherwise there would be a full week's discrepancy between the observances in New York and Tokyo.

Determining the date of Easter also brings up questions about the frequency of the dates and their modern calendar patterns, Francis said.

Some Easter dates appear with greater frequency than others. April 12, this year's date for Easter, is the most common of the 20th century, he said.

Easter can occur as early as March 22 or as late as April 25. But those two dates make scarce calendar appearances, he said.

"Evidently there are 35 possible dates on which Easter can occur," Francis said. If there are only 35 dates, "is it possible for all of them to occur in one lifetime?"

Francis says it is unlikely. There is only one span of 87 years when such a feat occurred -- 1799 to 1886. The Easter calendar cycle repeats itself every 5,700,000 years. The holiday cannot occur in March in two consecutive years.

By redefining the date of Easter, all these mathematical problems could easily be solved, Francis said.

"Yet in spite of the mathematical madness, formula frustration, complex computation, and difficult dilemma, the joy of the wonderful day remains," said Francis.