**John of Holywood**or

**Johannes de Sacrobosco**was educated at Oxford. He became a canon of the Order of St Augustine at the monastery of Holywood in Nithsdale.

In 1220 Sacrobosco went to study in Paris. Although almost all dates for Sacrobosco are guesses we do know one date precisely for, on 5 June 1221, he was appointed a teacher at the University of Paris. Soon after this he became professor of mathematics at Paris.

Sacrobosco promoted Arabic methods of arithmetic and algebra in his teachings. In *De Algorismo* he discusses calculating with positive integers. The work contains 11 chapters, one each on topics such as addition, subtraction, multiplication, division, square roots and cube roots.

In 1220 Sacrobosco wrote *Tractatus de Sphaera* a book on astronomy in four chapters. The first chapter deals with the shape and place of the Earth within a spherical universe. The second chapter deals with various circles on the sky. The third chapter describes rising and setting of heavenly bodies from different geographical locations while the fourth chapter gives a brief introduction to Ptolemy's theory of the planets and of eclipses.

The book, which predates Grosseteste's astronomy book, is well written and was widely used throughout Europe from the middle of the 13^{th} Century. Clavius used in the 16^{th} Century and it was still the basic astronomy text until the 17^{th} Century. It was essentially the first astronomy text to be printed in 1472. Despite its long life as a teaching book Barocius had pointed out 84 errors in the book as early as 1570.

Sacrobosco wrote *De Anni Ratione* in 1232. This book deals with time, studying the day, week, month, year as well as the Moon and the ecclesiastical calendar. He maintains that the Julian calendar is 10 days in error and should be corrected. He suggest a reform of the calendar achieved by omitting one day every 288 years.

He wrote a number of other book including *Tractatus de Quadrante* on the quadrant.

**Article by:** *J J O'Connor* and *E F Robertson*