This extraordinary publication, the work of the printer, mathematician and cosmographer, Peter Apian, is predominantly an instrument in the form of a book. By realising the Ptolemaic planetary theory in moveable paper volvelles, with small circles (epicycles) that could be rotated and their centres moved along large ones (deferents), Apian produced a set of ‘equatoria’ – instruments, a pair for each planet, for calculating the celestial longitude and latitude of the planets for any time and date. The book was dedicated to the Hapsburg rulers, Emperor Charles V and his brother Ferdinand I of Spain, which explains its adulatory title, ‘The Astronomy of the Caesars’. It was truly a product of astronomical craft.
