Astrolabe by Heinrich Höing

The astrolabe is a combination of a rotatable star map, a visor instrument for angular measurement and an astronomical slide rule. It was developed by Greek astronomers between 150 BC and 150 AD. In the Middle Ages, Islamic scholars improved the astrolabe and made it known in Europe. The astrolabe can be compared to a rotating star map. From the south pole of the imaginary celestial sphere, all celestial circles and stellar positions are projected onto a plane laid by the equator (stereographic projection). On a single washer, designed for a certain geographical latitude, height and azimuth lines of the horizon system are depicted. Above it is an artfully perforated disc, the rete, rotatable around the central axis and with the most important fixed star positions as well as the zodiac (ecliptic). A fluctuating ruler (alhidade) with two sighting devices, diopters, on the back of the instrument allows the measurement of elevation angles of celestial bodies. Various hour and date scales allow the universal use of the astrolabe in tasks of position and time determination in astronomy, astrology and surveying.