iv The Earth a Spinning Ball 61 



observation of the Sun, or by observing when the Sun attains 

 equal altitudes, before or after crossing the meridian, and 

 halving the interval of time. To get Greenwich time in 

 remote places is more difficult. Accurate chronometers, 

 very carefully regulated and rated, are usually relied on, the 

 average time shown by two or three instruments being taken 

 as correct. If at noon local time, when the Sun is on the 

 meridian, the chronometer shows that it is 1 1 A.M. Green- 

 wich time, it is evident that an hour must elapse before the 

 Earth has turned sufficiently far toward the east to bring 

 the meridian of Greenwich under the Sun. The interval 

 between the local meridian and that of Greenwich is there- 

 fore I hour's turning or 15 ; and since the Earth is turning 

 toward the east the local meridian must lie 15 E. of that of 

 Greenwich. If at local noon in another place the chrono- 

 meter showed 2 P.M. Greenwich time, it is evident that the 

 Earth has been turning for 2 hours toward the east since 

 Greenwich was under the meridional sun, and the place of 

 observation lies 2 hours of turning or 30 W. The apparent 

 position of the Moon on the star-dome at successive intervals 

 of Greenwich time is given in \heNautical A lmanac,\he Moon 

 thus serving as a clock-hand pointing to the hour. But seen 

 from different parts of the surface of the Earth the Moon is 

 displaced to one side or another, and it is necessary to 

 calculate the angular distance of the Moon from certain stars 

 as it would appear if measured from the centre of the Earth, 

 just as correct time is only shown by a clock when the 

 observer stands in front of it ( 33). When this correction 

 for parallax, as it is termed, is made, the lunar distances 

 give the Greenwich time by a simple calculation and the 

 longitude can be found at once. Since the great circle of 

 the equator, the circle of only half the size of the parallel of 

 60, and the minute circle immediately surrounding the pole 

 are all divided into 360 of longitude, it is evident that 

 while the arc subtending I on the equator is equal to that of 

 a degree of latitude, a little over 69 miles, the arc subtend- 

 ing i of longitude at the parallel of 60 is only 34^ miles, 

 and that close to the pole only a few feet or inches. The 

 parallels of latitude are equidistant from each other, but 



