Year: 1874

The Approaching Transit


The army of astronomers destined for the
observation of the great transit OF Venus Is
in motion and rapidly advancing to the vari-
GUS positions assigned it. No astronomic
phenomenon of the century has attracted SO
much attention or called forth such lavish
labor. The United States, by the confession
of all, has most largely embarked in this high
scientific emprise, and her observers Will
occupy stations the most difficult OF access
and least pleasant of tenure, but most promis-
ing Of rich results, both in the Northern and
Southern Hemisphere. These stations are dis
tributed, three in the Northern and five in the
Southern Hemisphere, and selected as suitable
for observing the entire passage Of Venus
across the suns face. Russia's corps OF OBJ
servers is next in importance, and though
their stations are to be all within Russian
territory tho field will be ample for the most
satisfactory research. The English will have
seven parties at work, the Germans at least
four and the French five or, possibly, six.


To comprehend the nature of this interna-
tional cooperative work it is necessary to note
the progress made in transit observations
since 1859, when the famous jeremiah Hor-
rocks and his friend Crabtree witnessed the
first transit which astronomy had predicted.
The prime object of the transit observation is
to determine the exact distance Of Venus from
the earth, since in the determination of this
distance are wrapped up the solutions OF the
several important problems Of solar astronomy
SO long sought after by science. There are
four methods devised for obtaining the grand
result, oil, however, based on the fact that
Venus as seen from different terrestrial view
points at any instant of her transit will ap-
pear projected on different parts of the suns
face. If two widely separated observers
should at the same instant find the exact ap-
parent place of Venus on the suns face
comparison of observations would show how
much the star was displaced in direction, and
the distance between the observers being
known and serving as a base-lne the required
distance Of Venus from the earth is easily
calculated. This is the direct method Of
transit observation.


But the nicely required in measuring the
distance between the two observers and the
difficulty of obtaining it in the last century
led HALEY to devise another This famous
scientist proposed that two observers. one at
northern and the other at southern
station, should note the exact time
the star consumes in completing its transit
across tho suns face, along the two different
paths she will seem (as viewed by them) to
follow, and thence to deduce the length Of
these two apparent paths, which affords the
very displacement Of the star required. This
method is dependent for success on conditions
which make the entire transit visible, condi-
tons hard to realize, since, barring cloudy
weather, the transit lasts several hours, and
the axial whirl Of the earth may take one or
both of the observers out of position to see
the whole transit. The Delisle method aims
to escape the contingencies which jeopardjze
the success of the Halleyan method, by SIM
plifying the observations to the determination
Of the moment when, from opposite sides of
the globe, the transit is seen to begin earliest
and to begin latest. If one station observes
when the transit begins first and another sta
ton on the opposite side of the earth notes
when it begins latest comparison OF records
affords the data for computing the distance of
Venus from the earth, and thus, consequently,
of ascertaining the exact distance of the sun.
But to put this method into successful opera
tion each observer must have previously as
certained with great exactness his longitude
and also his local time, which are hard to OBJ
Al: ~
what precarious. The last, or Photographic
method, consists in making the sun picture
his own disk, with Venus on the face at any
instant, which furnishes data for compari-
son similar to that obtained, but without pho
tography, by the direct method.


The American observers will rely mainly
on the Halleyan and Photographic methods,
but not to the exclusion of both the others.
The Russians will employ the Halleyan
method in their most important expeditions,
and the English the Delislean and Halleyan.
The French will combine the Halleyan with
the Photographic apparatus, and have already
prepared for very extensive observations It
is pretty we'll agreed upon that, OF all the
methods to be employed, the Photographic
has to contend with the fewest difficulties, and
the value of its results will be largely en-
hanced by the fact that the actual measure
ments will be made after the transit, when the
excitement of the observer has cooled down.
Apart from the elucidation of many other
practical problems, the success of the present
observations will aid materially, it is said, in
perfecting the lunar tables of our nautical
almanacs, SO esse ntial to navigation. The
motions of the moon are disturbed by the
suns attraction, and when this is accurately
determined by ascertaining the suns distance
the moon will become to the mariner grand
celestial clock, giving him the absolute time.
An INTERESTING INQUIRY. New York
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