THE DETERMINATION OF PRECISE TIME! 
By Str Haroup SPENCER JONES 
Astronomer Royal of Great Britain 
Of the three fundamental physical units, there is an essential 
distinction between the unit of time and the units of mass and length. 
The units of mass and length are represented by material standards 
to which any mass or length can be related, either directly or indirectly. 
But the unit of time cannot be represented by any material standard. 
For practical purposes time can be thought of in the Newtonian sense 
as something which flows uniformly. The passage of time can be 
marked by a clock, and any simple natural phenomenon which obeys 
one definite law without perturbation might be used to mark off equal 
intervals of time and therefore to serve as a clock. The rotation of 
the earth provides us with a natural clock. We shall see later that it 
is not a perfect clock, but that it is sufficiently uniform for almost all 
practical purposes; it has, moreover, the great advantage of never 
stopping. 
We can therefore define the unit of time as the period of rotation 
of the earth. Some reference object must be selected against which 
to measure the rotation. For the purposes of everyday life, time 
must be related to the sun, whose rising and setting gives the alterna- 
tion of daytime and nighttime. The day defined by the rotation of 
the earth with respect to the sun is called the true solar day; it is the 
interval between two consecutive transits of the sun across the 
meridian of any place. With this unit, true solar time is obtained 
by dividing the true solar day into 24 hours and calling the instant of 
meridian passage of the sun 12 hours. The time given by a sundial 
is true solar time. For practical purposes, however, true solar time 
is not convenient; because the motion of the sun across the heavens 
is not uniform, the length of the solar day varies in length throughout 
the year. For civil purposes, therefore, a mean solar day is used, 
whose length is equal to the average length of the true solar days 
throughout the year. The time based on the mean solar day as unit 
is called mean solar time. The relationship between mean solar time 
and true solar time at some particular instant is defined by means of 
1 Sixteenth Arthur lecture, given under the auspices of the Smithsonian Institution April 14, 1949, 
189 
