thumb|Heavily annotated copy of [[De sphaera mundi|De Sphaera of Sacrobosco.]]
Johannes de Sacrobosco, also written Ioannes de Sacro Bosco, later called John of Holywood or John of Holybush ( 1195 – 1256), was a scholar, Catholic monk, and astronomer who taught at the University of Paris.
He wrote a short introduction to the Hindu-Arabic numeral system. Judging from the number of manuscript copies that survive today, for the next 400 years it became the most widely read book on that subject. He also wrote a short textbook which was widely read and influential in Europe during the later medieval centuries as an introduction to astronomy. In his longest book, on the computation of the date of Easter, Sacrobosco correctly described the defects of the then-used Julian calendar, and recommended a solution similar to the modern Gregorian calendar three centuries before its implementation. That could be true, yet there is neither good supporting nor good contradicting evidence for it. Based on Anglicus writing so soon after Sacrobosco's death, a birthplace in England may deserve greater credence than later suggestions.
Among those other possibilities, several different tenuous efforts have been made to figure out his birthplace from his appellative name de Sacrobosco. Long after his death, Johannes de Sacrobosco was called and sometimes is still called by the name "John of Holywood" or "John of Holybush", a name which was constructed by post-hoc reverse translation of the Medieval Latin sacer boscus, "holy (sacred) wood". Sacer Boscus or Romance Sacro Bosco as such is an unknown town or region. One traditional report, that he was born in Halifax, West Yorkshire, is the speculation of a 16th-century antiquary, John Leland, which was discredited by William Camden: Halifax means "holy hair", not "holy wood". The historian John Veitch claimed that he was born in Galloway and studied the classics among the monks of Whithorn and Dryburgh.
Based on a suggestion by Stanihurst, Holywood, County Down also claims Sacrobosco. However, Pedersen attributes this assertion to Holywood being familiar to Stanihurst. A similar claim is made that he was born in Holywood, County Wicklow, but there is no known supporting historical document.
Pedersen mentioned that James Ware, writing in 1639, believed that the birthplace of Sacrobosco was near Dublin. Local historical records in Ireland seem to indicate that Johannes de Sacrobosco was a member of the Hollywood family, born in Artane Castle.
The "sphere" Sacrobosco was referring to is the celestial sphere – an imaginary backdrop of the stars in the sky – which was the meaning of the word mundi ("world") at that time, not the planet Earth. Though principally about astronomy, in its first chapter the book also contains a clear description of the Earth as a sphere. De Sphaera Mundi was required reading by students in all western European universities for the next four hundred years.
Algorismus
Sacrobosco's Algorismus a.k.a. De Arte Numerandi is thought to have been his first work, written c. 1225. 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 arithmetical procedures into the European university curriculum.
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