116 THE OCEAN RIVER 



These are the Four Positions in which the North Star 

 is helow the Pole 



When the Guards are on the East Arm, the Star is below the 

 Pole one degree and a half. 



When the Guards are on the Line above the East Arm the Star 

 is three degrees and a half below the Pole. 



When the Guards are at the Head, the Star is below the Pole 

 three degrees. 



When the Guards are on the Line over the West Arm the Star 

 is below the Pole half a degree. 



The measurement of north-south position or latitude by 

 meridian altitude of the sun, moon, or stars is fairly simple, 

 because they apparently move only very slowly in a north and 

 south direction. The measurement of longitude or east-west 

 position is far more difficult, since at the equator the heavens 

 appear to travel at the rate of about 15 nautical miles per 

 minute. Fortunately astronomers have from the earliest times 

 been aware of this, and even in the days of Hipparchus and 

 Ptolemy a great deal was known about the time at which heav- 

 enly bodies appeared overhead and the rate at which they 

 moved. Thus the problem of determining longitude really 

 resolved itself into a problem of knowing the exact time taken 

 for the sun or star to reach the longitude of the observer after 

 passing a fixed point. For some time longitude was defined as 

 the distance east or west of Paris, but after some argument on 

 both sides the longitude of Greenwich, England, was generally 

 accepted as the prime meridian, and today longitude is calcu- 

 lated by the time the sun takes to travel from Greenwich to 

 the meridian of the observer. 



The first sound proposal for measuring longitude was that 

 of Galileo, who in 1616 suggested that since Jupiter's satellites 

 moved around it with great regularity they formed, as it were, 

 a universal time standard. Thus the time of passage of the sun 



