113 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



suspended from the upstretched hand, and directed to any point of 

 the compass, the observer could readily measure the angular differ- 

 ence of elevation of any two stars, mountains, or other objects by sub- 

 tracting their respective angular distances above the horizon line. 



In addition to these features, the astrolabe had a complex frame, 

 called the spider, or rete, upon its face. This frame was filled with 

 points representing the positions of important stars. There were, 

 besides, a number of metal plates packed within the body of the 

 instrument. The proper one to suit the observer's latitude could be 

 selected and exposed next under the spider. On this plate were drawn 

 families of curves used for various purposes. For instance, after 

 measuring the altitude of the sun, a setting could be made whereby 

 the time of day was given. This setting involved the constellations 

 of the zodiac which appeared on a circle, also part of the face of the 

 instrument. On the back of the astrolabe was drawn a system of 

 cross-section lines so that from any setting of the pointers there could 

 at once be graphically computed some desired mathematical result. 



The astrolabe maintained itself as an instrument of astrology, 

 astronomy, and navigation for nearly 2,000 years. It was displaced 

 by the mariner's sextant, invented by John Hadley about 1731. So 

 higlily was the astrolabe appreciated that it is still spoken of with 

 regret that so choice an instrument is not quite up to modern require- 

 ments of accuracy. It is interesting to note that the poet Chaucer 

 wrote a treatise on the astrolabe and its uses. 



Although both the sun dial and the astrolabe were used from 

 antiquity to tell the time of day, neither they nor the clepsydra, or 

 water clock, reached the precision needed for fundamental progress 

 in the theory of astronomy. The positions of the stars are known 

 when their distances north or south of the celestial equator, that is, 

 their declinations, are determined, and in addition the times at which 

 they cross the meridian, which fixes their right ascensions. There- 

 fore, not only must the astrolabe, or its equivalent, be used to measure 

 the culminating angular distance above the horizon of a star to fix its 

 declination, but it is no less important to measure the exact time 

 when the star makes its meridian transit. There was no instrument 

 in the hands of the ancients which could keep accurate time for 24 

 hours. An imperfect substitute for a true clock available to the old 

 astronomers would be to note the relative lengths of clepsydra time 

 between meridian passages of stars, and reduce this as nearly as pos- 

 sible to true time by a network of astrolabe measurements of angles 

 between stars, reduced by trigonometry to east-west projection. The 

 accuracy available in star positions could not have reached a 

 thousandth part of present-day precision. 



