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9.1.4.2
A Growing Problem |
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It is not my intention to attempt a reprise of medieval history in this section. Virtually the sole concern of medieval savants in the area of calendrics was to calculate the date of Easter properly. The history of this endeavor already has been discussed in section 6.2.3. |
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By A.D. 730, the Venerable Bede, historian in the English monastery of Jarrow, observed that the vernal equinox had advanced by three days from the date of 21 March mandated by the Council of Nicaea in A.D. 325. |
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In Paris, professor Johannes de Sacrobosco noted in the 13th century that the discrepancy had increased to seven days. In the same century, Roger Bacon wrote a treatise, De Reformatione Calendarii, which he did not publish but transmitted to Rome. No action resulted. |
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In A.D. 1472, Pope Sixtus IV invited the German astronomer Johann Muller (Regiomontanus) to supervise a reform of the calendar. Regiomontanus was assassinated in 1476, and no action resulted. |
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In the 16th century, Pope Gregory XIII approached the various governments of the Holy Roman Empire and obtained agreement that all would accept alterations. After considerable haggling "in committee," the reform that bears his name was promulgated in a papal bull issued in March 1582. |
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9.1.4.3
The Gregorian Reform |
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Gregory was advised by the Neapolitan astronomer and physician Luigi Lilio Ghiraldi (Aloysius Lilius). Lilius died in 1576 before the reform was completed, so Gregory turned to the German Jesuit mathematician Christopher Clavius, who verified all of the calculations and developed the rules for the reform. Clavius published his Romanii Calendarii a Gregorio XIII P.M. restituti Explicatio in A.D. 1603. The reform had the expected bipartite form of a one-time correction and a new set of rules. The reform was actually quite detailed, since it treated a number of issues of calculation that had assumed importance in the Church. |
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The one-time correction was eventually accomplished very smoothly, although much of the discussion preceding the reform had focused on ways to spread the correction over a large period. At the time of the reform, the discrepancy totaled 10 days. In the end, Gregory simply directed that the day following the Feast of Saint Francis (5 October) should be reckoned as 15 October. The simplicity of this act is deceptive. It was actually a bold and elegant stroke, thought through very carefully. The day of 4 October 1582 was a Thursday, and the day of 15 October 1582 was designated as a Friday. The structure of the seven-day week was thereby maintained for the week of the reform and all succeeding weeks. Preservation of the cycle of seven days was essential to the proper positioning of Sunday. The reader may encounter some convoluted arguments in the calendar reform literature that attempt to argue that Gregory altered this sequence. He did not. |
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