Aristarchus was a copious writer but only one of his works has survived, which is based on a geocentric view of the Cosmos. The title of his treatise was “On the Sizes and Distances of the Sun and the Moon” and it was published in Oxford in 1688. Using correct geometric arguments, he deduced that the Moon’s diameter was one third that of the Earth’s, while the Sun’s diameter was 20 times that of the Moon, since their apparent angular sizes seen from the Earth were just the same. Although his reasoning was correct, his results were erroneous due to lack of accurate observations. Following Eratosthenes’ calculations of the circumference of the Earth, he concluded that the Moon’s circumference was 14000 km, its true value being only about 11000 km. The fact that the Sun is larger than the Earth was taken as a mere indication of a heliocentric model. The work “On a System of the Cosmos” has been ascribed to Aristarchus. We know via citations in the writings of other scientists that he presented another book in favour of a heliocentric universe, though little evidence can be found about the origin of his ideas on this model.