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.