Hartman's Planispheric Astrolabe

You measured a star's angle above the horizon, then set the rotating star map to read the hour or find other stars.

Date: 1537
Maker: Georg Hartman
Place: Germany: Bavaria, Nuremberg
Materials: brass (overall material)
Dimensions: overall: 22.5 cm x 16.1 cm x 1.3 cm; 8 27/32 in x 6 11/32 in x 1/2 in
Others named in the record: Galilei, Galileo (possible owner)
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. Engraved plates that fit below the rete have scales of altitude and azimuth (arc of the horizon) for specific latitudes. This brass astrolabe has four plates; one may well be a replacement. It was made in Nuremberg by Georg Hartman in 1537. An inscription on the inside of the instrument states that it once belonged to the Italian mathematician and astronomer Galileo Galilei (1564-1642).

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. 146-150. The object is referred to in the catalog as CCA No. 262.

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