1927 The Texas Almanac and State Industrial Guide Page: 46
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46 THE TEXAS ALMANAC.
MEASUREMENT OF TIME.
In books of science one finds references
to three kinds of time: (1) Sidereal, (2)
true solar and (3) mean solar. In com-
mon usage there are (1) local civil, or
sun, time, and (2) standard, or railroad,
Sidereal Time-With reference to any
fixed star, the earth rotates on its axis
approximately 366.25 times in a year. It
is commonly measured from the vernal
equinrox, -a point in the heavens in the
constellation Aries, the first sign of the
zodiac. It is used in astronomy and many
clocks in observatories are regulated to
True Solar Time-Since the earth re-
volves once around the sun, while rotat-
ing 366.25 times on its axis (rotating in
the same direction that it revolves) it is
plain that the earth rotates on its axis
only 365.25 times-approximately-in a
year with reference to the sun. True solar
time is measured by the passing of the
sun from zenith to zenith. Due to the
obliquity of the elliptic and the lack of
uniformity of the motion of the earth in
its orbit, the true solar day is of uneven
length, hence not practical as a standard
Mean Solar Time-The mean solar day
is obtained by striking a mean (or aver-
aging) the true solar days of the year. It
is measured by a fictitious body known as
the "mean sun," which is supposed to
move from zenith to zenith with abso-
lute uniformity. Thus mean solar days
are of even length and, like sidereal time,
may be measured by clocks and chro-
In Common Usage.
Local Civil Time-This is commonly
known as "sun" time; it coincides with'
mean solar time. By sun time, we mean
that at any point on the earth it is noon
when the sun is at the zenith, and mid-
night when the sun is at the exact op-
posite point. Sun time varies four min-
utes for each degree of longitude. Thus
when it is 10 o'clock at Greenville near
the 96th meridian, it is 9:56 at Dalworth
on the 97th meridian; 9:52 at Mineral
Wells near the 98th meridian, and 9:48 at
Cisco near the 99th meridian.
Standard Time-Local civil, or sun, time
was sufficient for practical needs until
widespread means of rapid transportation
came into use. In 1883, standard, or rail-
road, time was adopted by "leveling off"
sun time into four zones, centering about
the 75th, 90th, 105th and 120th meridians,
respectively. These belts are known as
Eastern, Central, Mountain and Pacific
(see map) and time throughout each is
uniform, and varies by one hour from the
time in the next belt. When it is 12
o'clock noon in the Eastern belt it is 11
a. m. in the Central, 10 a. m. in the Moun-
tain and 9 a. m. in the Pacific zone. Log-
ically, the division lines between the belts
should be straight lines, running from
north to south, half way between the den-
tral meridians, but, due to local prefer-
ences, there is considerable lack of uni-
formity. Thus El Paso is actually west of
the 105th meridian, but due to local pref-
erence, it is included in the Central zone
with the remainder of Texas. Incidental-
ly, El Paso has the distinction of being
that city in the United States in which
there is the widest difference between
standard and sun time. Here it is 12
o'clock noon, standard time, at about
10:55 a. m., sun time.
The regular movements of the earth and
moon have, throughout the history of
man, furnished suitable natural divisions
of time for day, month and year, but his-
torians have had some trouble in arriving.
at a suitable basis for numbering years.
The earth has one long time cycle, the
25,000-year undulation of its poles, but
this was unknown until recent years and,
furthermore, the skip of astronomically
fixed periods from one year to 25,000
years' duration is too great to be of prac-
tical consequence. There have sprung up,
as a result, numerous chronological eras
based upon important events in the his-
tory of mankind. The Romans dated
from the supposed founding of the city;
the Greeks from a certain year of the
Olympian games; the Babylonians from
the era of Nabonazzar, the Jews from the
exodus from Egypt, and the Moham-
medans from the Hegira, or flight of Mo-
hammed from Mecca to Medina. The
chronological era in use throughout the
Christian world today-and many Eastern
countries as well-is the Christian era
dating from the birth of Christ. Many of
the chronological eras have fallen into
disuse except for historical reference;
others are employed primarily in religious
observances, while still others, such as the
Mohammedan, are commonly used today.
It is interesting to note that the
Christian era was not employed until the
sixth century and did not come into wide
usage until the ninth century, the cycle
of Indiction bei.g generally used in
Europe prior to that time.
Below are given correlation for the year
1927, between some of the more promi-
nent eras and the Christian era as meas-
ured on the Gregorian and Julian calen-
dars. The Gregorian calendar is em-
ployed throughout Western Christencom,
the Julian in the East, particularly
Greece and Russia. Jan. 1, 1927, Julian
calendar, corresponds to Jan. 14, 1927,
Gregorian calendar. (See explanation in
preface to Calendar.)
Byzantine Era-Year 7436 of this era
begins Sept. 1, 1927, Julian calendar.
Jewish Era-Year 5688 begins at sunset
Sept. 26, 1927, Gregorian calendar.
Era of Rome-Year .2680 -begins Jan. 1,
1927, Julian calendar.
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1927 The Texas Almanac and State Industrial Guide, book, 1927~; Dallas, Texas. (texashistory.unt.edu/ark:/67531/metapth123785/m1/50/: accessed October 21, 2017), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting Texas State Historical Association.