Claude Sanderson, age 15, of Lake Charles, La., for his question:

WHEN WAS THE ASTROLABE FIRST USED?

An astrolabe is an instrument used for measuring the position of heavenly bodies. It was probably first used by the ancient Greek astronomer Hipparchus.

An astrolabe consists of a circle or section of a circle, marked off in degrees, with a movable arm pivoted at the center of the circle. When the zero point on the circle has been oriented with the horizon, the altitude or azimuth of any celestial object can be measured by sighting along the arm.

In the 16th Century, shortly before the invention of the telescope, the great Danish astronomer Tycho Brahe, whose amazingly accurate observations made possible the formation of the present theories of the solar system, constructed an astrolabe with a radius of 10 feet.

Until superseded by the sextant during the 18th Century, smaller types of astrolabes were the principal instruments used by navigators.