Tycho Brahe's mural quadrant. Engraving of the Danish astronomer Tycho Brahe (1546-1601) sitting by his mural quadrant at his Uraniborg observatory on the island of Hven between Denmark and Sweden. The mural quadrant was a device used, together with clocks (lower right) to measure the positions of the planets and stars in the sky. This artwork, dated 1587, is from the 1602 edition of Brahe's book Astronomiae instauratae mechanica. Brahe is pointing at a star seen through the observation hole (upper left), while assistants in the foreground read the clocks, the angle, and note the readings. The background illustrations show various astronomical, mathematical and chemical work. Brahe is said to have carried out the most accurate astronomical observations of his time.