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Mathematics and Religion...

Paul Hazard, in his illuminating study on the European Crisis of Consciousness of the 1680s [published in 1935], highlighted the central role of chronologists in shaking the cultural certainties induced by the Catholic Church: complex calculations and some fruitful comparisons called into question the more than six thousand years that had elapsed since Creation.

Neperian logarithms were invented to help calculate the date of the Apocalypse.

And do you know the history of the Gregorian calendar? In 1582, the offset of the spring equinox [21 March], a day of equal duration between days and nights, had shifted by about ten days. Failure to take account of such a drift threatened to shift all religious feasts and first of all Easter.

Easter is set on the first Sunday after the 14th moon of the first month, from which it follows that Easter can only be after the equinox. From the adoption of this rule at the Council of Nicaea in 325 to the year 1582, the gap between the civil equinox (March 21) and the celestial equinox was 10 days.

To correct this anomaly, a choice had to be made to compensate for the delay [in one or several years] and to propose a schedule that offered greater stability over time. Alois Lilius had a brilliant idea: to remove three leap days every 400 years, so that the secular years would no longer be leap years, except for one every four centuries: 1700, 1800 and 1900 were 365-day years, 2000 366.

To arrive at this result, which consumed many astronomers, Lilius based himself on known measurements of the duration of the natural solar year. Ptolemy had said that a year was worth 365 days 5 hours 55 minutes and 12 seconds; Albategne, seven centuries later, offered 365 days 5 hours 46 minutes and 24 seconds. Copernicus gave a variation between 365 days 5 hours 55 minutes 57 seconds and 40 thirds and 365 days 42 minutes 55 seconds and 7 thirds. The Alphonsine tables, made by order of the King of Spain in 1250, showed 365 days 5 hours 49 minutes and 16 seconds. That is, approximately the middle of the interval proposed by Copernicus: it was the value of the alphonsine tables that was used to establish the new calendar.

Everyone knows that the delay was made up at once in October 1582 , by the succession of October 15 to October 4, leaving the Protestant countries with the Julian computation with a lag that will tend to increase in time.

Visit to discover the book by Pierre Dupont, a Toulouse priest, who explains all these subtle calculations.

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