Tycho Brahe's Azimuth quadrant, 16th century.
© Science Museum / Science & Society Picture Library
'Quadrans - volubilis azimuthalis': engraving taken from Tycho Brahe's (1546-1601) 'Astronomiae Instauratae - Mechanicae' (1602), showing an azimuth quadrant used by the famous Danish astronomer 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 the actual measurements taken from the sighting arm (A-K) along the radial scale (P-S). His precise measurements of their positions formed the basis of the first new star catalogue to be compiled since Antiquity.