Star Points for April, 2003; by Curtis Roelle Date of Easter Many annual events that we celebrate, such as birthdays, occur on a specific calendar date. New Years Day is always on January 1. Independence Day is July 4. Traditionally, the only national holiday whose date changed from year to year was Thanksgiving which is pegged to the fourth Thursday in November. Several traditional holidays are now observed on Mondays and so their dates vary from year to year. Memorial Day, Labor Day, Columbus Day, President's Day, and MLK's birthday are all Monday holidays. Although their dates change from one year to the next, the Monday and Thursday holidays always occur about the same week each year. Merchants find the Monday holidays particularly useful for their annual three-day weekend sales events. However, Easter bucks the system. Easter can occur in March or April. The date is determined astronomically from the positions of the earth and moon in relation to the sun. Traditionally Easter is said to occur on the first Sunday, after the first full moon, following the vernal or spring equinox. The vernal equinox marks the start of spring and may occur on March 19, 20, or 21 in our currently used Gregorian Calendar. For simplicity the astronomical vernal equinox is ignored in the formula used for calculating Easter. The equinox is assumed to be March 21. That means that Easter depends primarily on determining when the next full moon - known as the "Paschal Moon" - occurs. Technically the astronomical definition of "full moon" isn't used either. Instead the "ecclesiastical" full moon used is defined as the 14th day of lunation, or 14 days past new. New Moon is determined by what is called the "Metonic cycle" which is named after the Greek philosopher Meton of Athens. (The Metonic cycle is approximately 19 years in length.) According to Guy Ottewell in his Astronomical Calendar, if March 21 is a Saturday and the Paschal Moon falls on that day then Easter comes earliest - on March 22. On the other hand, if April 18 is a Sunday and the Paschal Moon falls on that day, then Easter will come at its latest - on April 25. Once the date of Easter has been calculated, then the other related dates of observation can also be pegged. These include Good Friday, Maundy Thursday, Ash Wednesday, Shrove or Fat Tuesday, and the season of Lent. The first Easter coincided with the Jewish Passover. In subsequent years Easter was celebrated on the Sunday which followed Passover. The Jewish Calendar was reformed several hundred years later. In 1582 the Roman Catholic Church replaced the Julian Calendar with the Gregorian Calendar and thus the two religious holidays are now more or less independent. The association of Easter with the Passover and the Passover in turn based on astronomical criteria leads to an interesting question: Is it possible to determine the precise date of Christ's crucifiction? Many scholars have addressed the subject and several possible dates have been proposed. Astronomer Sten Odenwald has also addressed the issue. Based on contemporary political and religious information, biblical writings, and a lunar eclipse he has determined the most likely date of the crucifiction to be Friday, April 3, 33 AD. His analysis is interesting and can be found on-line at http://itss.raytheon.com/cafe/qadir/q867.html. For more astronomical information see Dr. Odenwald's "Astronomy Cafe" page at http://itss.raytheon.com/cafe/. As far as Christian holidays go, Easter is the only whose commemoration is done in the spirit of the 14th verse of the first chapter of Genesis.