382 POPULAR SCIENCE MONTHLY. 



meridian. A clock thus regulated is said to show sidereal time. Then 

 the right ascension of any star is equal to the sidereal time at which 

 it crosses the meridian of any point on the earth's surface. Eight as- 

 cension thus designated in time may be changed to degrees and minutes 

 by multiplying by 15. Thus, one hour is equal to 15°; one minute of 

 time is equal to 15' of arc, and one second of time to 1" of arc. 



It may be remarked that in astronomical practice terrestrial longi- 

 tudes are also expressed in time, the longitude of a place being desig- 

 nated by the number of hours it may be east or west of Greenwich. 

 Thus, Washington is said to be 5h. 8m. 15s. west of Greenwich. This, 

 however, is not important for our present purpose. 



The first astronomer who attempted to make a catalogue of all the 

 known stars is supposed to be Hipparchus, who flourished about 150 

 B.C. There is an unverified tradition to the effect that he undertook 

 this work in consequence of the appearance of a new star in the heavens, 

 and a desire to leave on record, for the use of posterity, such informa- 

 tion respecting the heavens in his time that any changes which might 

 take place in them could be detected. This catalogue has not come 

 down to us — at least not in its original form. 



Ptolemy, the celebrated author of the 'Almagest,' flourished a.d. 

 150. His great work contains the earliest catalogue of stars which we 

 have. There seems to be a certain probability that this catalogue either 

 may be that of Hipparchus adopted by Ptolemy unchanged, or may 

 be largely derived from Hipparchus. This, however, is little more than 

 a surmise, due to the fact that Ptolemy does not seem to have been a 

 great observer, but based his theories very largely on the observations 

 of his predecessors. The actual number of stars which it contains is 

 1,030. The positions of these are given in longitude and latitude, and 

 are also described by their places in the figure of the constellation to 

 which each may belong. Not unfrequently the longitude or latitude is a 

 degree or more in error, showing that the instruments with which the 

 position was determined were of rather rough construction. 



So far as the writer is aware, no attempt to make a new catalogue 

 of the stars is found until the tenth century. Then arose the Persian 

 astronomer, Abd-Al-Rahman Al-Sufi, commonly known as Al-Sufi, who 

 was born a.d. 903 and lived until 98G. Nothing is known of his life 

 except that he was a man celebrated for his learning, especially in 

 astronomy. His only work on the latter subject which has come down 

 to us is a description of the fixed stars, which was translated from the 

 Arabic by Schjellerup and published in 1874 by the St. Petersburg 

 Academy of Science. This work is based mainly on the catalogue of 

 Ptolemy, all the stars of which he claimed to have carefully examined. 

 But he did not add any new stars to Ptolemy's list, nor, it would seem, 

 did he attempt to redetermine their positions. He simply used the 



