DETERMINATION OF PRECISE TIME—-SPENCER JONES 191 
ception of mean sidereal time, measured relative to the mean position 
of the First Point of Aries. Actual observation of the stars provides 
the astronomer with true sidereal time, which he then has to correct 
for the nutation to obtain mean or uniform sidereal time. 
The determinations of time by astronomical observations are used 
to control the performance of a standard clock, determining its error 
at a specific instant and the rate of increase or decrease of that error, 
the clock then being used to obtain the time at other instants. This 
usually involves extrapolation to some time subsequent to the latest 
observation. For such extrapolation to be accurate, the time de- 
terminations must not be affected by serious errors and the standard 
clock must be of high precision. The determination of precise time 
therefore involves two problems, the determination with high ac- 
curacy of the time at specific instants and the development of time- 
keepers of very high precision. 
The sidereal time of the transit of a star across the meridian is 
equal to the right ascension of the star. Sidereal time can therefore 
be determined by observing the times of meridian transit of stars of 
known right ascension. The conventional method of making the 
observations has been to use a transit instrument. This consists of a 
telescope, mounted on an axis at each end of which is a cylindrical 
pivot. The pivots rest in fixed bearings, adjusted so that the common 
axis of the pivots is as nearly as possible horizontal and pointing in an 
east-west direction. If the axis of the pivots were exactly horizontal 
and in the east-west direction and if the optical and mechanical axes 
of the telescope coincided, the axis of the telescope would be in the 
meridian plane, whatever direction the telescope was pointing to. 
This ideal condition is never achieved and there are always small 
errors of level, of azimuth, and of collimation. These adjustments 
are liable to continual change; there are slow seasonal changes, as- 
sociated with changes of temperature and possibly also with sub- 
surface moisture; there are also more rapid changes, which are cor- 
related with changes of circumambient temperature and with the 
direction of the wind. To control these changes frequent observa- 
tions of level, of azimuth, and of collimation are essential, which take 
up a disproportionate amount of the observing time. The error of 
collimation can, however, be eliminated if the telescope is reversed 
in its bearings in the middle of each transit, half the transit being 
observed before reversal and the other half after reversal. It is not 
possible to reverse large transit instruments sufficiently quickly and 
it has accordingly become customary to use small transit instruments, 
which can be rapidly reversed, for the determination of time; as it 
is the brighter stars which are observed, a large aperture is not needed. 
There are other factors which have also to be taken into considera- 
tion. The pivots will never be absolutely cylindrical; their figures 
