182 EXPERIMENTAL GENERAL SCIENCE 



149. Distances on the Earth. Since the earth makes one 

 rotation on its axis every 24 hours, we can express distances 

 east and west around the earth in terms of either time, length, 

 or longitude. In north and south directions we can express 

 distance only in terms of length or latitude, because it is the 

 turning of the earth on its axis that gives us the measure of 

 time. The distance around the earth at the equator is about 

 25,000 miles, and since every circle has 360 degrees in it, the 

 length of one degree on the equator would be nearly 72 miles. 

 North and south of the equator, however, the circles grow 

 successively smaller and the length of a degree is necessarily 

 shorter also, but the same number of degrees pass beneath the 

 sun every hour, no matter where located. The great circles, 

 or meridians, which pass north and south through the poles, 

 are all of the same size and there is not this difference in the 

 length of the degrees north and south. Knowing the length 

 of a degree on a given circle, one can readily determine the 

 distance in miles between two places on it by the difference 

 in latitude or longitude. 



150. Time on the Earth. As the earth turns about in the 

 sunlight, one-half is always illuminated and one-half always 

 in shadow, but these are never stationary areas. Owing to 

 the earth's motion, light and darkness follow each other 

 around it at the rate of 15 degrees (^24 of 360) an hour, or 

 one degree every four minutes. To an observer on the earth, 

 the sun appears to move from east to west at the same rate. 

 When the sun is exactly overhead at a given place, it is noon 

 at that place as well as at all other places on the same meridian. 

 Fifteen degrees east of this meridian, it would be one hour 

 later, and 15 degrees west, it would be one hour earlier. This 

 is true noon by "sun time." 



161. Time Belts. In a great many places, true noon and 

 the noon registered by the clocks and watches are not the 

 same, the difference between them being often nearly an hour. 



