American citizen. [volume] (Canton, Miss.) 1864-1890, 1873-08-30
VOL. NXIII.
GREENWICH TINE.

treat IMAM treenWlch observatory appear
TO be, this bright summer morning.
For all its pleasant aspect, however
the idea of exploring it is decidedly
formidable one. At the very entranc
gates, one feels suddenly convicted of tn
most abject ignorance. Here are myster
ious metal pins fixed on the wall for TIL
determination of British measurements
and the question at once arises, What hav,
these to do with astronomy? Then ther,
is a great clock dial on which the hour
are reckoned from one TO twenty four, ant
which is popularity believed to be KEEP
going by the sun.
Determined to clear the way as he goe!
on, the visitor makes these outer Difficult
ties the subjects of his first inquiries OF
gaining admittance, and he discovers t.
his amazement that the very length of hi:
trousers, and the cut of his coat, and tho
height of his hat have all been determin
ed by measurements based upon the mo
tions of the heavenly bodies.
A tailor's yard measure, it appears
bears a certain proportion to the length Of
a pendulum which, under specified condi
tons, beats accurate seconds of time, and
seconds of time are determined by astro
nomical observation. If the tailor wishes
to verify his measure he has only to bring
it to the Observatory gate, where he will
to the clock, it is an astronomers clock
and astronomers know nothing of a. M 5
and p. ms; their calculations are suffic
iently complicated without them. The
notion that it is kept going by the sun is,
it need hardly be said, mere delusion.
On passing the outer portal of the OBJ
servatory, the visitor finds himself in an
open courtyard, with an irregular pile of
buildings on his left hand. Entering a
u . hoe Il of
interested to discover that he is really at
what may DE considered the fountain
head of all our computations of time. The
chief business of Greenwich, as all the
world knows, is to tell us the time of day,
and in this small and somewhat mean
looking apartment is the great telescope
by which observations for this purpose
are effected.
This instrument-the transit circle as it
is technically called-is twelve feet in
length, and its largest glass is eight inches
in diameter. 1t is suspended by the mid
die between two massive stone buttresses
in such manner as to permit of its
sweeping the sky in straight line over
head, though it cannot be veered round to
the right or left.
We have arrived, let us suppose, little
before noon the sun is about to cross the
and an observation is TO be
made. Shutters in the roof are thrown
open, the great telescope is swung up and
fixed in position, and an observer seats
himself at the lower end of it. While we
are waiting for the great luminary, let us
take poop through the instrument. All
that can be seen is number of vertical
called wires, though
they are in reality SO many pieces of cob
wed-stretched acr oss the field of obser-
walton at irregular distances. The center
one is the celebrated meridian of Greer
witch, or at all events it represents it, and
it is curious to reflect that from this cen
ter line ships of all civilized nations, and
in all parts of the known world, are reck-
Dning their distances; that this little piece
of cobweb is, practically, all that divides
The world into eastern and western hemi-
spheres.
While we are peering along the tele-
scope, the drowsy tinkling of innumerable
clocks is heard through the still summer
air, and we begin to think that for once at
least the sun is behind time. If not, then
it seems plain that all the Greenwich
clocks are wrong, supposition which is
quite at variance with all our traditional
ideas of the place. On inquiry, it is graft
flying to find that ::our faith in Greenwich
timepieces is perfectly justified, and that
it really is the sun that is behind time.
The apparent motion of the sun, as every
body knows, is really the motion of the
earth. Now the earth moves round the
sun in kind of oval pathway. When
she is on either side of this oval her
motions are accelerated, and the sun will
cross the meridian before he is due. Just
now, however, we are at one end of the
oval, and the earth moves slowly, and, as
we see, the sun is behind his time. 1t is
clear, therefore, that if the Greenwich
clocks were to be regulated according to
The time at which the lord of day puts in
an appearance at this little cobweb, they
would require constant alteration. They
ire, however, set to record the average
ime of his transit. This never varies,
md twelve O'Clock "Greenwich mean
line" is simply the mean or average time
1t which throughout the year the sun
meridian.
Let the observer now resume his watch
1t the instrument. What he has to do is
% record the precise instant at which the
suns edge or limb" as astronomers ex-
press it, passes that central wire" In
any single observation, however, he may
be a little at fault, and for the sake of
greater accuracy, therefore, he will note
the instant at which it passes over all the
wires" and then strike an average be-
tween them.
Slowly the sun creeps up to the first
line, and the observer lightly taps little
spring attached to the telescope. The
second wire" is reached, and again the
spring is tapped, and SO on throughout
the whole of the seven or nine webs en-
ployed in the observation.
The spring is connected with tele-
graphic wire extending to "chrono-
graph" in distant part of the building;
and in order to understand the method OF
the observation, we will now
follow the Geographic signal, or, as imag
ination is even swifter than the telegraph,
we will imagine that we have reached the
''chronograph" first, and are there ready
to receive the signals.
Accordingly we find ourselves in
queer little chamber, in which the most
prominent object is very beautiful speci-
men of clock whose pendulum, instead
of Oscil,ating backwards and forwards,
swings round in a circle, thus pr ducing
a motion perfectly uniform and unbroken.
This clock is revolving the "chrono-
graph" which consists of Il cylinder
around which sheet of white paper has
been strained. While we are watching
this revolving barrel, we see the observers
signals come. A little steel point, which
is travelling over the surface of the paper,
is in electric communication with the
spring attached to the great telescope;
and every time the observer taps the
spring, this little travelling point pricks
into the paper, thus recording that the
sun has just crossed a wire" This in
itself however, would not be a record of
the time of transit if it were not that
another little steel point, whish is in con-
nection with a galvanic clock in another
part of the building, has previously mark
ed the sheet of paper into spaces repre-
sentng precise seconds of time. On the
of the observation the paper
may he removed from the cylinder and