An astronomical sextant designed and built by Tycho Brahe, the last of the great naked-eye astronomers. It was built in 1572, in the garden of a house in Augsberg, Germany. The instrument takes its name from the fact that the arc (labelled CB) is one sixth of a circle. It was used for measuring the altitudes of stars (their angular heights above the horizon). The illustration is a copy of one which appeared in Tycho's book Astronomiae Instauratae Mechanica, published in Wandesburgi in 1598.