The western sentinel. [volume] (Winston-Salem, N.C.) 1887-1926, 1891-04-30
TAKING r.a.E FROM THE STARS.

How Cambridge Time" Is Secured in
the Observatory at Harvard.
No timepiece is perfect, and there
are no means on earth of keeping per-
feet time. The stars, however, furnish
the necessary means. At the observa-
tory in Cambridge there are two prin~
cipal clocks employed in keeping the
standard time-the standard mean
time clock, which telegraphs its signals
over the surrounding country, and the
normal sidereal clock, which is The
main standard of the observatory, to
which everything is referred. The 81
dcreal clock, as it's name implies, keeps
sidereal or star time. which gains about
three minutes and fifty eight seconds
per day over mean solar time, with
which we are all familiar
The clock is of the finest workman
ship and is kept in brick vault, I'M
derneath the observatory, where the
temperature is as nearly constant az
possible.
Every effort is made to protect it
from any influence which might affect
its rate" or. in other words, the
amount of its gain or loss per day.
This is necessary in order that tie rate
may be depended upon to give the COZ,
rect time during spells of cloudy wealth
err, when no observations can be made
On every clear morning the error of
this clock is carefully determined by
observing certain bright stars with an
instrument known as the meridian
circle.
This instrument consists of a telescope
mounted on trunnions like 4 cannon
and supported by a pier of solid ma
sonry. Ii is SO arranged that it can be
directed toward any point on the me
ridian, but cannot be pointed away from
the meridian line On looking into the
eyepiecc Of this telescope one sees a
series of fine parallel lines running
north and south across the field of
view, the middle line marking the
meridian.
When a time observation is to be
made the observer first selects a suit
able star from 4 printed star list, which
gives the exact time at which each of
the principal st,,rs crosses the meridian
He then sets the telescope at the proper
point on the meridian to intercept the
star, and putting his eye to the eye
piece, waits for the star to appear.
AS the star crosses each of the lines
before mentioned he presses an electric
key which he holds in his hand. The
signals thus given are recorded cleo
trically on registering instrument
called Il chronograph, on which are also
being recorded the seconds of the sid
ereal clock. The chronograph consists
of a cylinder made to revolve by clock
work at the rate of about once a min.
ute.
4 paper, wrapped around this cylin
der, receives the record traced by pen
which is connected by an electro mag
net in such a manner that any signal,
made either by the operator or the
clock, causes the pen to make mark
on the paper.
BY examining this paper the observer
is able to tell within tenth of a sec
ond the time which the normal clock
indicated when the star crossed the me
rdian.
Comparing this with the time taken
from the star list shows the error of the
clock. y comparison is then made be
tween the sidcrcal and mean time
clocks, which, after allowing for the
difference between mean and sidereal
time, shows the error of the mean time
clock.
Ii the clock is slow slight weight i.
placed on the top of the pendulum
bob which causes the clock to gain
slightly. If, on the other hand, the
clock is found to be fast n cor respond
ing weight is removed, making the
clock lose slowly. In this way the
standard signals are kept within a few
tcnths of Il second of the correct time.
-NOuth'S ColuraDioD