Computus in its simplest definition is the art of ascertaining time by the course of the sun and the moon. This art could be and was a theoretical science, such as that explored by Johannes of Sacrobosco in his De sphera--a science based on arithmetical calculations and astronomical measurements derived from use of the astrolabe or, increasingly by the end of the 13th century, the solar quadrant. In the context of the present exhibit, however, computus is understood mainly as the practical application of these calculations. To reckon time in the broadest sense and to determine the date of Easter became one and the same effort. And for most people, understanding the problem of correct alignment of solar, lunar, yearly and weekly cycles to arrive at the date of Easter was simply reduced to a question of "when?" rather than "why?". The result was a profusion of calculation formulae, charts and memory devices.

Accompanying these handy mechanisms for determining the date of Easter were many other bits of calendrical information that faith, prejudice and experience leveled to the same degree of acceptance and necessity: the lucky and the unlucky days for travel or for eating goose; the prognostications of rain or wind; the times for bloodletting; the signs of the zodiac; the phases of the moon; the number of hours of sunshine in a given day; the feasts of the saints; the Sundays in a perpetual calendar.

Smith Western Add. MS 01, ff. 60v-61:A 14th century Italian manuscript of Johannes de Sacrobosco, Computus with a diagram of the celestial bodies. Plimpton MS 170, f. 64v: A copy of Bono da Lucca, Computus solaris et lunaris produced in Italy during the 1390s with an Easter chart to the year 1882 (new style). Plimpton MS 211, f. 137v: An early 16th century Italian treatise on arithmetic with a mnemonic hand for the date of Easter.
HSA09A necrology copied in Spain ca. 1536 for the use of the Comendadores de Santiago Mayor y de Santa Eufemia, with an Easter wheel dated 1536.
Hispanic Society of America, MS B2929: A breviary from late 14th / early 15th century Spain, possibly Avila:
HSA08Leaf with explanation of the Dominical Letters. HSA21Detail of a leaf with two Easter wheels (but with the year of inception erased); in Spanish.