Johannes de Sacrobosco or Sacro Bosco (John of Holywood, c. 1195 – c. 1256) was a scholar, monk, and astronomer (probably English, but possibly Irish or Scottish) who taught at the University of Paris and wrote the authoritative mediaeval astronomy text Tractatus de Sphaera.
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Although described as English, his birthplace is unknown because Sacrobosco is an otherwise unknown town or region. The belief on his nationality is based on a 1271 statement by Robertus Anglicus.[1] Traditional theories were based on suggestions of antiquarians, without documentary proof.
Sacrobosco is also called John of Holywood, by reverse etymology from the Latin sacro bosco. A traditional belief, that he was born in Halifax, is due to the 16th century author, John Leland,[2]:176–177 and was discredited by William Camden: Halifax[3] means 'holy hair', not 'holy wood'.[2]: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 presuming him a Scot.[2]:179[4] 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. [2]:177–178[5] A similar claim is made for Holywood, County Wicklow, though there is no known historical document which supports this.
The story that he was educated at the University of Oxford is no more documented than the stories on his origin.[2]:177
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.[2]: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.[2]: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.[2]:181
The lunar crater Sacrobosco is named after him.
About 1230, his best known work, Tractatus de Sphaera, 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).
In his Algorismus, theorized to have been his first work, Sacrobosco showed himself to be a strong proponent of the Arab methods of numerical calculation, his Algorismus being the first text to introduce Hindu-Arabic numerals and procedures into the European university curriculum.[2]:199–200
What Sacrobosco may be most famous for is his criticism of the Julian calendar. In his book on computus, De Anni Ratione (1235), he maintained that the Julian calendar was ten days off and that some correction was needed. He made no proposal to correct the accumulated error of ten days but looking to the future, he proposed to leave one day out of the calendar every 288 years.[2]:209–10 In this book, he invented the false notion that Caesar Augustus took a day from February to give to August (see Julian calendar).[6]