PART II 

 EARTH RELATIONS 



CHAPTER X 

 FORM, MOTIONS, LATITUDE, AND LONGITUDE 



Form. The form of the earth is very much like that of a 

 sphere, but since it is not exactly a sphere, it is called a spheroid. 

 The form has been determined in various ways: (1) Ships have 

 sailed quite around it. This proves that it is everywhere bounded 

 by curved surfaces, but it does not prove that it is a sphere or even 

 a spheroid, for it would still be possible to sail around it if it had 

 the shape of an egg. (2) When a vessel goes to sea, its lower part 

 disappears first, and when a vessel approaches land, its highest part 

 is seen first from the land. By people on the vessel, the highest 

 lands are seen first, and the low ones later; and the spires and 

 chimneys of houses appear before the roofs, and the roofs before the 

 lower parts. Like (1) above, these facts show only that the earth 

 has a curved surface. But from whatever port vessels start, and in 

 whatever direction they sail, objects on land disappear at about 

 the same rate. This means that the curvature is nearly the same in 

 all directions. A body whose curvature is the same in all direc- 

 tions is a sphere, and a body whose curvature is nearly the same 

 in all directions is nearly a sphere. This is the condition of the 

 earth. (3) Again, the earth sometimes gets directly between the 

 sun and the moon. It then casts a shadow on the moon, making 

 an eclipse of the moon, and this shadow always appears to be cir- 

 cular. In these and other ways it is known that the form of the 

 earth does not depart greatly from that of a sphere. 



Size. The circumference of the earth is nearly 25,000 miles, 

 and its diameter nearly 8,000 miles. Since the earth is not a per- 



205 



