355 
GREENWICH TIME AND ITS TELEGRAPHIC 
DISTRIBUTION. 
By WILLIAM ELLIS, F.R.A.S., Superintendent oe the Time 
Department, Royal Observatory, Greenwich. 
HE object of our present paper is to describe that system by 
which Greenwich time, as found by astronomical observa- 
tion at the Royal Observatory at Greenwich, is daily transmitted, 
by telegraphic aid, to distant parts of the kingdom ; a subsidiary 
use of the telegraph which, although not directly contemplated 
in its establishment, yields practical advantages of no small 
But before directly proceeding to consider its utilitarian 
applications we must give some account of the manner of 
reckoning and determining time. The astronomical considera- 
tions involved are, however, so fully treated in works on astro- 
nomy that we need only concern ourselves here with the more 
practical aspect of the subject. 
When the sun reaches its greatest daily altitude in the 
heavens we call the time noon, and the interval which elapses 
between one noon and the next we call a solar day. But the 
natural solar day thus measured is (owing to the varying motion 
of the earth in its elliptic orbit, and the inclination of its 
axis of revolution to the same orbit) to a slight extent variable. 
Its length oscillates between certain small limits, which renders 
the ordinary use of such a day for many reasons inconvenient. 
The inequality is fortunately small as compared with the length 
of the day, so that its use in practice is avoided by assuming 
the existence of an artificial solar day — one of uniform length, 
and consequently better adapted to the wants of mankind. It 
is known as the mean solar day. Natural solar time (that 
shown by a sun-dial and variously called “ true ” or “ apparent ” 
solar time) is sometimes rather before and sometimes rather 
after mean solar time (that shown by a clock). Four times in 
each year they are together. The difference usually existing 
between them, which amounts to as much as 16 minutes in 
value. 
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