Archimedes was fascinated by large numbers and addressed the subject in a treatise entitled The Sand Reckoner. In it, he tells King Hiero II of Syracuse that he can calculate how many grains of sand are contained in a sphere with the Sun at its centre and circumscribed by the sky of the fixed stars - in other words the huge sphere of the heliocentric universe of Aristarchus, an astronomer active in the mid-3rd century BC. Eager to conduct far more complex calculations than were possible with the traditional numeral system based on instruments such as the abacus, Archimedes demonstrated the possibility of working with colossal figures.