Easter day and the foundation of western science

Predicting the date when Easter Sunday will fall is necessary when the church decided to place a 40 day fast in advance of it. Such an endeavour is not as easy as it may seem as it involves reconciling two irreconcilable calendars the lunar and solar. The month is named after the moon because each month is approximately the duration of a single moon cycle. The average number of days between moon phases is actually 29.5 days and months were devised to have 29 (hollow month) or 30 (full month) days. This Romulus calendar had only ten months with the spring equinox in the first month. It had only 304 days and it seems that the Roman just ignored the remaining 61 days which fell in winter. The calendar was reformed many times but it was not reliable. According to tradition under the second emperor of Rome Numa Pompilius c. 700BC two extra months were added (Januarius and Februarius) and this accounts for the mismatch in the month names Sept, Oct, Nov & Dec which translate as month numbers 7, 8, 9 & 10 but are now actually months 9, 10, 11 & 12. Confusion about when to add leap days meant that it was not till about 4AD that the Julian calendar had become reliable with leap days inserted every 4 years but it was still out of sync with the solar year by between 10 & 11 minutes. Thus the accumulation of this error over time meant that after about 131 years the calendar was out of sync with the equinoxes by one day. The date of the spring or vernal equinox is the basis from which the calculation of the date of Easter Sunday is taken. Thus errors in the calendar mean the calculation of Easter day may also be in error.

In 325AD the Council of Nicaea established that Easter would be held on the first Sunday after the first full moon occurring on or after the vernal equinox. Easter is based on the Jewish Passover which occurs on the first full moon after the vernal equinox by watching the phases of the moon the date of Passover can be determined easily a few weeks before. However the Christians had to know many weeks in advance when Easter Sunday was going to occur in order to calculate the date of Ash Wednesday. Mistakes in such calculations might mean that one parish could be celebrating Easter a week or two after their neighbours. Because fasting and abstinence was quite austere in medieval times the Easter celebration was anticipated like no other. If the neighbours got to celebrate first then they could legitimately celebrate it again in the next parish and it is fair to assume there would not be many happy Easter bunnies in a particular parish.

In order to accurately calculate the date of Easter Sunday it required figuring out how the universe worked.

“In fact, Ireland in the Early Middle Ages led the way in terms of serious scientific engagement with the physical universe and the attempt to understand the nature of the created world. The famous studies of Archbishop James Ussher in the seventeenth century have their antecedents in the efforts of Irish scholars, 1000 years before him, to offer rational explanations of the natural phenomena that they observed around them in their everyday world. More than anywhere else in Europe at that time, the Irish in the seventh century succeeded in figuring out ‘how things worked’ in the universe. They did so not only in the field of technical chronology (in which they were THE masters), but also in those areas of study that the modern world calls Science.” From Saints, scholars and science in early medieval Ireland – Prof. Dáibhí Ó Cróinín, NUI Galway