Hevelius and His WifePortable sextants were used to chart a ship’s position at sea by measuring the altitude of celestial bodies above the horizon. Astronomers used much larger sextants to measure the angular distances between two bodies. Here, Johannes Hevelius uses the great brass sextant at the Danzig observatory, with the help of his wife, Elisabetha. Six feet in radius, this instrument was closely modeled on the one used by Tycho Brahe. (f Typ 620.73.451, Houghton Library, Harvard University)