In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is in Byrhtferth 's Enchiridion (a science text) of 1010–1012, where it was defined as 1/564 of a momentum (1 1 ⁄ 2 minutes), and thus equal to 15/94 of a second. It was used in the computus, the process of calculating the date of Easter.