Mary Ellen Waithe, Hypatia used an unusual algorithm for division (in the then-standard sexagesimal numeral system), making it easy for scholars to pick out which parts of the text she had written. The consensus that Hypatia's commentary is the source of the additional material in the Arabic manuscripts of the Arithmetica has been challenged by Wilbur Knorr, a historian of mathematics, who argues that the interpolations are "of such low level as not to require any real mathematical insight" and that the author of the interpolations can only have been "an essentially trivial mind... in direct conflict with ancient testimonies of Hypatia's high caliber as a philosopher and mathematician." Cameron rejects this argument, noting that "Theon too enjoyed a high reputation, yet his surviving work has been judged 'completely unoriginal.'" Cameron also insists that "Hypatia's work on Diophantus was what we today might call a school edition, designed for the use of students rather than professional mathematicians." Hypatia also wrote a commentary on Apollonius of Perga's work on conic sections, but this commentary is no longer extant. She also created an "Astronomical Canon"; this is believed to have been either a new edition of the Handy Tables by the Alexandrian Ptolemy or the aforementioned commentary on his Almagest. Based on a close reading in comparison with her supposed contributions to the work of Diophantus, Knorr suggests that Hypatia may also have edited Archimedes' Measurement of a Circle, an anonymous text on isometric figures, and a text later used by John of Tynemouth in his work on Archimedes' measurement of the sphere. A high degree of mathematical accomplishment would have been needed to comment on Apollonius's advanced mathematics or the astronomical Canon. Because of this, most scholars today recognize that Hypatia must have been among the leading mathematicians of her day. Reputed inventions One of Synesius's letters describes Hypatia as having taught him how to construct a silver plane astrolabe as a gift for an official. An astrolabe is a device used to calculate date and time based on the positions of the stars and planets. It can also be used to predict where the stars and planets will be on any given date. A "little astrolabe", or "plane astrolabe", is a kind of astrolabe that used stereographic projection of the celestial sphere to represent the heavens on a plane surface, as opposed to an armillary sphere, which was globe-shaped. Armillary