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.!. The equation of true time is therefore the amount of 
time which it 1s 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 which 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 ibeality to that corresponding to 
the same moment at another place, although a very elementary 
1 The word equation is not used here in its mathematical sense; it is equ'valent to the word ‘‘error.” 
