ni. in TIME, VELOCITY AND ACCELERATION 35 



Solar time is known as apparent time, and clock time as mean 

 time. 



Sidereal Day. Just as in the case of the sun so with all the 

 stars, they rise, south, and set. But whereas with the sun the 

 interval between two successive southings varies throughout the 

 year, it is found that the time which elapses between two 

 succeeding southings of a star at any season of the year is 

 always the same. This interval constitutes a star or sidereal 

 day. 



Period of Rotation of the Earth. As the apparent motions 

 of stars across the sky are produced by the rotation of the earth, 

 it will be evident that the exact time of rotation can be deter- 

 mined by finding the interval which elapses between two 

 successive returns of any particular star to the same point of the 

 sky. A star may, indeed, be regarded as a fixed reference mark 

 under which the earth turns ; so that by observation of it w T e are 

 able to determine the time taken by the earth to spin round once. 

 The interval between two successive transits of the same star, or, 

 as it is called, a sidereal day, is the time of such rotation. 



Is'o matter w r hat star is selected for observation the interval is 

 the same, thus showing that the earth is a rigid body, and that 

 all parts of its surface have the same angular velocity. 



Units of Time. The sidereal day, like the mean solar day, 

 is subdivided into hours, minutes, and seconds, but as the latter 

 is four minutes longer than the former, the units are not of the 

 same value. We may take either the mean solar second as the 

 unit of time, or the sidereal second. In the former case the unit 

 is founded on the average length of the solar day, and in the 

 latter upon the length of the invariable star day, or the time of 

 rotation of the earth upon its axis. But in either case the 

 second, that is, the unit of time, is the 86,400th part of the 

 day used. 



In physical measurements the unit of time adopted is the 

 mean solar second, that is, it is founded on the average time 

 required by the earth to make one complete rotation on 

 its axis relatively to the sun considered as a fixed point of 

 reference. 



Instruments for Measuring Time. We need only concern 

 ourselves with the modern contrivances for measuring time, viz., 

 clocks and watches. It will be sufficient to regard these as in- 

 struments for measuring intervals of time in terms of the mean 

 solar day to which attention has been directed, In a clock the 



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