248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



is constant in length and is called the mean solar day. It com- 

 mences at the moment when the center of the sun passes the upper 

 meridian of the place. Mean solar time is used in all the affairs of 

 civil life and our clocks are therefore regulated to it, not to true solar 

 time. In astronomical use, the beginning of the mean solar day is 

 at the upper passage of the center of the mean sun across the meridian; 

 that is, at mean noon; whereas the civil day commences at the mo- 

 ment of the lower passage (under the earth), midnight. In the first 

 instance, we speak of mean astronomical time; in the second, of civil 

 time. The latter is exactly 12 hours earlier than the former. 



The difference between true and mean time is known as the equa- 

 tion of time. 1 The equation of true time is therefore the amount of 

 time which it is necessary to add algebraically to true time in order 

 to get mean time; the equation of mean time, what we must add to 

 mean time to have true time. Accordingly, for the same moment, 

 the equations of true time amd mean time are equal in amount, but 

 opposite in sign. The equation of time varies from day to day, but 

 its greatest value is a little less than 17 minutes. 



SOCIAL CONSIDERATIONS. 



The principal affairs of daily life go on while the sun is above the 

 horizon; that is, during the daytime. The sun, therefore, controls 

 most of our actions, and it is but natural that it should serve to meas- 

 ure our time. Since the equation of time is always less than 17 

 minutes, the difference between the true and mean times is of little 

 importance and brings no inconvenience into civil life. 



All the general facts just stated apply to any place upon the globe. 

 If each place were to adopt the time appropriate to its own meridian, 

 called local time, the consequent diversity of time would result in 

 great confusion. It is therefore advisable for the convenience of 

 social life to adopt some conventional system of time for all the people 

 of a certain region. Their clocks must be regulated to the time of 

 some conveniently chosen meridian; there must be some standard 

 time fixed either by law or custom. The choice of this depends on 

 various considerations. The principal consideration seems to be 

 that this time shall depart as little as possible from local time. In 

 our choice of the meridian by which to regulate our clocks, we should 

 therefore limit ourselves to one winch passes through some central 

 part of the region under consideration; then the difference between 

 the local and the official time will be as small as possible in the ex- 

 treme parts of that region. 



The changing of the time at one locality to that corresponding to 

 the same moment at another place, although a very elementary 



i The word equation is not used here in its mathematical sense; it is equ : valent to the word "error." 



