The Republican journal., 1873-01-02
Venus and Photography.

| some particulars OF the preliminary pro-
ceedings with reference to observations
on the transit of Venus--an event to take
place Dec. s. 1874. 1t will be recollected
that small appropriation has been made
by Congress for this purpose The great
interest which is felt by scientific men all
over the world in obtaining careful
series Of observations of this transit, is
more than justified by the rarity of the
event, and its Importance in furnishing
astronomers with actual measures of ce-
lestial distances. These transits occur at
alternate intervals of eight and more than
a hundred years. The last was in 1869 :;
the next will be in 1874; one follows that
in 1882, but after that there will be none
till 2004. The distance Of the earth from
the sun, which transit observations give
the best means for determining, is used as
the unit of measure in astronomy 1t is
one of the most illustrative evidences OF
the uncomfortable spaces with which that
science deals, that when the whole d.
ameter of the earth's orbit is used as a
base-hne, we yet can scarcely perceive
the parallaX of the nearest fixed star
That is to say, that although the motion
OF the earth around the sun alters the
formers position in space more than 100, -
100000 miles in six months, it has only
been with the most accurate observations,
and the best OF instruments that the differ
ence OF the apparent position of any OF
the stars, which results from such a
change of place on the part of the observ-
Ell, has been detected and measured The
! distances of the members of the solar
| system have been ascertained principally
upon this basis, and are astronomically
reckoned as being such and such portions
of the radius of the carth's orbit; the cal
culations being largely facilitated by Keep
| ler's laws. which lie at the foundation of
the doctrine Of gravity
| 1t is difficult to give in Il few words
| even an approximate motion of the pro-
cess of calculation by which observations
on the transit Of Venus are made available
| for measuring our distance from the sun
Nevertheless we shall make the attempt.
If person seated in n room within few
yards Of an open window, should hold
horizontally and level with his eyes a
sheet OF paper, and draw upon it from
each eye line toward some one point On
building on the opposite side Of the
street, say distance of one hundred feet,
his results would not be unlike those of
astronomers when attempting to obtain by
direct observation the distance of the sun.
He would have the distance between his
eyes as the base of a triangle hut the two
lines upon the sheet OF paper toward the
object, would flow toward each other so
slightly that he would find great difficulty
in estimating their rate of approach or,
IN other words, in measuring the angle
which the sides of this triangle form with
It's base. SO, though astronomers may
take the whole diameter Of the earth as a
base-llne and observers on opposite sides
OF the globe may furnish their angular
measurements of lines drawn toward the
SUM- those lines will be found 50 nearly
parallel that a mistake in observation to
the extent Of the thickness Of a hair seen
at the distance OF i mile makes an error
IN the calculation of The distance OF the
sun of 50, 900 miles
But now If the observer ut the window
closes 1t and selects a bar OF the sash for
an intermediate point of observation, he |
may draw lines from his eyes towards the |
bar which will approach each other 50 /
rapidly that their angle is easily measur- |
able. Looking with one eye and then with !
the other, he will see the bar projected on |
the building opposite, in TWO different |
places. He may measure the apparent |
space between these places, and can be |
certain that all the parts OF a triangle of |
which that space is the base and the bar |
the apex, are exactly proportionate to
those OF the measurable triangle of which |
the space between his eyes is the base. |
II, now, he has the means OF knowing the |
proportion which his distance from the |
opposite buildln. bears 10 the wi 18th of the |
street, fe will have no difficulty in calcu-
lating by the rule of three his absolute |
distance from that building and ascertain
ing all its visible dimensions. The bar of
the sash represents, in the larger problem,
the planet in transit. keper's laws, and |
innumerable observations, have already |
provided the proportion Of the earth's
orbit to that CT Venus. Difference Of |
time and the motions OF the earth and the !
planet add to the intricacies but not to
the difficulties OF calculation. But ii is
evident that stations for observation in the
track OF shadow Of the transit need to be
wide apart, so as to give a long base-line
OF measurement.
Within a few years, and notably in the
eclipse observations OF 1869, American
astronomers have taken the lead in the
use OF photography for celestial observa-
tons. BY stellar photography maps have
been obtained that meet the precise tests
OF mlcrOmetrical measurement The diff
ticuities were enormous. In taking such
the most delicate clockwork
must carry the ponderous instruments in
a direction contrary to the motion of the
earth before exposure. Very Slight differ
ences OF temperature, to say nothing of
the heat accompanying an image OF the
sun, modify the movement, as we'll as the
shape Of the instruments and carefully !
adjusted focus which must be actinic, not
visual. The highest order of skill and
patience were called forth in the construc-
ton OF the Photographic lens; the story OF
that is alone, a history. 1t remains only
to be said, that while photography is to
play the most prominent part in the OBJ
servations Of the transit, its use for this
purpose is largely due to the unobtruslve
labors Of a private citizen, who has been
content to devote his time and health to
celestial photography, without other re-
ward than the satisfaction which the sue
cessful pursuit of science affords
Delaware Discipline.