Clearfield Republican. [volume] (Clearfield, Pa.) 1851-1937, 1879-03-19
DIVISIONS OF TIME.
EARLIEST MEASURES oFTIME-wHY TINE
VARIES WITH LONGITUDE.


EARLIEST MEASURES oFTIME-wHY TIMI
VARIES WITH LONGITUDE.
Astronomers usually distinguish three
kinds Of time: Sidereal, or star time
that which is obtained by observing
the disappearance of the same star on
successive evenings behind some sta.
tionary object; mean, or clock time
which is shown by well-regulated
clocks and watches apparent, or dial
time, which is indicated by a meridian
mark on q sun dial.
If the disappearance of star behind
a chimney or other permanent object
is observed several evenings in success.
sion, the intervals will be found
to be exactly the same. Since
the disappearance of the star is caused
by the rotation of the earth on its axis
this shows that its rotation is uniform
and is taken as the unit of time
But civil affairs are regulated by
the rising and setting of the sun; or
in other words, by day and night, and
sidereal time, therefore, is not entirely
suited for the purpose. Recourse, con.
sequently, was had, at very early
period, TO methods for marking the
progress of the sun in the sky. One
of these was perpendicular pole,which
threw its shadow on a perfectly level
surface. The time at which the shadow
was shortest was noon. The distance
between this and the disappearance Of
the shadow was divided into eQual
spaces. This was the origin OF the
sun.dal, Of which we have the first
record in the Bible. But owing tC
the unequal advance OF the earth in
an elliptical, or oval, instead of a cir
cular, orbit, the sun does not arrive al
a meridian mark, or noon, at equal in.
tervals. The Inconvenience arising
from this Irregularity has been over
come by the adoption of mean or clock
time, that is, such as would be showr
by the sun if the earth moved around
il in a circle and at uniform rate.
The difference between mean ol
clock time and sidereal time Is nearly
four minutes, and this gives rise to the
apparent shifting of the heavens in the
course of year. A star rises nearly
four minutes earlier every evening, ol
two hours sooner in q month, and, o
course, sets that much earlier. As {
consequence, those stars which appear
overhead or on the meridian at mid
night at a particular time will, in 81,
months, pass that meridian at noon.
If the earth, while it makes rota
ton on its axis, did not advance in it
orbit, solar time and sidereal tim,
would agree and mean time would no
be needed. This may be illustrated
by the hands Of a clock or watch
The hands are together at twelv,
o'clock, and if the hour hand did no
move they would be together again a
the end OF an hour. But instead o
this, the minute hand has to pass owe
space equal to that which the hou
has advanced before they are in tn
same relative position As the minut
and the hour hand OF a clock NOV,
uniformly, the space is always equal
This is the case with sidereN time, oh
: tanned from the stars, and mean tim
|, derived from an imaginary sun; botl
! are uniform, and the difference be
; tween these, as already stated, is tn
same, nearly four minutes. But if th
hour hand OF a clock, instead OF NOV
ing at uniform rate, moved sometime
slower than at others, or, instead c
moving forward or toward the right
seemed to move backward, it is ev
dent that the time in which the hand
would be together would vary, bein
8 sometimes less than an hour and som;
times greater. Now, this is preeisel
q what takes place with ordinary Q
mean time and apparent or dial tim,
The latter is unequal-at some period
ahead of mean time, at others benin
il hence the expressions, sun fast @
sun slow, which may be found in mos
almanacs. This difference betwce
:, dial time and clock time is called tb
equation of time.
Astronomically the day begins Q
noon, because the sun is then on th
: meridian and can be observed, but tb
day ending at noon would be Uncover
lent and lead to endless disputes in th
affairs Of life. The civil day is Their
fore made to begin at midnight b
most nations, as it is period whe
business has ceased and when th
question whether it is today or ye
terday seldom enters into the transa'
tions Of civilized life.
. Time also varies with a difference
longitude, but not with difference
/. latitude. In other words, places ea'
or west of a given locality have diffe
cnt time, but not those north or soutl
When it is twelve o'clock at Philade
phia, 1t is that hour at every place d
rectly north or south OF it as far Q
the Arctic circles, but nowhere els
This gives rise to the difference b.
: tween schedule time and local tim
q which may be noticed in our railway
J, by local time, or the time Of the plat
!. from which thev start and arrive I