Previously Unpublished Works, Fascicles VI-VII

xxi
A.

Boethius, Martianus Capella, and Bernardus Sylvestris, and takes the length of his years, months, etc. from the only Arabic sources he knows, the Almagest The influential 2nd-century astronomical treatise by Ptolemy. and Alfraganus The 9th-century Persian astronomer al-Farghani.. He takes the mean error of the year as one day in 288 years, and suggests leaving out the leap-year day in the last year of the period. He is also aware of error in the lunar month, which could be remedied by altering the golden number golden number A number used in the Metonic cycle to determine the date of the new moon and, consequently, Easter. "but since it was forbidden in a general council to change anything regarding the calendar, it is necessary for those today to still endure errors of this kind" original: "sed quoniam in concilio generali aliquid de kalendario transmutare prohibitum est, oportet modernos ad huc hujusmodi sustinere errores". He approved of Gerland’s correction of Dionysius Dionysius Exiguus, the 6th-century monk who invented the Anno Domini dating system; Gerland (11th century) argued that Dionysius had miscalculated the year of Christ's birth., calling his the natural system; putting, however, the difference between the two systems as eleven years, a mistake in which he is followed by Grosseteste and Campanus of Novara, though not by Bacon.

The Computus Computus A medieval science or textbook dedicated to the calculation of time and the dates of religious feasts. of Grosseteste Robert Grosseteste (c. 1175–1253), Bishop of Lincoln and a key figure in the development of scientific thought in England. seems to date from about the same time, but displays more knowledge of Arabic astronomy. He quotes from Ptolemy, Albategni The astronomer al-Battani (c. 858–929)., Thebit Thabit ibn Qurra (c. 826–901)., Azarchel The Spanish-Arab astronomer al-Zarqali (1028–1087)., and Alpetrangius The astronomer al-Bitruji (died c. 1204)., and uses the Toletan tables Also known as the Toledo Tables, these were used to predict the movements of the sun, moon, and planets.. He is the first to propose the use of the Arabic cycle of thirty years for lunar calculations; he points out the errors of the ecclesiastical calculation, but remarks (p. 259) that as the Church has not altered the ancient method of finding the movable feasts he will include it in his treatise.

The last medieval work on the subject is the Great Computus original: "Computus Maior" of Campanus of Novara, which may be dated approximately as about 1270 A.D. It is printed with other works in The Sphere with commentaries original: "Sphera cum commentis", Venice, 1518. The work is somewhat similar to that of Bacon, quoting the same Arabic authors and giving the same tables. Grosseteste is named, but not Bacon. The differences lie in the fact that Campanus had no practical knowledge—he puts the winter solstice at 15 December when it fell on the 14th (in 1270 A.D.)—and that his arguments are needlessly wordy.

The Computus of Roger Bacon is known in two complete manuscripts of the end of the thirteenth century and in an incomplete extract made at the end of the fourteenth century. These manuscripts are the British Museum Royal MS. 7 F. viii, Bodleian MS. Ashmole 347, and Erfurt, Amplon. MS. F. 394. There are also some seventeenth-century excerpts in Oxford and Douai of no authority.

After first four articles in MS. 7 F. viii, which are:
(1) Part five of the compendium of the study of theology original: "Pars quinta de compendio studii theologie",
(2) a passage about the comet of 1264,