Johannes de Sacrobosco  Johannes de Sacrobosco, also written Ioannis de Sacro Bosco ( 1195 – 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, and was discredited by William Camden: Halifax means 'holy hair', not 'holy wood'. 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.