10 



THE SKY. 



started. A third proof of the earth's rotundity is found in 

 the form of its shadow, when the moon is eclipsed by it ; 

 this shadow, as thrown by the earth upon the moon, is 

 circular, in whatever position the earth may be, and a sphere 

 is the only solid form which can in all positions cast a 

 circular shadow. 



The earth turns upon its axis every 23 hours, 56 minutes, 

 and 4 seconds, which constitutes a day, and makes one 

 revolution round the sun in every 365^ days, which make 

 up a year; but the earth's axis is not at a right angle 

 to an imaginary line drawn from the earth to the sun, but 

 at an angle of 23 deg. 28 min. to it, as shown at fig. 8, 



PIG. 8. 



where the line from N to s is the earth's axis of rotation, 

 the dotted line leading to s is the sun's direction, and E Q 

 is the equator. 



The earth travels through space at aerate of more than a 

 million-and-a-half miles per day, besides the distance which 

 each object upon it is carried in its diurnal rotation (24,000 

 miles) . At first thought it would seem impossible that such 

 a rapid motion should not be felt ; but as the air and clouds, 

 and every object belonging to the earth, moves with it, and 

 this motion is perfectly smooth, it can only be perceived by 

 looking at objects independent of the earth. If sailing on 

 the water, when it is quite smooth and the land not very 

 near, the ship seems to be stationary and the objects on 



