160 Trees, Stars, and Birds 



FIG. 109. Yerkes Astronomical Observatory, Lake Geneva, Wisconsin. 



Time measured by the movements of the heavenly 

 bodies. If your clock gained 5 minutes a day, would 

 it matter, provided that every other clock and watch 

 did the same? People could keep appointments be- 

 cause their timepieces would agree. School would begin 

 each morning when the clock pointed to nine o'clock. 

 But after a few weeks a clock gaining 5 minutes a 

 day would point to nine o'clock before sunrise. A little 

 reflection will show that the length of a day is not deter- 

 mined by our clocks and watches, but that our time- 

 pieces must be made to conform to the actual length 

 that the day has. Likewise the length of a year does 

 not depend on any human law or decree. We might 

 agree to call 360 days a year. If we did, after a while 

 Christmas would come in summer and the Fourth of 



