EXHIBITS > The astronomical measuring instruments of the ancient Greeks >
Tetrantas of Hipparhos (Quadrant)
It was a measuring instrument which was used (in astronomy and navigation) for the calculation of astronomical sizes and (in topography and in construction) for the measurement of terrestrial distances (e.g. the height of a building).
It consisted of graduated in degrees quadrant which had an aiming device at one edge and from its centre hung a plumb bob. The geographic latitude of each place could be found directly by aiming the Pole Star (it was equivalent to the complement angle of the angle that shaped the aiming line with the thread) and indirectly by the measurement of the zenith of some other celestial body (e.g. the Sun).
Later straight lined scales were engraved on the surface of the instrument for the conversion of the (terrestrial) angles in the proportions of lengths but also monthly arcs with curved hour lines for one (or more) latitudes. A mobile button was also added which slid at length of the thread and was regulated depending on the month which was indicated by the zodiac at the edges of the instrument.
The instrument at any given hour of the day could be used as a locator of place while with any given latitude as a sundial. The level astrolabes constitute its evolution.