Evening star. [volume] (Washington, D.C.) 1854-1972, 1927-07-31

however, ever to construct a time
piece which will remain Indefinitely ir
absolute harmony with star time.
Sun Has Complications.
The use of one star-the sun-for
establishing a time standard is very
ancient and has become fixed in the
human mind. When this star Is
directly overhead it is high noon in
the popular mind. Time is measured
by the progress of this star so easily
observable, from sunrise to sunset.
The sun is very good watch, indeed
but is subject to various complications
which make the other and more dis
tant stars better guides when absolute
accuracy is sought.
For the earth not only is turning
on its own axis but is moving around
the sun once year over q vast circle
approximately 100000 miles in dame
ter. SO the suns apparent position
among the stars changes from day to
(LJ. Even if the earth did not rotate
at all on its own axis the sun would
rise and set once 4 year because Of
the earth'S motion around it. Hence
inevitable corrections would be neces-
sary If time observations were taken
from the sun. Some days, even some
hours, would be longer or shorter
than others.
All time calculations are based on
the fact that the earth makes a com
plete revolution in slightly less than
"a hours. The uniformity holds true at
least to the thousandth of a second.
1t will remain the same for centuries
before a correction will be necessary.
Consequently the length Of the
sidereal day. the interval between the
passage of a selected star over the
meridian of Washington and the next
succeeding passage of the same star
over the same meridian, is uniform.
The length of the solar day is not
uniform because Of the apparent
movement of the sun in the heavens.
There are 1854 solar days in a year
and 1864 star days, the extra day
being due to the rotation of the earth
around the sun. The "star" wear is
the time required for the earth to
move from any position in respect to
the fixed stars as seen from the earth
back to the same position again Ir
actual time, it is 20 minutes longer
than the solar year. due to the process
sion of the equinoxes.
1BttlE convenient.
The business of the world, however
must be conducted on solar time. II
is more convenient and has been estab
lished by generations. Sideral time
signals coming over the radio would
mean very little in the average house.
hold or business Office and some math.
ematical calculations would be requir
ed to set watches by them. The Naval
Observatory staff perform these caicu
lations and translate the sideral clock
into terms of the sun clock generally
used. But the signals are based on
mean, rather than apparent solar
time. The sun is treated as il it were
one OF the fixed stars and not q NOV.
ing object.
To make possible the universal use
OF this mean solar time the earth is
divided into time zones extending
westward from the zero meridian al
Greenwich, England, near London,
and arranged approximately one hour
apart. Washington is five zones from,
Greenwich and hence, by mean sola;
time. always is five hourse behind
when the star time is translated into
sun time. Actually, of course, it is
never exactly the same time in Rock
vile as in Washington, but uniforms
ity between zones is assumed by the
time map. Otherwise there would re
an inextricable miX-up and endless
mathematical calculations The time
zone lines of which there are -four
across the United States. do not Pre-