28 FIRST YEAR SCIENCE 



again vertical over the equator, the inclination of the north 

 pole away from the sun decreases. The days when the 

 sun is over the equator are called the autumnal (Sept. 23) 

 and vernal (March 21) equinoxes, since the days and nights 

 are then of equal length all over the earth. 



The greater heating of the hemisphere at one part of the 

 year than at another gives us the changes which we call 

 the seasons. Since the change in the length of the day 

 and in the direction of the sun's rays is very small 

 within the tropics, the change in the amount of heat 

 received is very slight, so that in this region there is 

 almost no change of seasons. But at the poles, where 

 for six months there is continuous night and for six 

 months' continuous day, the change of seasons is exceed- 

 ingly great. At middle latitudes the changes, though 

 marked, are not excessive. 



There are then two causes which combine to give us our 

 change of seasons: the revolution of the earth around the 

 sun, and the inclination of the earth's axis to the plane of 

 its orbit. 



13. The Measurement of Time. Experiment 9. On a fair 

 day place a sundial in an exposed position, and after carefully adjust- 

 ing it, compare its readings with those of an accurate watch. Prob- 

 ably your watch is set to railroad time and the readings therefore are 

 not alike, unless you are on the time meridian. 



Although the exact determination of time is a difficult 

 task and requires great skill and very accurate instru- 

 ments, yet it is not very hard to determine quite satis- 

 factorily the length of a solar day. Before there were any 

 clocks, people told the time of day by sundial (Fig. 11), 

 which consisted of a vertical rod, the shadow of which 

 fell upon a horizontal plane. From local noon, or the 

 time the sun cast the shortest shadow on a certain day, 



