IO 



OUR PHYSICAL WORLD 



ancients, of the earth as floating, half -submerged, on a great 

 sea and as moving about the sun, also floating, half -submerged, 

 the surface of this sea would represent the plane of the ecliptic. 

 The planes of the orbits of the other planets are all nearly in the 

 plane of the ecliptic, that of Mercury being inclined to it at an 

 angle of 7, the others at much smaller angles. 



The axis of the earth, the imaginary line on which it seems to 

 rotate, so producing day and night (see diagram, Fig. 4), does 



FIG. 4. Diagram of the earth in four positions in its orbit about the sun. 

 a and c the summer and winter solstices; b and d, the equinoxes. The relative 

 sizes of the sun, the earth, and its orbit are necessarily incorrect. 



not stand vertically to this plane of the ecliptic but is inclined to 

 it at an angle of 23^. Note that the earth's axis is an imaginary 

 line. The North Pole is not a real pole sticking up out of the 

 earth. When Peary stood at the Pole there was nothing to 

 mark the spot. If he had stood there long enough he would 

 merely have turned about as one would if standing over the 

 pivot of a turntable. 



It is evident that at position a in the diagram the days are 

 long and the nights short in latitude 40 in the Northern Hemi- 



