Prismatic astrolabe

(engineering) A surveying instrument that makes use of a pan of mercury forming an artificial horizon, and a prism mounted in front of a horizontal telescope to determine the exact times at which stars reach a fixed altitude, and thereby to establish an astronomical position.

A surveying instrument used to make the celestial observations needed in establishing an astronomical position. The instrument (see illustration) consists of an accurate prism, a small pan of mercury to serve as an artificial horizon, an observing telescope with two eyepieces of different power, level bubbles and leveling screws, a magnetic compass and azimuth circle, adjusting screws, flashlight-battery power source, light, and a rheostat to control the intensity of illumination.

A prismatic astrolabe, used to make celestial observations. (U.S. Naval Oceanographic Office)

By using a fixed prism, the instrument measures a fixed altitude, usually 45°. As a rising star increases altitude past that for which the instrument was constructed, the direct image appears to move upward from the bottom of the field of vision to the top. The image reflected by the mercury horizon appears to move downward from top to bottom. At the established altitude the rays produce images at the center of the field of view. A fixed altitude is used to minimize error due to variations from standard atmospheric refraction. Each accurately timed observation provides one line of position.