De temporum ratione (English: On The Reckoning Of Time) is a treatise written in Latin by the Northumbrian Anglo-Saxon monk Bede in 725. The treatise includes an introduction to the traditional ancient and medieval view of the cosmos, including an explanation of how the spherical earth influenced the changing length of daylight, of how the seasonal motion of the Sun and Moon influenced the changing appearance of the New moon at evening twilight, and a quantitative relation between the changes of the tides at a given place and the daily motion of the moon.[1]
De temporum ratione describes a variety of ancient calendars, including the Anglo-Saxon calendar.[2] The focus of De temporum ratione was calculation of the date of Easter, for which Bede described the method developed by Dionysius Exiguus. De temporum ratione also gave instructions for calculating the date of the Easter Full moon, for calculating the motion of the Sun and Moon through the zodiac, and for many other calculations related to the calendar.
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