Robert Brooks, age 13, of San Francisco, Calif., for his question:

What is an astrolabe?

Its name is coined from two Greek words meaning a star and to make or place. The astrolabe was used by Copernicus and Galileo, by Kepler and newcon and countless major and minor astronomers through many centuries. The chief duties of this astounding instrument are now performed by the sextant.

We space agers are reaping the patient work of centuries. Every event in our space program fills our minds with awe and wonder, but the study of the works of past astronomers tends to fill us with wonder of another kind. We feel respect and deep gratitude for the great ones of history who gathered our cosmic data, scrap by patient scrap. The amazing instrument called the astrolabe is part of their story.

Basically, the astrolabe is a metal disk with a movable arm. The metal on both sides is engraved with strange lines and curves. There are graduated degrees, signs and figures. A metal ring is fixed at one point along the rim of the disk, and the unusual instrument looks somewhat like a large, old fashioned vest pocket watch.

The astrolabe can indeed tell the time, the season and the exact date to the expert who can read it. The strange looking instrument is both a clock and a calendar ¬and this is not all. It tells latitude and informs the expert who uses it exactly where he is on the face of the globe.

Such a wondrous gadget, you would think, should be in every home and certainly in the pocket of every traveler. But its long term of service to mankind has finished and its duties have been taken over by other instruments. We have clocks and watches to tell us the time and calendars to tell us the date. Pilots use the up to date sextant to locate their positions. We measure time and the geographical lines of the earth's surface by the motions of heavenly bodies. The astrolabe was an instrument for measuring the exact height or altitude of the sun, the planets and the wheeling stars. Even a beginner could use it to locate the sun's zenith at high noon and figure the compass points. Such experts as newton could use an astrolabe to compute the cosmic laws from scratch

Mankind began to chart the wheeling heavens before the dawn of history, and his instruments have improved through the ages. The complex astrolabe grew from the simple shadow stick. Mankind had to create a mechanical clock to mark the passing hours, and the completed astrolabe rewarded him with a more perfect clock based on the parade of heavenly bodies around the vast dial of the sky.