Tycho Brahe's double-arc instrument for measuring angular distances, 1590.
© Science Museum / Science & Society Picture Library
'Arcus bipartitus minoribus - siderum distantus inferviens': engraving taken from Tycho Brahe's (1546-1601) 'Astronomiae Instauratae - Mechanicae' (1602), showing the double-arc instrument that the famous Danish astronomer used at his purpose-built observatory on the island of Hven, near Helsingborg in Sweden. Brahe used this instrument to make naked eye observations of the stars, Moon and planets. It could be moved in both vertical and horizontal directions, with actual measurements taken from the pair of sighting vanes (A-B & I-K) at each end. His precise measurements of their positions formed the basis of the first new star catalogue to be compiled since Antiquity.