El Paso herald. (El Paso, Tex.) 1901-1931, 1915-02-01
Sun Is Not Always on the
Meridian When the Clock
Says 1t Is Noon.
By Prof. S. ALFRED MITCHELL


Director Mccormick Observatory, Uni
versity of Virginia.
IF A LARGE number of people were
asked, "What is meant when in
= daytime a clock says u o'clock -
about 80 out of every 100 would an
swer 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. Nearly
every one has a friend who owns a
valuable watch that keeps excellent time,
and many a time this friend, on being
asked the exact time, has looked at his
watch with a great deal of pride, and
has told the hours, at the same time
Temarking, "This watch regulates the
SUM- If the watch keeps as poor time
as does the sun, it should be sent to
81 matchmaker, for the sun is a wretch
edly bad timekeeper. 1t sometimes Is
as much as a quarter of an hour fast,
/ and at another season it may be an
equal amount slow. in fact, it may be
said thata clock of extreme precision,
said that a clock of extreme precision,
exactly at II when the sun is on the
meridian.
But it will be asked. eff the sun
does not give US our time, what then
MNtG -Ao ENtl NOllE hllOMHs inc LOlt-
ton OF the earth on its axis is our
measure of time, each complete revOlu-
ton marking a day. As the earth rO-
tates, it carries our meridian around
with it, and as the meridian crosses
through the heavenly bodies in success
sion, they are said to transit The in
terval of time between two successive
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 stars are fixed, the earth rotates
uniformly 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? Instead Of selecting a star.
the astronomers have picked upon the
Nernal equinox, the intersection in the
sky Of the equator and elliptic. When
it is on the meridian the astronomers
clock says Oh. ON. Os. the beginning
OF, the sideTcal dav, whose hours run
from Oh. to oath. In a great observatory
the positions 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 journey once in
year. The sun moves in what is called
the "ecliptic," and its motion is from
west to east among the stars This APP
parent motion of the sun has very i'm
portant consequences. Let US consider
those only that refer to time. Imagine
the sun and star on the meridian
together at noon on a certain day.
when the Earth RevOIves.
On9 complete revolution Of the earth
on rs axis would bring the earths
meridian back to the star, and sidereal
! day would have elapsed. IN this inter
val Of time, the sun has apparently
moved eastwards among the stars, and
it is necessary for the earth to rotate
a little farther on it's axis to overtalSe
the sun. bring it on the meridian. and
complete the solar day. Hence sidereal
days are each shorter than solar days
AS the sun completes its journey in
! year. it crosses the meridian one time
less in year than does star, and
with respect to the sun, the stars gain
one revolution and hence i'm a year
there are S65 Ii solar days but 366 IA
sideTeal days. Consequently, it is readi-
ly seen that in , months 3 sidereal
clock viii gain n day or "a hours on
z a solar clock Twenty hours per
year means two hours per month, or
| nearly four minutes a day. a second
every six months.
Sidereal time keeps track Of the mo-
lions of the stars (Which is its sole
purpose). and hence is seen that each
of the stars viii come on the meridian
two hours earlier every month. The
| bright star Sirius is on the meridian at
10 p. m. on the first Of February, on the
first of March it will be on the merid-
ian at p. m. The same thing holds
good for all the fixed stars and con-
stellations.
The Sky Mau.
The Sky Map as here printed, is an
attempt to represent the blue vault OF
heaven by drawing on a piece of pa
per. The heavens form a sphere 1t
is found impossible to correctly repie-
! sent on a plane without distortions the
relative distances and direct ;ons on a
sphere. Various attempts nave been
made but the one given here is about
the most satisfactory. As the readers
well know, the sky Map gives the posi-
ton of the heavens at p. m. on the
first Of the month. At the end of the
month the stars viii have fhe same po-