French Planispheric Astrolabe

Sighting a star's height on the back, then turning the sky map on the front, told you the time and where other stars were.

Date: 1548
Materials: brass (overall material)
Dimensions: diameter: 15 cm; 5 29/32 in
Museum: National Museum of American History
Credit line: Gift of International Business Machines Corporation

The astrolabe is an astronomical calculating device used from ancient times into the eighteenth century. Measuring the height of a star using the back of the instrument, and knowing the latitude, one could find the time of night and the position of other stars. The openwork piece on the front, called the rete, is a star map of the northern sky. Pointers on the rete correspond to stars; the outermost circle is the Tropic of Capricorn, and the circle that is off-center represents the zodiac, the apparent annual motion of the sun. This brass astrolabe has a body and throne plate, a handle, ring, rete, alidade, pin, wedge, and index arm. It is signed I. Galois. Other inscriptions are in Latin. The scales inscibed on the front of the body is for a latitude of 48 degrees.

Reference:

For a detailed description of this object, see Sharon Gibbs with George Saliba, Planispheric Astrolabes from the National Museum of American History, Washington, D.C.: Smithsonian Institution Press, 1984, pp. 140-144. The object is referred to in the catalog as CCA No. 204.

The museum's own record (Smithsonian, National Museum of American History)