The Washington herald., 1911-02-26
WHY THE CLOCKS
DO NOT TELL THE
REAL TIMEOF DAY
By PROF. S. ALFRED MITCHELL,
OF Columbia University.

By PROF. S. ALFRED MITCHELL,
OF Columbia University.
II a large number of persons were
asked the question: What is meant
when in the daytime a clock says IL
o'clock?" about ninety-nine out OF
every hundred would answer that at
exactly that hour the sun is on the
meridian, and it is noon. But the sun
is not on the meridian each day when
the clock is at Il.
But" it will be asked, eff the sun
does not give US our time. what. then
does?' AS every one knows, the rota-
tion OF the earth on it's axis is our
measure of time, each complete revo-
luton marking a day. AS the earth TO
tates it carries our meridian around
with it. and as the meridian crosses
through the heavenly bodies in sue
cession they are said to transit" The
interval of time between two success
siye transits of the same celestial body
across the meridian is called a day.
If the body considered is a star, we
have what is known to the astronomer
as a star day. or sidereal day. The
stars are fixed. the earth rotates uni
formly on its axis, and as a result the
sidereal days for all stars are of equal
length. But which star shall be chosen
as the most important in the heavens,
so that time shall be reckoned from IT?
The "SidereaI Day.
Instead of selecting a star the as
tronomers have picked upon the vernal
equinox, the intersection in the sky of
the equator and ecliptic. When it is
on the meridian the astronomers clock
says Oh. ON. Os., the beginning of the
sidereal day, whose hours run from Oh.
TO 24th. In great observatory the po-
sitions of the stars are recorded in
time kept by such a sidereal clock. and
sidereal time is of great Importance
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 a
year. The sun moves in what is called
the "ecliptic." and its motion is from
the west to east among the stars. This
apparent annual motion of the sun has
very important consequences. Let us
consider those only that refer to time.
Imagine a sun and a star on the me-
One complete revolution of the earth on
its axis would bring the earth's meridian
back to the star, and a sideral day would
have elapsed. In this interval of time
the sun has apparently moved eastward
among the stare, and it is necessary for
the earth to rotate a little farther on
its axis to Overtake the sun. bring it out
on the meridian. and complete the solar
day. Hence, sidereal days are each
shorter than solar days.
AS the sun completes its journey in a
year, it crosses che meridian one time
less in year than does a star, and with
respect to the sun the stars gain one
revolution, and hence in a year there
are 365h solar days, but 366A sidereal
days. Consequently, it is readily seen
that in twelve months a sidereal clock
will gain day, OF twenty-four hours on !
a solar clock. Twenty-four hours per
year means two hours per month, or |
nearly four minutes day, a second every |
six minutes.
Keeps Track of the Stars.
Sidereal time keeps track of the mo-
tions OF the stars (Which is its sole pur-
pOse), and hence it is seen that each of
the stars will come on the meridian two
hours earlier every month. The bright |
star Sirius was on the meridian at 10 D. m |
on February l: on March l it will be on |
the meridian at p. m. The same thing |
holds good for all the fixed stars and |
II all sidereaI days are equal to each |
other, are not all solar days likewise? ? |
Solar days. though longer than sidereal !
days, could be equal in length to each
ltker only if the sun moved uniformly
among the stars in circle at right an
giles TO the axis of the earth. This circle
in the sky is tho equator. But the sun
elliptic, and with motion in this latter
5.rcle which is not unii, I'M Hence a
1ouble cause exists which alters the in
ervals of time between successive tran
its of the sun across the meridian and
which we call apparent solar days As
4 matter of fact, solar day at one time |
1f the year may be as mGck as fifty sec
ends longer than at another time of tie
ear
1t is manifestly Impossible to manu-
acture a clock which would keep track
With the actual time kept by the sun
md it became necessary to invent r:n:-
orm sun time that a clock could follow.
Consequently, the average, or the mean
# the lengths of all the apparent solar
lays for the year, was taken, and this
vas called the mean solar day" An
sun called the mean SUM- is
opposed TO move in the sky which keeps |
his sort of time. The real sun does not /
i've directly the time of noon as kept
" our clocks.
Clock noon at any observatory differs ,
rom apparent noon," when the actual !
un is on the meridian, by an amount | !
nown as the equation of time which
nay be called as great as sixteen min ]
ties.