Triquetrum

The triquetrum, whose name comes from a Latin word meaning three cornered, was an ancient astronomical instrument first described by Ptolemy in the Almagest in the second century AD. It consisted of a vertical post carrying a graduated scale and two pivoted arms hinged at top and bottom, the upper arm fitted with sights that an observer aligned on a celestial body so that the angle of the lower arm could be used to calculate the object's position, particularly the parallax of the Moon. It remained one of the most widely used instruments for measuring the altitude of celestial objects until the invention of the telescope, and astronomers including Copernicus and Tycho Brahe relied on it in their observations.