The Texas Almanac for 1870, and Emigrant's Guide to Texas Page: 11
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ASTRONOMICAL CALCULATIONS.
THE SPOTS ON THE'SUN,
which can often be seen without a telescope, through a smoked glass, are commonly sup-
posed to be black, but are said by a recent careful observer to be of a deep red- color. On
one occasion, as two observers at different points were looking at a spot on the Sun, they
saw a brilliant body pass across it, like a large Ifeteor; and at the same instant the mag-
netic needles at the observatory at Kew were violently deflected--a fact which, with others,
is supposed to indicate the incessant magnetic action-of the Sun upon the Earth.
TO GET CORRECT TIME.
WrEN the shadow cast by the Sun reaches the noon-mark, set the clock, before or after
12, at the time given in calendar pages of this Almanac, in the column of "Sun Fast or
Slow," and it will be exactly right. If a meridian line is used instead of a noon-mark, the
passing the lines by the Sun's centre is the moment for setting the clock. Any skillful sur-
veyor can make a noon-mark or meridian lines of small bras or copper wires. In doing
so, he must allow for the variation of the magnetic needle from a true or astronomical north
and south line.
TRANSITS OF VENUS.
VENUs revolves round the Sun in about seven and a half of our months, at the distance
of about 66,000,000 of miles, while that of the Earth is about 92,000,000. Theplane of Venus's
orbit cuts that of the Earth's at an angle of only 3 23' 30'; and though enus crosses the
Earth's plane twice in each of its years, it is only at long intervals that the Earth is in a line
with Venus and the Sun at the hour of this intersection--when that planet is seen crossing
the face of the Sun, like a small, round, black spot. These transits occur at regular intervals
of 121 years, 8 years, 105M years, and 8 years; then 121N years again. The first that was
observed was on December 6th, 1639; the next on June 3d, 1761; and the last on June 3d, 1769,
one century ago. The next will occur in December, 1874, after a lapse of 1053 years from
the last. In 1769, such a deep interest was felt in the expected transit, and in the questions
of science it would help decide, that the European sovereigns sent out scientific observers
to Lapland and Kamtschatka, to St. Helena, India, and the Cape of Good Hope, and the
celebrated Captain Cook to the Sandwich Islands. A deeper and more general interest will
doubtless be shown in the approaching transit in 1874.
TRUE TIME.
Two kinds of time are used in Almanacs--clock or mean-time in some, and apparent or
sun-time in others. Clock-time is always right, while sun-time varies every day. People
generally suppose it is twelve o'clock when the sun is due south, or at a properly made
noon-mark. But this is a mistake. The Sun is seldom on the meridian at twelve o'clock;
indeed this is the case only on four days of the year: namely, April 15th, June 15th, Sept.
1st, and December24th. In this Almanac, as in most other almanacs, the time nsee is clock-
time. The time when the Sun is on the meridian or at the noon-mark is also given to the
nearest second for every day in the year in the fifth column of each calendar page. This
affords a ready means of obtaining correct time and for setting a clock by using a noon-
mark, adding or subtracting as the Sun is slow or fast.
Old-fashioned almanacs, which use apparent time, give the rising and setting of the Sun's
centre, and make no allowance for the effect of refraction of the Sun's rays by the atmo-
sphere. The more modern and improved almanacs, which use clock-time, give the rising
and setting of the Sun's upper limb, and duly allow for refraction. The practice of setting
timepieces by the rising or setting of the Sun or Moon is not strictly correct, as the uneven-
ness of the Earth's surface and intervening objects, such as hills and forests, near the
points of rising and setting, occasion a deviation in every place from the time expressed
m the almanac, which time is adapted to a smooth, level horizon. The only means of
keeping correct time is by the use of a noon-mark, or a meridian line.
TO ASCERTAIN THE LENGTH OF THE DAY AND NIGHT,
AT any time of the year, add 12 hours to the time of the Sun's setting, and from the sum
subtract the time of rising, for the length of the day. Subtract the time of setting from 12
hours, and to the remainder add the time of rising next morning, for the length of the night.
LEAP-YEAR.
Evary year the number of which is divisible by 4 without a remainder, is a leap-year,
except the last year of the century, which is a leap-year onlywhen divisible by 400 without
a remainder. Thus the year 1900 will not be a leap-year.
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The Texas Almanac for 1870, and Emigrant's Guide to Texas, book, January 1870; Galveston, Texas. (https://texashistory.unt.edu/ark:/67531/metapth123775/m1/13/: accessed May 7, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting Texas State Historical Association.