Yorkville enquirer. [volume] (Yorkville, S.C.) 1855-2006, 1911-02-21
of the same celestial body across the
meridian Is called a day.

have what is known to the astronomer
as a star day, or sidereal day. Tht
stars are fixed, the earth rotates uni
formly on its axis, and as a result the
| sidereal days for all stars are Of equa
length. But which star shall be choser
as the most important in the heavens
so that time shall be reckoned from it
Instead of selecting a star the as
| tronomers have picked upon the verna
equinox, the intersection in the sky
| Of the equator and ecliptic. When 4
! is on the meridian the astronomer!
clock says Oh. ON. Os., the beginning o.
the sidereal day, whose hours run from
Oh. to oath. In a great observatory
the position of the stars are recorded
in time kept by such a sidereal clock
and sidereal time Is of great import.
ance to the astronomer.
Due to the annual revolution Of the
earth about the sun the sun appears
to move continuously eastward among
the stars, completing a journey once
in a year. The sun moves in what Is
called the "ecliptic," and its motion Is
from west to east among the stars
This apparent annual motion of the
sun has very important consequences
Let us consider those onry that refer tc
time.
Imagine q sun and a star on the
70 q r
day. One complete revolution Of the
earth on its axis would bring the
earth's meridian back to the star, and
sldereal day would have elapsed. Ir
parently moved eastward among the
stars, and it is necessary for the earth
to rotate a little further on its axis
to overtake the sun, bring 1t out on the
meridian and complete the solar day
Hence sidereal days are each shorter
than solar days.
AS the sun completes it's Journey ir
year, 1t crosses the meridian one time
less in a year than does a star, and
with respect to the sun the stars gain
one revolution, and hence in a year
there are 1854 solar days, but 1861
sidereal days. Consequently 1t Is read
fly seen that in twelve months a side
real clock will gain day in twenty
four hours, on a solar clock Twenty
four hours yer year means two hours
per month, or nearly four minutes g
day, a second every six minutes.
Sidereal time keeps track Of the mo-
tiohs of the stars (Which is its sole
purpose). and hence it is seen that each
of the stars will come on the meridian
at 10 p. m. on the 1st of February on
the 1st of March it will be on the eri-
dian at p. m. The same thing holds
good for all the fixed stars and con-
stellations.
II all sidereal days are equal to each
other, are not all solar days likewise
Solar days, though longer than side
real days, could be equal in length tc
each other only If the sun moved uni
formly among the stars in a circle al
right angles to the axis Of the earth
This circle in the sky Is the equator
But the sun does not move In the
equator but in the ecliptic, and with
motion in this latter circle which Is not
uniform. Hence a double cause exists
which alters the intervals of time be-
tween successive transits Of the sur
across the meridian, and which we cal
apparent solar days. AS a matter Of
fact, a solar day at one time OF the
wear may be as much as so SECONDS
longer than at another time of the
year.
1t is manifestly impossible to man.
ufacture clock which would keep track
with the actual time kept by the sun
and it became necessary to invent
uniform sun time that clock could
follow. Consequently, the average, OF
the mean of the lengths Of all the ap-
parent solar days for the year, was
taken, and this was called the mean
solar day." An imaginary sun called
the mean sun" is supposed to move IN
the sky which keeps this sort or time
The real sun does not give directly the
time of noon as kept by our clocks.
Clock noon at any observatory diff
fers from apparent noon," when the
actual sun is on the meridian, by an
amount known as the equation OF
time" which may be as great as 10
minutes. A "clock which regulates
the sun" would always be wrong. Ac-
curate, mean solar time may be deter
mined by the astronomer from ooser-
vations on the sun, but. strange as II
may seem, a great observatory like
the United States naval observatory OF
Washington determines it's precise So
lar time by observations on stars.-Ne"
York Yorld.