DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY
Astronomical sextant. Historical artwork of Johannes Hevelius (1611-1687) and his wife Elisabetha (1647-1693) observing with a sextant designed by Johannes in about 1658. Sextants were used for measuring the angle of an object, usually the Sun, above the horizon. Here, Elisabetha is aligning the instrument, while Johannes sets the alidade (index arm).
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