The Great Equatorial Armillary Instrument, one of the instruments at Tycho Brahe's great observatory at Uraniburg on the island of Hveen, Denmark (now Ven, Sweden). The instrument was used to determine the declination (celestial latitude) of stars. The circle was 272 cm in diameter and its edge was calibrated to one quarter of a minute of arc. The instrument could also be used to measure right ascension (celestial longitude) by comparison with a star of known right ascension. The illustration is from Tycho's book Astronomiae Instauratae Mechanica, published in Wandesburgi in 1598.