A tradition of natural science books continued in the middle ages from ancient Greek and Roman writings through the early middle ages, when writers such as Isidore of Seville (6th century) and Bede (early 8th century) write treatises on cosmology and calculation of time. Later expanded at Fleury and taken further still by Abbo of Fleury, who lived at Ramsey in the 10th century and was an important intellectual link to the continent, the tradition flourished in Britain's monasteries. To these natural science books belong a tradition of diagrams which incorporate Christian cosmological doctrine into classical theories on the structure of the heavens and earth, uniting geography, physics and computation of time to demonstrate the harmony of creation. Books such as this one (which is in two separate manuscript fragments in the British Library) would have been textbooks for training monks and churchmen. This part of the textbook contains several diagrams and tables concerned with the calculation of dates. The text on this page prefaces a treatise on the calculation of the movable date of Easter. It takes the form of a letter written by the early 6th-century Roman abbot Dionysius Exiguus ('Denis the Little'), who was responsible for the introduction of the Christian Era, still in use today as the means of numbering years, which counts them from the supposed year of Christ's birth, rather than from the reign of the Roman emperor Diocletian.