The Master Grande Tradition Grande Complication is fully dedicated to astronomical timekeeping. Its main asset therefore – and its most striking feature – is the flying tourbillon acting as hour hand, showing not the mean time as found on all wristwatches, but the time unit used by astronomers to keep track of the direction of the stars: sidereal time. A day is basically defined as one rotation of the earth around its own axis. Depending on which point of reference is used, astronomy distinguishes between a solar day and a sidereal day. The former is measured with the sun as reference, the latter uses other, more distant fixed stars. Since the earth orbits around the sun as well as upon its own axis, the Solar day is slightly longer than the Sidereal day, differing by about one degree or almost four minutes. To underline these celestial intentions that make the watch a perfect tool for astronomical observations, a zodiac calendar is placed on the outside edge of the dial together with a month indication to further depict the constellation of the stars at any given time of the year, by means of annual calendar regulating the time measurement and display system.