The link between the eclipse and Oswy's instigation of the Synod of Whitby seems clear. The subsequent history of Britain, and a wide variety of other matters, hinges upon his decision to switch allegiance to the Roman Church. Above, though, I wrote "it seems remarkable that the possible link between the synod and the eclipse had not previously been examined because the events occurred close in time and both involved the Moon." How did the synod involve the Moon? The answer here is simple: through the dependence of the Easter computus upon the lunar phase.
The central argument at the synod revolved around whether the 84-year cycle used by the Celtic Church or the 19-year cycle of the Roman side provided a better representation of the lunar brightness variation, coupled with the assumed full moon date (fourteenth or fifteenth day after new moon). It seems pretty obvious that the recent eclipse that had spawned the synod provided a rather concrete test. How did the opposing parties' lunar tables compare?
We are sure that the eclipse occurred on May 1, both from modern astronomical calculations and also accounts of it preserved in monastic annals from Ireland and elsewhere in mainland Europe. But the surviving English account has it on May 3. This could not have been a simple slip of the quill because the ancient dating system passed on from the era of the Roman republic was still in use. In that system there is no possible ambiguity, no chance of a simple mistake having been made. Some deliberate manipulation must have taken place.
The explanation for this erroneous English account seems to be that the Roman Easter table had a date for the new moon given as May 3, and that is what was recorded as the date of the eclipse after the fact despite it having actually occurred on May 1. A solar eclipse can only occur at conjunction, and the sighting of the new moon is typically not until 30 hours later (refer, for example, to Figure A-6 in the Appendix). But new moon can only be seen just after sunset: if it is not quite visible one evening, you must wait another full day until your next chance. (Similarly, if you miss an hourly train by 5 minutes, then you must wait another 55 minutes for the next departure.) A tabulated date for new moon on May 3 could therefore be consistent with the eclipse having occurred on the first day of that month. This is all known with hindsight and modern technical knowledge though. At the time it seems that the Roman party purported the eclipse to have occurred on May 3, in accord with their lunar tables.
The May 1/3 discrepancy has long been a puzzle to chro-nologists, having been pointed out at least as early as 1590. Under the circumstances it might not be too strong a statement to say that the record seems to have been falsified, and we would like to know how this came about. Who was responsible?
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