Johannes de Sacrobosco
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 is notable for three things. First, 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 (to judge from the number of manuscript copies that survive today). Second, he 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. Third, in his longest and most original book, written more than three centuries before the introduction of the Gregorian calendar, Sacrobosco correctly described the defects of the then-used Julian calendar and with a pretty good degree of precision he recommended what was essentially the 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 with plausibility and each with lack of adequate evidence.
Place of birth 
The country in which he was born is uncertain. In 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 appelative de Sacrobosco. Long after his death, Johannes de Sacrobosco was called and sometimes still is called by the name John of Holywood, 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. Pedersons 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
- De Sacrobosco qui computista Joannes
- Tempora discrevit, iacet hic a tempore raptus.
- Tempora qui sequeris, memor esto quod morieris.
- Si miser es, plora: miserans pro me procor ora.
Tractatus de Sphaera 
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. It was required reading by students in all Western European universities for the next four centuries. Though principally about the heavens it contains a clear description of the Earth as a sphere and its popularity shows the nineteenth-century opinion that medieval scholars after this date thought the Earth was flat as a fabrication (See: Flat Earth).
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 
De Anni Ratione 
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 AD. 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 365.2425 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 was not quite as accurate as that 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 astronomy sources, but not the same sources, and there is no indication in his writings that he could read Arabic.
- Tractatus de Sphæra - in English translation
- Algorismus, or De Arte Numerandi, printed without date or place [1490?], and at Vienna, 1517, by Hieronymus Vietor; Cracow, 1521 or 1522; and Venice, 1523
- De Anni Ratione, or De Computo Ecclesiastico, printed, Paris [1538?], 1550, 1572, 8vo.
Notes and references 
- Pedersen, "In Quest of Sacrobosco"
- "The Spread of the [Hindu-Arabic] Numerals in Europe" in The Hindu-Arabic Numerals, by D.E. Smith and L.C. Karpinski, year 1911, pages 58-59 and 134-135.
- Johannes de Sacrobosco biography by the University of Cambridge.
- "Joannes de Sacrobosco". Catholic Encyclopedia. New York: Robert Appleton Company. 1913.
- Johannes de Sacrobosco biography by the University of St. Andrews
- Parish of Holywood
- Pedersen, Olaf (1985). "In Quest of Sacrobosco". Journal for the History of Astronomy 16: 175–221. Bibcode:1985JHA....16..175P.
- Holywood or Halifax, John in Dictionary of National Biography, 1885–1900.
- Online Galleries, History of Science Collections, University of Oklahoma Libraries High resolution images of works by and/or portraits of Johannes de Sacrobosco in .jpg and .tiff format.
- Tractatus de Arte Numerandi by Johannes de Sacro Bosco (in Latin).
- (Dutch)Dutch translation of De Anni Ratione