After having discovered Jupiter's satellites in January 1610, Galileo assiduously observed them through his telescope to determine their orbital periods, with the aim of using them to measure cosmic time. The brass Jovilabe, which contains four tables showing the mean motion of the satellites, is an extraordinary calculating instrument - which can be used virtually on the touch screen - for predicting the apparent configurations of Jupiter's system at any given moment.

By using the Jovilabe and the telescope in combination, Galileo believed he could solve the problem, crucially important to oceanic navigation, of precisely determining a ship's point of longitude.