American Heritage® Dictionary of the English Language, Fourth
Edition - n. A medieval instrument, now replaced by the sextant,
that was once used to determine the altitude of the sun or other
celestial bodies. Century Dictionary and Cyclopedia - n. An
obsolete astronomical instrument of different forms, used for
taking the altitude of the sun or stars, and for the solution of
other problems in astronomy. The name was applied to any
instrument with a graduated circle or circles, but more
especially to one intended to be held in the hand. Some
astrolabes were armillary spheres of complicated construction,
while others were planispheres intended to measure the altitude
only. One of the most important uses of the astrolabe was in
navigation, for which it was superseded by Hadley's quadrant and
sextant. - n. A stereographic projection of the sphere, either
upon the plane of the equator, the eye being supposed to be in
the pole of the world, or upon the plane of the meridian, the
eye being in the point of intersect