160 A Short History of Astronomy [CH. vi. 



abandon the said opinion," which the Cardinal did.* 

 Immediately afterwards a decree was issued condemning 

 the opinions in question and placing on the well-known 

 Index of Prohibited Books three books containing Copper- 

 nican views, of which the De Revolutionibus of Coppernicus 

 and another were only suspended " until they should 

 be corrected," while the third was altogether prohibited. 

 The necessary corrections to the De Revolutionibus were 

 officially published in 1620, and consisted only of a few 

 alterations which tended to make the essential principles 

 of the book appear as mere mathematical hypotheses, 

 convenient for calculation. Galilei seems to have been 

 on the whole well satisfied with the issue of the inquiry, 

 as far as he was personally concerned, and after obtaining 

 from Cardinal Bellarmine a certificate that he had neither 

 abjured his opinions nor done penance for them, stayed 

 on in Rome for some months to shew that he was in 

 good repute there. 



127. During the next few years Galilei, who was now 

 more than fifty, suffered a good deal from ill-health and 

 was comparatively inactive. He carried on, however, a 

 correspondence with the Spanish court on a method of 

 ascertaining the longitude at sea by means of Jupiter's 

 satellites. The essential problem in finding the longitude 

 is to obtain the time as given by the sun at the required 

 place and also that at some place the longitude of which 

 is known. If, for example, midday at Rome occurs an 

 hour earlier than in London, the sun takes an hour to 

 travel from the meridian of Rome to that of London, and 

 the longitude of Rome is 15 east of that of London. 

 At sea it is easy to ascertain the local time, e.g. by 

 observing when the sun is highest in the sky, but the 

 great difficulty, felt in Galilei's time and long afterwards 

 (chapter x., 197, 226), was that of ascertaining the time at 

 some standard place. Clocks were then, and long after- 

 wards, not to be relied upon to keep time accurately during 



* The only point of any importance in connection with Galilei's 

 relations with the Inquisition on which there seems to be room for 

 any serious doubt is as to the stringency of this warning. It is 

 probable that Galilei was at the same time specifically forbidden to 

 " hold, teach, or defend in any way, whether verbally or in writing," 

 the obnoxious doctrine. 



