78 A Short History of Astronomy [Cu. ill, 



their courts were proficient in astrology as in astronomy 

 proper. 



The first translation of the Almagest was made by order 

 of Al Mansur's successor Harun al Rasid (A.D. 765 or 766 

 -A.D. 809), the hero of the Arabian Nights. It seems, 

 however, to have been found difficult to translate ; fresh 

 attempts were made by Honein ben Ishak (P-873) and 

 by his son Ishak ben Honein (P-Qio or 911), and a final 

 version by Tabit ben Korra (836-901) appeared towards 

 the end of the 9th century. Ishak ben Honein translated 

 also a number of other astronomical and mathematical 

 books, so that by the end of the 9th century, after which 

 translations almost ceased, most of the more important 

 Greek books on these subjects, as well as many minor 

 treatises, had been translated. To this activity we owe 

 our knowledge of several books of which the Greek originals 

 have perished. 



57. During the period in which the Caliphs lived at 

 Damascus an observatory was erected there, and another on 

 a more magnificent scale was built at Bagdad in 829 by the 

 Caliph Al Mamun. The instruments used were superior both 

 in size and in workmanship to those of the Greeks, though 

 substantially of the same type. The Arab astronomers 

 introduced moreover the excellent practice of making 

 regular and as far as possible nearly continuous observa- 

 tions of the chief heavenly bodies, as well as the custom 

 of noting the positions of known stars at the beginning 

 and end of an eclipse, so as to have afterwards an exact 

 record of the times of their occurrence. So much import- 

 ance was attached to correct observations that we are told 

 that those of special interest were recorded in formal 

 documents signed on oath by a mixed body of astronomers 

 and lawyers. 



Al Mamun ordered Ptolemy's estimate of the size of the 

 earth to be verified by his astronomers. Two separate 

 measurements of a portion of a meridian were made, which, 

 however, agreed so closely with one another and with 

 the erroneous estimate of Ptolemy that they can hardly 

 have been independent and careful measurements, but 

 rather rough verifications of Ptolemy's figures. 



58. The careful observations of the Arabs soon shewed 



