Evening star., 1901-11-15
TIME FROM STARS
The Sidereal Clock at the Naval
Observatory.
TRANSIT OF SHINING SPHERES
Methods Employed by Government
Astronomers,
BSOLUTE ACCU r RACY

| Modern methods of reckoning and Keep
ing time are a product Of three commercial
and scientific factors-the railroad, the
telegraph and the telescope. To the rail-
road we owe our uniform system of stand
ard time, before the adoption of which, in
1888, every city and farmhouse had
standard of its own. or course, until that
| order of affairs was superseded there was
no such thing as a moment common to
| everybody's clock dial Nor would it be
possible to mark a common moment fnstan-
taneously throughout a broad area Ike that
| of the United States without the aid Of the
| telegraph.
But it is in the preparation of the time
message that the telescope does its essen
| tial work; for it is of prime Importance
| that the noon second telegraphed out daily
from the naval observatory should be the
collect second. 1t is calculated. therefore.
not from the sun, nor from the plants, but
1--WE go an infinite distance into the heavens
and get it directly from the fixed stars.
| Telegrams From the skies.
Devoted exclusively to this purpose at
| the naval observatory is a transit instru-
| ment having a five-and-a-t third-inch object
lens-a telescope mounted firmly on hori-
4 zontal axis extending east and west, so
| that when it is revolved thereon its range
| of vision describes the meridian of the place
| where il is located. Just to one side lies
| an open volume of the American Nautical
| Alamac in which is given large list or
| stars convenient for transit observations
| together with the exact time that each
| crosses the Washington meridian every
night or day in the year When the OBJ
| server is ready for his work, one of these
| stars is selected which is scheduled to
| cross the overhead circle within q min
! ute or two, reasonable time being left in
| which to set the telescope across the stars
path.
On the night that are visiting the
] observatory, the first little stellar world
| chosen to be watched happens to be the
| third star in the constellation of Taurus,
| which will cross the meridian eight and
| three-tenths seconds after Ala o'clock,
| siderial time-ten minutes after pm on
| this particular night by a watch keeping
| eastern standard time. Now, if the sidereal
| clock pendulum shall make the eighth
| stroke after Ala just three-tenths OF a
| second before Gamma Tauri crosses the
meridian. then the clock is correct; but If
| the eighth second be licked before or after
| that instant, then the clock is so much too
4 fast, or too slow, as the case may be.
! This is the point to be determined.
On the lower end of the transit instru-
| ment is a circular scale, the degrees OF
| which are marked in such small subdivi-
| sions that a microscope is attached ia or
| der to read them. The angle of the stars
] height in the heavens is given in the nau-
/ tical almanac. To this angle the gradu-
| ated scale is set. The great spy glass IS
| now turned on its axis until the spirit level
| on the graduated circle lies exactly hori-
| zontal, whereupon the telescope is SO posed
that the star will pass centrally across Its
| face.
The observer wheels his reclining chair
| underneath and takes his place 1t the eye
piece, holding In one hand portable teie-
| graph key. AS yet he sees nothing but the
thirteen lines stretched across the object
| glass, but in reality strands of spider
| silk just inside the eyepieca. Eleven OF
| these lines are perpendicular and are placed
| at unequal distances apart in order TO pre-
vent the observer from unconsciously tcy-
| ing to mark time as the star passes be-
| tween them. Two are horizontal. Pres
| entity there appears between the horizontal
! lines the horned face Of the little Taurus,
| who unhesitatingly proceeds on his journey
| across the field apparently about as fast
as down feather falls from the ceiling
| toward the floor. As he passes behind a
| vertical line the observer presses the cleo
! trio key; meanwhile a neighboring chrono-
| graph has recorded precisely the moment
| at which the key was pressed, and SO on
| till the eleven wires are crossed. The
| middle spider line marks the exact rEerid-
| ian, but the relative positions Of the others
| are accurately known, and the eleven rec
| ords made in order to decrease possf-
| bic error by averaging. Watching the
| transit Of one star with an instrument hav-
| ing eleven spider lines is equivalent to
| watching eleven stars with an instrument
| having only the middle line. And in this
| manner about six stars are usually OBJ
served. From these there is total of
sixty-six chronograph readings to be aver
| aged, in which result the probability of er-
| ror Is reduced to the minimum
Office of the Chronograph.
| The chronograph is a very interesting In
strument. AS its name implies, its business
| is to record time. A cylinder between six
! and seven inches In diameter lin the par
| tcular instrument describedD is made by
| clockwork to rotate on its axis at the uni
| form rate of one revolution in just sixty
seconds. Around this cylinder IS wrapped
| a sheet of paper resting lightly against
| which is the point of an ink bulb, or foun-
.ni \, -
| cylinder a screw motion carries this Zen
| forward about the eighth Of an inch; so
| that if 1t were not disturbed # would make
perfect spiral line round and round the
| cylinder
But the iron holder of this pen is the ar
| mature of an electro-magnet, which In turn
| is in connection with the sidereal clock.
| So that at each stroke of the seconds PEN
| dulum the electric circuit is momentarily
broken, at which moment a spring draws
| the fountain pen sharply to one side and
] instantly releases Ii. Accordingly, the
pendulum beats its successive seconds a
line is drawn around the cylinder show
| ing saw-tooth intentions about one-third OF
\ an inch apart. This distance is the writ
| ten measure of a second of time.
The fifty-ninth beat does not break the
| circuit, and thereby leaves double-length
space. Thus a glance at the chronograph
record will readily locate the first second
| Of each minute and from this the others
may be numbered consecutively.
Now, the telegraph key In the hand of
| the observer also breaks the electric cir
cult. So that every time this key is pressed
an extra saw tooth is written between two
| Of the regular teeth made by the clock.
These extra teeth are the important ones.
They show at Just what part of what SEQ
ond the star passed each spider line, ac-
cording to the sidereal clock and since the
true time of the stars passage is already
known 1t Is now soon seen just how far
wrong the clock is.
"Personal Equation
The average man is in the habit of think
ing that when a simple occurrence happens
right before his eyes he knows what he
sees and sees it just It is. But astron-
omy has learned that this is not 30. When
men watch so plain thing as a star pass
! ing behind wire they differ In their opin-
ions to the exact moment when II
passed. and 1t is necessary to make mathe-
matical allowance for their personal pecu-
! tiarities I'VE press the key when We
think 1t is there. but 1t IL always somewhere
CONTINUED Oh Nineteenth Page)