172 A Short History of Astronomy [CH. vi., $ 133 



may be of interest to note that these five books still remained 

 in the edition of the Index of Prohibited Books which was 

 issued in 1819 (with appendices dated as Lite as 1821), 

 but disappeared from the next edition, that of 1835. 



133. The rest of Galilei's life may be described very 

 briefly. With the exception of a few months, during which 

 he was allowed to be at Florence for the sake of medical 

 treatment, he remained continuously at Arcetri, evidently 

 pretty closely watched by the agents of the Holy Office, 

 much restricted in his intercourse with his friends, and 

 prevented from carrying on his studies in the directions 

 which he liked best. He was moreover very infirm, and 

 he was afflicted by domestic troubles, especially by the 

 death in 1634 of his favourite child, a nun in a neighbouring 

 convent. But his spirit was not broken, and he went on 

 with several important pieces of work, which he had begun 

 earlier in his career. He carried a little further the study 

 of his beloved Medicean Planets and of the method of finding 

 longitude based on their movements ( 127), and negotiated 

 on the subject with the Dutch government. He made also 

 a further discovery relating to the moon, of sufficient 

 importance to deserve a few words of explanation. 



It had long been well known that as the moon t!e>cribes 

 her monthly path round the earth we see the same markings 

 substantially in the same positions on the disc, so that 

 substantially the same face of the moon is turned towards 

 the earth. It occurred to Galilei to inquire whether this 

 was accurately the case, or whether, on the contrary, some 

 change in the moon's disc could be observed. He saw 

 that if, as seemed likely, the line joining the centres of the 

 earth and moon always passed through the same point 

 on the moon's surface, nevertheless certain alterations in 

 an observer's position on the earth would enable him to 

 see different portions of the moon's surface from time to 

 time. The simplest of these alterations is due to the daily 

 motion of the earth. Let us suppose for simplicity that 

 the observer is on the earth's equator, and that the moon is 

 at the time in the celestial equator. Let the larger circle 

 in fig. 58 represent the earth's equator, and the smaller 

 circle the section of the moon by the plane of the equator. 

 Then in about 12 hours the earth's rotation carries the 



