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Astronomical Images : Manuscript perpetual calendar from a Book of Hours


Astronomical Images

<p style='text-align: justify;'>The most typical perpetual calendar in the Middle Ages was the Book of Hours. They were based on the divine office said by a priest and were compiled for the daily devotion of the laity. In addition to a calendar, these normally also contained prayers for the canonical hours of the day, prayers to the Virgin Mary and for the dead. As in this example, perpetual calendars marked each day of the month with a dominical letter, a golden number and a saint's name, which enabled the calendar to be used year after year. Dominical letters (A to G) indicate the days on which Sunday falls in any particular year. Starting with 1 January as 'A', the first seven letters of the alphabet are iteratively assigned to subsequent days, with 29 February left blank. Sundays could then be specified each year by one of these dominical letters (or two letters in the case of leap years). The golden number - usually indicated by the Roman numerals I to XIX - gives the position of the New Moon. Because the lunar month and calendar month do not coincide, a particular phase of the Moon will not fall on the same day every year. But a phase of the Moon falls on the same day of the calendar month every nineteen years (a Metonic cycle), thus the numbers one to nineteen were allotted to the days on which the New Moon occurred in that particular year. Easter (the first Sunday after the Full Moon that occurs on or after the Spring equinox) and other moveable feasts were indicated by a combination of a golden number and a dominical letter. The conventions from perpetual calendars like this one were often carried over into printed, annual calendars or almanacs.</p>

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