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Jimmy Westlake

Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.

Have you ever wondered why the date of Easter Sunday bounces around from year to year? Sometimes it falls in March and sometimes in April. In fact, Easter Sunday can come as early as March 22 or as late as April 25. In 2008, Easter Sunday falls on March 23, almost as early as it can be.

The often-stated rule for determining the date of Easter Sunday is this: Easter is the first Sunday after the full moon that falls on or after the March equinox, the first day of spring in the northern hemisphere. This rule will usually predict the correct date for Easter, but sometimes it doesn't.

The ancient (and correct) rule for determining the date of Easter Sunday is, indeed, tied to the date of the full moon, but not to the date of the March equinox. Easter is the date on which Christians traditionally celebrate the resurrection of Jesus Christ.

Jesus is generally thought to have been crucified between the years 27 and 33 A.D., around the time of the Jewish festival of Passover, celebrated during the time of the first full moon of spring. In the year 325 A.D., early Christians decided they wished for all Christians in the world to celebrate Easter at the same time, preferably on a Sunday. They created a table of predicted full moon dates, called Paschal or Easter full moons, which stretched far into the future. The actual dates of the full moons can differ from these calculated dates by several days, but they are generally close. At that time, Easter Sunday was declared to be the Sunday immediately after the Paschal full moon each year.

The Paschal full moon in 2008 falls on Saturday, March 22, making March 23 Easter Sunday. The real full moon falls on Friday, March 21, and follows the equinox March 19, so, both rules point to the same date for Easter Sunday in 2008 - March 23. We haven't had an earlier Easter since the year 1818 when Easter fell on March 22, and that won't happen again until the year 2285. There will be one more Easter on March 23 before then, in the year 2160.

The bottom line is this: no one alive today will experience an earlier Easter Sunday than this year's date of March 23. Happy early Easter!

Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA's "Astronomy Picture of the Day" Web site, Spaceweather.com, Space.com, Discover.com, MSNBC.com, NationalGeographic.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines. His "Celestial News" article appears weekly in the Steamboat Pilot & Today. His "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, check out his Web site at www.jwestlake.com.