MOV 6 1915 



THE 



POPULAR SCIENCE 

 MONTHLY, 



NOVEMBER, 1909 



THE SHIFTING OP THE EARTHS AXIS 



By Dr. SIDNEY DEAN TOWNLEY 



STANFORD UNIVERSITY 



r~pHE earth has two principal motions, one of revolution about the 

 -L sun, the other of rotation upon an axis. The revolution about 

 the sun is accomplished in 365| days at an average speed of nineteen 

 miles per second, or thirty-three times the speed of the swiftest modern 

 projectile. The rotation upon its axis is accomplished in twenty-four 

 sidereal hours, and since the equatorial circumference of the earth is 

 nearly 25,000 miles, a point on the earth's equator has a speed of rota- 

 tion of over one thousand miles per hour. 



In form the earth is an oblate spheroid, a flattened sphere, and the 

 axis about which it rotates coincides very nearly with the shortest axis 

 of the body. If a plane be passed through the center of the earth 

 perpendicular to the axis upon which it rotates, not perpendicular to 

 the shortest axis, this plane will cut the surface in a circle which is 

 known as the equator. One of the two coordinates by which the loca- 

 tion of a place on the earth's surface is designated is its distance north 

 or south of the equator — measured in degrees, not in miles — and this 

 coordinate is called latitude. 



Let the small circle at the center of Fig. 1 represent a section of 

 the earth through the plane of any meridian and the large circle the 

 line in which this plane extended cuts the celestial sphere, supposedly 

 at an infinite distance, P'P" being the direction of the axis upon which 

 the earth rotates and CE the line in which the plane of the equator cuts 

 the given plane. Let be the place of observation and NS the line in 

 which a plane through the center of the earth parallel to the horizon 

 plane at cuts the plane of the meridian. According to the definition 

 the arc EO is the latitude of the place and it is easily seen from the 

 figure that this arc is equal to the corresponding arc on the sky E'Z, 

 VOL. lxxv. 58. 



