The Calendar Read Free

concerned. Therefore, since all things that are in the calendar are based on the length of the solar year, they of necessity must be untrustworthy, since they have a wrong basis.
    Bacon also condemns a second calendric mistake that comes out of the first. ‘There is another greater error,’ writes Bacon, ‘regarding the determination of the equinoxes and solstices. For . . . the equinoxes and solstices are placed on fixed day . . . But astronomers are certain that they are not fixed, nay, they ascend in the calendar, as is proved without doubt by tables and instruments.’
    This second point was critical, Bacon notes, because the spring equinox--astronomically the point between winter and summer at which the sun strikes the equator--is the date used by Christians to determine Easter. According to Church rules, Easter is celebrated on the first Sunday after the first full moon after the spring equinox.* In Bacon’s day the equinox was permanently fixed on 21 March by order of the Church, as established by an important Christian council held at Nicaea in Turkey in AD 325. But since 325, as Bacon notes, the equinox had been ‘ascending in the calendar . . . and likewise the solstices and the other equinox’ by 1/130 of a day each year, or just over 11 minutes. He set the true date of the equinox for the year he was writing, 1267, on ‘the third day before the Ides of March’, or 12 March--a nine-day difference. ‘This fact cannot only the astronomer certify,’ says Bacon, ‘but any layman with the eye can perceive it by the falling of the solar ray now higher, now lower, on the wall or other object, as anyone can note.’
    *The actual calculation of Easter is considerably more complicated than this, but this simplification will suffice for now.
    He calculates that by 1361 the calendar would drop back another whole day, throwing the entire progression of dates and sacred days further into disarray. The friar concludes with an appeal to Clement to embrace the ‘truth’ offered by science, and to fix the mistake:
    Therefore Your Reverence has the power to command it, and you will find men who will apply excellent remedies in this particular, and not only in the aforesaid defects, but in those of the whole calendar. ... If then this glorious work should be performed in your Holiness’ time, one of the greatest, best, and finest things ever attempted in the Church of God would be consummated.
    Bacon’s own solution was to drop a day from the calendar every 125 years. But he adds a word of warning, noting that ‘no one has yet given us the true length of the year, with full proof, in which there was no room for doubt’--a reality that would continue to complicate a final solution for the calendar problem for centuries to come.
    Roger Bacon was hardly the first to realize the calendar’s drift against the solar year. A millennium earlier the Greek astronomer Claudius Ptolemy (c. 100-178) had noted that the calendar year fell short against the true year, though his calculation differed substantially from Bacon’s. In The Almagest, a work on astronomy widely read (if not fully understood) during the Middle Ages, Ptolemy sets the drift at about one three-hundredth of a day, a slippage of an entire day every 300 years. This amounts to a five-minute shortfall, or a year of 365 days, 5 hours and 55 minutes, rather than Caesar’s year of 365 days and 6 hours (365 1/4 days). ‘And this number of days,’ writes Ptolemy, ‘can be taken by us as the nearest approximation possible from the observations we have at present.’ Considering that Ptolemy, like Bacon, had no telescope and believed that the sun revolved around the earth, this calculation was a reasonably close approximation, though less accurate than Bacon’s year of 365 days, 5 hours and 49 minutes.
    Between Ptolemy and Bacon, scholars in Europe and Asia tinkered with solutions, attempting to refine earlier estimates of the true year--but always falling short (or