24 FIRST YEAR COURSE IN GENERAL SCIENCE 



15. The Cause of Seasons. The unequal length of day 

 and night and the change of seasons depend on the position 

 of the earth's axis. To illustrate conditions, draw upon a 

 table as large a circle as possible, to represent the orbit or 

 path of the earth around the sun. The table is the plane 

 in which the circle lies, that is, the plane of the orbit. 

 At opposite sides of the circle, mark Spring and Autumn. 

 Darken the room and place a candle or low lamp in the 

 center of the circle to represent the sun. Let a tennis ball 

 represent the earth. Mark opposite points on the ball N 

 and S. Pass a knitting needle through these two points, as 

 the axis. Holding the axis perpendicular to the table at 

 Spring, we see that the ball is lighted from the point N to 

 the point S, most brightly half-way between. Keeping the 

 axis perpendicular, move slowly around the circle, turn- 

 ing the ball at the same time. It is all the time lighted 

 just as at first one half light, the other half dark, with 

 the points N and S marking the extent of light in each 

 direction. 



If, now, the axis is tipped so that it makes less than a right 

 angle with the table, and the ball is placed at Spring, we see 

 that it is, as before, lighted from N to S and most brightly 

 at the line half-way between. But as we move around the 

 circle, keeping the axis always pointing in the same direction, 

 conditions change. At one position the north pole is turned 

 away from the sun; then there is no light at the north pole, 

 but there is light all around the south pole. At the opposite 

 point on the circle, the reverse is true. 



The facts in regard to the earth are these: its axis is in- 

 clined 23J to the plane of the earth's orbit, and always 

 points in the same direction almost directly to the North 

 Star. Hence, the north pole is sometimes turned away from 

 the sun, sometimes toward it. On two days, one in March 

 and one in September, the sun's rays fall vertically upon the 

 equator and obliquely 90 north and south of the equator, 



