DETERMINATION OF PRECISE TIME—SPENCER JONES 2(1 
Though the changes in the length of the day have been fully estab- 
lished by these observations, the data are not sufficiently accurate to 
decide whether the changes occur suddenly or whether they are spread 
over a few days, a few weeks, a few months, or even over a year or two. 
If they occur rather suddenly, they could be detected with ease by 
quartz clocks in their present stage of development; if spread over a 
few months, the larger changes could be detected, but changes of 
smaller amount would be likely to escape detection. Since a few 
years ago, when quartz clocks were adopted at Greenwich as the 
basis for the time service, a close watch has been kept for any evidence 
of a change in the earth’s rotation. Once or twice small changes 
have been suspected but there has always been some factor which 
has made a definite conclusion impossible—perhaps one of the clocks 
has changed its rate or has stopped at the crucial time, or there has 
been some uncertainty in the determination of frequency drift. The 
evidence provided by the observations of occultations of stars by 
the moon is that there has been no major change in the earth’s rate 
of rotation since about 1918. There may possibly have been small 
changes, but no definite conclusions are as yet possible. 
It is not inconceivable that there may be small annual variations 
in the rate of rotation of the earth. There are seasonal displacements 
of matter over the earth’s surface; there is, for instance, a high-pressure 
region over Siberia at one season of the year and a low-pressure region 
at another season, entailing the displacement of large atmospheric 
masses, with corresponding change in the moment of inertia. Such 
effects would be tangled up with effects due to periodic errors in star 
places and with the effects of the polar motion. Much more is likely 
to be learned about these matters when the atomic clock has reached 
a further stage of development, so that the frequency drift of the 
quartz crystal can be eliminated. Observations with photographic 
zenith telescopes should gradually smooth out any residual periodic 
errors in star places, while the information they provide about the 
variation of latitude will furnish basic data which can be used subse- 
quently to separate polar motion effects from small variations in the 
earth’s rotation. It may prove, however, that the earth itself is 
rather like a pendulum clock in its behavior and that its rate of rota- 
tion is liable to frequent and small irregular changes, so that we can 
at present merely observe their integrated effect. 
The question may arise in the near future how the unit of time 
should be defined. Clocks are now at a stage when their stability 
for short periods is of a higher accuracy than the earth’s rotation 
itself. The earth, however, has the advantage over any clock that 
it has no liability to a stoppage. It may be possible to develop 
atomic clocks to a stage at which they can be run for several years 
