Johannes de Sacrobosco, also written Ioannis de Sacro Bosco (c. 1195 - c. 1256), was a scholar, monk and astronomer who was a teacher at the University of Paris. He wrote a short introduction to the Hindu-Arabic numeral system which became the most widely read introduction to that subject in the later medieval centuries (judging from the number of manuscript copies that survive today). He also wrote a short astronomy textbook, Tractatus de Sphaera, which was widely read and influential in Europe during the later medieval centuries as an introduction to astronomy. In his longest and most original book, Sacrobosco correctly described the defects of the then-used Julian calendar, and, three centuries before its implementation, recommended a solution much like the modern Gregorian calendar.
Very little is known about the education and biography of Sacrobosco. For one thing, his year of death has been guessed at 1236, 1244, and 1256, each of which is plausible and each lacking adequate evidence.
The country in which he was born is uncertain. In the year 1271 Robertus Anglicus stated that Sacrobosco was born in England. That could be true, yet there is neither good supporting nor good contradicting evidence for it. On the basis of the testimony of someone writing in year 1271, a birthplace in England can be taken as having more likelihood than other possibilities. Among other possibilities, several different tenuous efforts have been made to figure out his birthplace from his appellative de Sacrobosco. Long after his death, Johannes de Sacrobosco was called and sometimes still is called by the name John of Holywood or John of Holybush, a name which was constructed by post-hoc reverse translation of the Latin sacro bosco, where sacro = "holy" (sacred), and bosco = "wood". "Sacrobosco" as such is an unknown town or region. One traditional report, that he was born in Halifax, West Yorkshire, is due to a 16th-century author, John Leland,:176-177 and was discredited by William Camden: Halifax means 'holy hair', not 'holy wood'.:177 Sacrobosco has been identified, by Thomas Dempster, with an Augustinian canon from Holywood Abbey, Nithsdale (in fact a Premonstratensian house); which would be a reason for imagining him to have been born in Scotland.:179 He is also claimed by Holywood, County Down, this being based on a suggestion of Richard Stanihurst. However, Pederson attributes this assertion to Holywood being known to Stanihurst. Pederson's book mentioned that in 1639 James Ware assumed that the birthplace of Sacrobosco was near Dublin. Stanihurst and even Pederson were probably unaware that the seat of the Sacrobosco/Hollywood family in Ireland was in Artane, a suburb of Dublin ("The History of the County of Dublin" by John D'Alton published in 1838). Local historical records in Ireland seem to indicate that Sacrobosco was a member of the Artane Hollywoods and was born in Artane Castle.:177-178 A similar claim is made for Holywood, County Wicklow, though there is no known historical document which supports this.
According to a seventeenth-century account, he arrived at the University of Paris on 5 June 1221, but whether as an arts student or as a licentiate (one having a Master of Arts degree from another university and thus qualified to teach) is unclear.:175-182 In due course, he began to teach the mathematical disciplines at the University of Paris.
The year of his death is uncertain, with evidence supporting the years 1234, 1236, 1244, and 1256.:186-189, 192 The inscription marking his burial place in the monastery of Saint-Mathurin in Paris described him as a computist, one who was an expert on the calculation of Easter.:181
About 1230, his best-known work, Tractatus de Sphaera (On the Sphere of the World) was published. In this book, Sacrobosco gives a readable account of the Ptolemaic universe. Ptolemy's (updated) Almagest had been translated into Latin in 1175 by Gerard of Cremona from the Arabic translation held in Toledo and copies had quickly found their way to Paris. In addition Sacrobosco was able to draw on translations of the Arabic astronomers Thabit ibn Qurra, al-Biruni, al-Urdi and al-Fargani.
Sacrobosco's sphere was the imaginary backdrop of the stars in the sky, which was the meaning of the word 'world' at that time, not the earth as we know it. Though principally about the heavens it also contains a clear description of the Earth as a sphere, in the first chapter. The Sphere was required reading by students in all Western European universities for the next four centuries.
Sacrobosco's Algorismus aka De Arte Numerandi is thought to have been his first work, and the date is estimated at about 1225, and before 1230. The Hindu-Arabic methods of numerical calculation had arrived in Latin Europe during the previous fifty years but had not been disseminated on a wide scale. Sacrobosco's Algorismus was the first text to introduce Hindu-Arabic numerals and procedures into the European university curriculum.:199-200
What Sacrobosco may be most famous for is his criticism of the Julian calendar. In his book on computus, entitled in Latin De Anni Ratione (English: On reckoning the years), dated circa 1235, he maintained that the Julian calendar had accumulated an error of ten days and that some correction was needed.
The Julian calendar was instituted in the 1st century BC. The Julian calendar year contained 365.25 days, with the 0.25 day provided for by a Leap year once every fourth year. However, the more precise length of a solar year is about 365.2422 days. The imprecise 365.25 days had resulted in an accumulated error of about 10 days by the 13th century. Sacrobosco made no proposal on how to get rid of the accumulated error. But looking to the future, he proposed to leave one day out of the calendar every 288 years to prevent continuing error.:209-10
Sacrobosco's data were not quite as accurate as those of the Alfonsine tables which were produced a few decades later in the 1270s based on the Arabic Tables of Toledo. The astronomer Campanus of Novara in 1268 did similar work, again making use of Arabic astronomy sources. Sacrobosco too was influenced by Arabic astronomical sources, but not the same sources, and there is no indication in his writings that he could read Arabic.