Daily Yellowstone journal., 1891-04-01
TAKING TIME 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 t~elegraphs 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 ST.
dereal clock, as its 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 a brick vault, I'M
deneath the observatory, where the
temperature is as nearly constant as
possible.
Every effort is made TO protect 1t
from any influence which might Affect
its rate" OF, in other words the
amount of its gain or loss per day.
This is necessary IL order that the rate
may be depended upon to give the cor
rect time during spells of cloudy wealth
en-. when no observations can be made
On every clear morning the ITO of
this clock is carefully determined by
observing certain bright stars with an
instument known as the meridian
circle.
Thisinstrument~ consists Of a telescope
mounted on trunnions like a cannon
and supported by a pier of solid ma
sonry. Ii is 50 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
eyepiece 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 70 be
made the observer first selects a suit
able star from a printed star List, which
gives the exact time at which each OF
the principal stars crosses the meridian
He then sets the telescope at the proper
point on the meridian 70 intercept the
star, and putting his eye TO the eye
piece, waits for the star TO appear.
, As the star crosses each of the hne'
before mentioned he presses an electro
; KEY which he holds in his hand TIM
signals thus given are recorded cleo
tricaIly on a registering instrument
: called a ehroooraph. on which, are ais
being ~reoUried the seconds of the nd
; eueal clock. The chrunograph consists
of a cylinder made TO revofve by CLOCK
, work at the rate Of about once a min
ute.
u A paper wrapped around this cyln
= der. receives the record traced by apen
: which is connected by an electro mag
G net in such a manner that M SIGNAL
II by the operator OF tn
= Joct causes the pen to make a mari
4 on the paper.
/, By examining this paper the observe
Is able to tell within a tenth Of sec
snd the time which the hOrnd clocl
= indicated when the star crossed the me
hdian.
Comparing this with the time take
: from the star list shows the error of tn
clock. A comparison Is then made b
tween the sidereal and mean tim
Il clocks, which, after allowing for tn
difference between mean and sideres
time. shows the error Of the mean tim
clock.
, II the clock is slow a slight weight
" placed on the top Of the pendulum
:, bob. which causes the clock 70 gal
slightly. II, on the other hand, Il
- clock is found 70 be fast a correspond
ing weight is removed, making Il
a clock lose slowly. In this way q
O. standard signals are kept within a fe
tenths of a second Of the correct tim
Youths Companion.