bill (04/12/83)
Joe P. quotes the Book of Common Prayer on the date of Easter, and he is of course correct. What he didn't say is that Easter is NOT determined by the time of the ACTUAL full Moon, but rather by a mathematical approximation known as the "Paschal Full Moon". The tables in the Book of Common Prayer are constructed according to this rule, rather than the best known astronomical rule. Given time, the rule for the Paschal full Moon will diverge more and more from the time of the actual full Moon (although the error is only a few hours at present). People often wonder that Passover sometimes comes a month later than Easter. This is an interesting question because the two festivals were originally related, and the "Easter Rule" was intended at first to cause them to coincide. According to the Hebrew calendar, Passover starts on the 15th day of the Hebrew month Nisan. All months begin at New Moon, and in ancient times the Vernal Equinox always occured within Nisan. The method used to calculate the moment of New Moon is really quite accurate - it dates back to the Babylonian exile (being identical to the rule used by the Babylonians) and after all that time, is only a few hours off. The discrepancy arises because the Hebrew calendar uses a different rule to estimate when the Vernal Equinox occurs than does the Gregorian calendar. The Gregorian calendar is very accurate - the error is one day in about 3000 years. The Hebrew calendar uses an approximation known as the Metonic Cycle to estimate the date of Vernal Equinox (a Solar calendar concept) relative to the lunar calendar. According to the Metonic Cycle, there are 235 lunar months in 19 years, and this is a pretty good approximation. It is not exact, however, and at the present time it predicts the Vernal Equinox a few days late. As a result, several times during the 19 year cycle, the beginning of the month of Nisan is postponed to the new moon AFTER the actual Vernal Equinox. An extra "intercalary" or "leap" month is inserted. (These leap months are added routinely in years #3,6,8,11,14,17 and 19 of the Metonic cycle, since 19 years * 12 months falls seven months shy of 235 months.) When this happens, Passover will be a month later than Easter. This discrepancy will become worse and worse with time. If my notes are correct, at the present time 3 years out of 19 have Passover a month later than Easter. They are the 8th, 11th and 19th years of the cycle, so not every year which has an intercalary month will have this discrepancy between the dates of Easter and Passover. (The latest cycle began with its year 1 in 1978-9, so the next year in which this will happen should be 1986). 1000 years ago, Easter always came a few days after Passover. In the future the discrepancy will become more and more obvious, as the two calendars slip more and more out of phase. What is remarkable about all this is the accuracy with which the ancients were able to determine the lengths of the fundamental astronomical cycles that went into this calculation. When one realizes that the rule for Passover is of the order of 2500 years old, and only now is getting noticeably out of phase with the actual astronomical events it attempts to track, you really have to give the people who made up the rules a lot of respect. Bill Jefferys (-!hao!cires!nbires!un-ngp!bill) Astronomy Dept University of Texas