26 OUR OWN WORLD 



As the axis of the earth is inclined to a line drawn from 

 the earth to the sun, the light the earth receives is similar 

 to that received by the globe in the last part of the experi- 

 ment. Thus the days and nights vary in length during 

 the year, because in summer the northern hemisphere is 

 inclined toward the sun and in winter away from it. 



The Movement of the Earth around the Sun. The earth 

 not only turns on its axis every day, but it travels around 

 the sun, continually changing its position in relation to 



the stars. It moves with the 

 tremendous average velocity 

 of about 19 miles a second. 

 It is this revolution around 

 the sun which gives us our 

 measure of time which we 

 call a year. It takes 365 

 days and a fraction to com- 

 FIGURE 6. DRAWING AN ELLIPSE P lete this revolution; and so 



we consider 365 days to be a 



year, and add a day practically every fourth year to 

 account for the fractions. 



In the journey around the sun, the earth does not move 

 in a circle but in an ellipse. To draw this figure, stick 

 two pins into a piece of cardboard, a short distance apart. 

 Place over the two pins a loop of string, and with the 

 point of a pencil draw the loop taut as in Figure 6. If the 

 loop is kept taut as the pencil point moves around the two 

 pins, the .resulting curve will be an ellipse. 



The points where the pins pierce the cardboard are called 

 the foci. Draw a straight line to join the foci, and extend 

 the line to cut the ellipse at two points. Now place a small 



