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The Astrolabe
A Portable Calculator and Measuring Tool

Astrolabes served as mechanical maps of the Universe. Sophisticated, hand-held instruments, they were used for centuries to teach people about the sky. The back side had a moveable sighting arm and a scale of degrees for measuring altitude. The front side was engraved with a flattened map of the heavens, which was used with other moveable parts to solve practical astronomical problems.

   
Astrolabe Image (96k JPEG) Islamic Astrolabe, 1090 A.D.

This astrolabe has several interchangeable plates, each engraved with the celestial coordinates for a different latitude. The pointers on the top plate indicate the positions of 22 bright stars. The top plate can rotate to show where those stars will appear at different times or dates, much like a modern paper or plastic star finder. The instrument could also be used to predict when the Sun or certain bright stars would rise or set on any date.

Lent by the National Museum of American History

   
French Astrolabe, circa 1600s

When this astrolabe was suspended from the ring at the top, the sighting arm and scale of degrees on the back could be used to measure the height of the Sun or a star above the horizon. Such measurements could be used to determine latitude or the time of day or night. This astrolabe was made about six centuries after the one above. Has the basic design changed much?

Lent by the National Museum of American History
Astrolabe Image (62k JPEG)
   
Globe Image (96k JPEG)
Islamic Celestial Globe, 1630 A.D.
This brass globe served both as a map of the heavens, as viewed from outside the starry sphere, and as a precision tool for making astronomical calculations. Engraved on its surface are various coordinate lines, constellation figures, and Arabic inscriptions. The stars are made of embedded bits of silver. The globe is hollow and was cast in one seamless piece. It was originally set in a cradle of rings, which depicted the horizon and other astronomical circles.

Lent by the National Museum of American History
 
The Quadrant

A Precision Tool for Mapping the Heavens
Quadrants were used to measure altitudes of celestial objects and were especially useful for mapping the sky. They consisted of a sighting device mounted on a quarter circle, or quadrant, which was engraved with degree markings. Quadrants ranged in size from small hand-held or table-mounted versions to large mural quadrants mounted on walls.

   
Quadrant Astrolabe, circa 1325 A.D.

This rare instrument-only seven dating from medieval times are known to exist-combines the features of a quadrant and an astrolabe. Essentially, the circular face of an astrolabe has been "folded over" twice to create a quarter-circle. This instrument could serve as a measuring tool and perform many of an astrolabe's calculation functions as well.

Lent by the National Museum of American History

Globe Image (96k JPEG)

 

Other Featured Artifacts in this section of the exhibit:
Tycho Armillary Sphere

 

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