GREENWICH TIME AND ITS TELEGRAPHIC DISTRIBUTION. 357 
the sidereal clock is made to register its seconds by punctures 
on paper fixed on a cylinder which revolves uniformly, and on 
which the observer at the transit-circle is able similarly to 
register any transits he may make. He has only to press a 
finger-piece attached to the transit- circle to effect the necessary 
registration. The punctures made by the clock form a scale 
by which the times corresponding to the punctures made by 
the observer are easily ascertained. The times for any transit 
being extracted from the register, and the mean taken, it is 
further corrected as necessary for the small deviations of the 
instrument, and finally for 66 personal equation,” that slight 
constant difference found to exist between even the best ob- 
servers, by taking account of which observations are reduced to 
one standard. By this treatment we obtain the clock times of 
transits such as would have been found had the transit-circle 
been in perfect adjustment and all observations been made by 
one person. 
Now it is of course possible to observe the sun at noon with 
the transit instrument and mean-time clock (taking the mean of 
the transits of the preceding and following borders), and, by aid 
of the equation of time, infer the error of the mean-time clock. 
But this is not the way an astronomer proceeds : he refers to 
the stars. Time can be thereby more accurately determined, 
and stars may be seen at some part of most nights, whilst the 
sun will often be invisible at noon for many days together. 
But the sidereal day differs from the solar day. The length of 
the solar day depends on the revolution of the earth on its axis 
and its advance in its orbit round the sun ; that of the sidereal 
day on the revolution of the earth on its axis alone. The con- 
sequence is that the sidereal day is shorter than the solar day 
by nearly 4 minutes of time ; and therefore a sidereal clock, 
or one that completes 24 hours in a sidereal day, must be used. 
The sidereal day commences when the “ first point of Aries ” 
(on a celestial globe one of the points in which the ecliptic 
cuts the equator) is on the meridian. Mean solar and sidereal 
time coincide once only in each year, on some certain day in 
spring. At other times they differ ; for as the stars shift once 
round in a year as regards the sun or solar day, so the sidereal 
clock in the same period shifts once through the 24 hours 
as respects the mean solar clock, the relation between the 
two being very exactly known. The position of any star is 
known by its right ascension and declination. In a general 
sense these correspond in the heavens to longitude and 
latitude as measured on the earth. In determining time we 
have to do with its right ascension, which is reckoned from the 
celestial meridian of the first point of Aries. And although 
the stars are, as it were, “ fixed,” it is still matter of calculation 
