CHAPTER III 



UNITS OF TIME, VELOCITY AND ACCELERATION 



The Earth's Rotation. The apparent daily motion of the 

 sun. and stars across the sky is a direct consequence of the 

 earth's rotation on its axis. The sun appears to regularly go 

 through certain periodic changes of position. It rises, travels 

 higher and higher into the sky, reaches its highest position, 

 sinks lower and lower, and finally sets. When the sun is at its 

 highest altitude on any day it is due south, and is said to south 

 or be southing. The interval of time between the sun's highest 

 position on any one day to its corresponding position on the 

 next succeeding day is an apparent solar day. These apparent 

 solar days vary in length throughout the year. 



EXPT. 44. Fasten a small rod at right angles to a flat 

 board. Place the board flat on a table so that the rod is 

 vertical. Move a candle in a semicircle above the table, and 

 note the change in the angle that the shadow of the rod 

 makes. Compare the conditions of the experiment with the 

 measurement of the solar day by means of a sun-dial. 



Mean Solar Day. It has been pointed out that the length of 

 days measured by the sun varies throughout the year, hence no 

 single one of these days will do for a convenient standard of time. 

 But if the lengths of all the days in the year be added together, 

 or the length of a year measured by the sun be divided by the 

 number of days in the 3'ear, we obtain an interval of time which 

 is always the same. Such a day, which is of course an imaginary 

 one, is called a mean solar day. Sometimes the mean solar day 

 will be longer than the solar day, sometimes it will be shorter, 

 and occasionally both days will be of exactly the same length. 



