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THE POPULAR SCIENCE MONTHLY 



the declination of the zenith — declination being defined in a way 

 exactly similar to latitude, i. e., the angular distance of a point on the 

 sky north or south of the celestial equator. Latitude is usually desig- 

 nated by the Greek letter $ and it may be seen from the figure that a 

 third definition of latitude is the angular distance of the celestial pole 

 above the horizon — the altitude of the celestial pole. 



Many methods of determining latitude have been devised, some of 

 them coming down to us from the ancient Chaldean and Egyptian 

 astronomers. The simplest method is to measure the altitude of the 

 sun at noon on the day it passes through the equinox. On that day, 

 the sun will cross the meridian at the point E', and its altitude will then 

 be 90° — 4>, as may be readily seen from the figure. A rough value of 



Fig. 1. 



this angle may be obtained by measuring the shortest shadow of a 

 vertical stick on a level piece of ground on the day of the equinox. The 

 height of the stick divided by the length of the shortest shadow is the 

 tangent of the complement of the latitude. 



If the earth be considered a rigid body and the axis upon which it 

 rotates be fixed within the body of the earth, the latitudes of all places 

 upon its surface will remain always the same. If, however, the axis 

 should shift its position within the earth, then the equatorial plane, 

 which must be always perpendicular to the axis, must shift and conse- 

 quently the latitudes of all places on the earth's surface must change 

 accordingly. 



It is well established that, at least during historic times, no changes 

 of any considerable magnitude have occurred, in the latitudes of places 

 on the earth. It has long been suspected by astronomers, however, that 



