Phases of the Moon, in a Calendar
Medium: Ink and pigments on vellum
For the medieval thinker, time was a subject with a great deal of potential. Time was tied into the divine order of everything, making what may seem an heretical or pagan subject such as astrology actually perfectly alright. Medieval calendars were founded on the idea that the celebration of Easter was a kind of cosmological time fix and was related to the positions of the sun and moon (which were among the planets orbiting the earth). Calendars had a place in church service books and prayer books where they served as a kind of diary to keep track of feast and saints' days, but also they would be included in scientific and astrological books along with complex diagrams showing relationships and harmonies of the elements of creation. This calendar belonged to the priory of Christ Church cathedral, Canterbury.
The the moon with its waxing and waning phases was believed to exert definite influences upon earthly events. Certain phases were favourable for some activities but unfavourable for others. This page has a table giving a general indication for days of lunar phases, categorising them as entirely, good, good from a certain hour or not good.