An alidade is a sighting device or pointer for angle measurement for use with a quadrant. Danish astronomer Tycho Brahe (1546-1601) made naked-eye observations that formed the basis of the first new star catalogue since antiquity. These precise measurements allowed Johannes Kepler to accurately compute the planets' orbits. Though Brahe did not believe the Earth moved around the Sun, his work provided support for the opposing view, known as the heliocentric system, proposed by Copernicus in 1543. In contrast, Brahe thought that all the planets orbited the Sun, which rotated round a stationary Earth. Illustration from Brahe's 'Astronomiae instauratae mechanica' (in English, 'Instruments of the renewed astronomy' or 'Apparatus for the Foundation of Astronomy'), (Nuremberg, 1602).
© Science Museum / Science & Society Picture Library