DETERMINATION OF LONGITUDE 33 



divided into 60 equal parts called minutes, and each minute 

 can be divided into 60 parts called seconds. 



It will be seen at once that the lengths of the degrees 

 of longitude decrease as the pole is approached, since all 

 the meridians pass through the poles and the distance be- 

 tween the meridians, which is considerable at the equator, 

 becomes nothing at the poles. Of course these lines are 

 simply imaginary lines and do not really exist, but in 

 making a map or a globe we draw them as if they existed. 

 The length of a degree of latitude at the equator is 68.7 

 miles, at the poles, 69.4. The difference is due to the 

 flattening of the earth near the poles. The length of a 

 degree of longitude at the equator is 69.65 miles, at the 

 poles, 0. 



16. Determination of Longitude. Since the earth turns 

 on its axis once in 24 hours, the interval between the pas- 

 sage of successive meridians under the sun will be four 

 minutes (24 x 60 -5- 360 = 4). At a point one degree 

 east of us the noon by local time is four minutes earlier 

 than it is with us, and at one degree west it is four min- 

 utes later. If an accurate clock were set to twelve o'clock 

 when the sun was nearest vertical at a certain place, 

 and were then carried to a place 15 west, it would 

 indicate one o'clock at the more western locality when it 

 was noon at the first place. Or, changing the statement, 

 if the clock indicated one o'clock when the sun reached 

 the highest point, the place must be 15 west of the 

 place from which the clock started. 



Thus we see that by means of an accurate timekeeper 

 we can tell difference of longitude between different 

 places. Every sea captain is provided with one or more 

 accurate clocks called chronometers, which are usually set 

 to the time at Greenwich. Thus all he needs to do to 

 get the difference in degrees of longitude between his 



