Frank, "The Ecclesiastical Calendar" is a wonderful book and absolutely essential for anyone with an unhealthy interest in the labyrinthine workings of the church calendar. It was written by the Bishop of Meath during his spare time and describes in complete detail the development and theoretical foundations of the ecclesiastical calendar. Unfortunately, it is difficult to find and I have not seen a copy offered for sale but I did manage to borrow a copy from the British Library which I returned with great reluctance.
Google Books have scanned a copy of the book and its description can be found at: http://books.google.com/books?id=qbA-rzFsIoMC but it cannot be downloaded in UK - apparently for copyright reasons although this seems highly conservative as the author died before publication in 1877. However, anyone accessing Google Books from the US may download a high-quality PDF copy of the book and I strongly recommend them to do so. Geoff -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Frank King Sent: 16 March 2008 21:43 To: Mac Oglesby Cc: [email protected] Subject: Re: Easter Dear Mac, Many thanks for your message... > If you can find the time, this mailing list member ... > would much appreciate details about how you derived the list of dates. I am glad you asked this question and equally glad that Patrick Powers has supplied the algorithm! I had it in my head that this algorithm was first published in Butcher's Ecclesiastical Handbook in 1876 and I give this as the (incorrect) citation in one of my sets of student exercises. When I Googled "Butcher's Ecclesiastical Handbook" all I got was a couple of references to plagiarised versions of my exercises. Tee hee! I wouldn't have discovered my error had you not prompted me to look for something else! I first saw the algorithm in Meeus and now see that he gives the correct citation "Butcher's Ecclesiastical Calendar, 1876". There is a reasonably complete account in: http://en.wikipedia.org/wiki/Computus One of my favourite books is "Calendrical Calculations" by Edward M. Reingold and Nachum Dershowitz. It is full of good jokes! This book almost attributes the algorithm to Clavius and Lilius (key figures in the Gregorian reform of the calendar). Once you have implemented the algorithm you can then have fun determining the length of the cycle. A naive approach is to start at an arbitrary year and look for the next year that has Easter on the same date. You then look to see whether the two following years match too. They won't! So you look for the next matching year and keep going until you repeat indefinitely. Your program will take a long time!! "Calendrical Calculations" says: The dates of Easter repeat only after 5,700,000 years, the least common multiple of the 19-year Metonic cycle, the 400 years it takes for the Gregorian calendar to return to the same pattern of days of the week, the 4000 years it takes for the Gregorian leap-year corrections to add up to 30 days, and the 9375 years it takes for the correction to the Metonic cycle to amount to 30 days. If you get to understand the ecclesiastical moon and the ecclesiastical vernal equinox you will wonder whether the algorithm works at all. In fact it does a remarkably good job at predicting "the first Sunday after the first full moon after the vernal equinox" which is often given as the formal definition of Easter. What I should like to know is what is meant by Sunday! Sunday begins at different times in different places. Could it mean Sunday as it is timed at the longitude of Rome? Enough from me! Frank --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
