New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1872-12-11
gEyUS AND PHOTOGRAPHY.

TENUS AND PHOTOGRAPHY
A Washington correspondent furnishes us
with some particulars of the preliminary pro-
ceedngs with reference to observations on
the transit of Yenus-an event to take place
December g. 1874. 1t will be recolected that
n small appropriation has been made by Con-
gress for this purpose The great interest
which is felt by scientific men all over the
world in obtaining careful series OF obserya-
tons of this transit is more than justified by
the rarity of the event, and its importance In
furnishing astronomers with actual measures
of celestial distances. These transits occur
at alternate Intervals of eight and
more than o 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 2OO4. The distance of the
| earth from the sun, which transit observations
| give tho best means for determining, is used
as the unit of measure in astronomy. 1t is
| one of the most illustrative evidences OF tho
inconceivabe spaces with which that science
deals, that when the whole diameter of the
| earth's orbit is used for a base-ine we yet
can scarcely perceive the parallax of the near
| est fixed stars. That is TO say, that although
| the motion of the earth around the sun alters
| the formers position in space in six months
| to an extent of more than 1S00OO,0O0 miles,
it has only been with the most accurate OBJ
seryatiOns and the best of instruments that
the difference of the apparent position of any
of the stars which results from such change
of place on the part Of observer, has been
detected and measured. The distances of the
members of the solar system have been ascer-
tanned principally upon this basis, and are
astronomically reckoned as being such
and such proportions of the radius of the
earth's orbit; the calculations being largely
facilitated by Keper's laws, which lie at the
foundation of the doctrine of gravity.
| 1t is difficult to give in a few words even
| an approximate notion of the process of cal
| culation 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 a person seated
| in O room within few yards of an open win
| dow, should hold horizontally and level with
| his eyes 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 at distance of 100 feet his results
| would not be unlike those of astronomers
when attempting to obtain by direct Observa-
| ton the distance of the sun. He would have
| the distance between his eyes as the base Of a
| triangle but the two lines upon his 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 his triangle form with its
base. So. though astronomers may take the
whole diameter OF the earth as a base-line,
and observers on opposite sides of the globe
may furnish their angular measurements of
| lines drawn toward the sun, those lines will
| be found SO nearly parallel that mistake in
| observation to the extent of the thickness of
| hair seen at the distance of a mile makes an
| error in the calculation of the distance of the
sun Of 50 900 miles.
| But now il tie observer at the window
closes it, and selects bar Of the sash for an
| intermediate point of observation, he may
draw lines from his eyes toward the bar
which will approach each other so rapidly that
| their angle is easily measurable. Looking
with oncseye and then with the other he will
| sec the bar projected ON the building opposite,
in two different places. He may measure the
| apparent space between these places, and can
which that space is the base and the bar the
apex, are exactly proportionate to those of
the measurable triangle of which the space be-
tween his eyes is the base. IF, now, he has
the means of knowing the proportion
which his distance from the opposite
building bears to the width Of the street, he
will have no difficulty in calculating by the
rule of three his absolute distance from that
building, and ascertaining all its visible dimsn-
sions. The bar of the sash represents, in the
larger problem, the planet in transit. Kepler's
laws and innumern6le observations have al
ready provided the proportion of the earth's
orbit to that of Venus. Differences of time
and the motions of the earth and the planet
add TO the intricacies but not to the difficulties
of calculation. But it is evident that stations
for observation in the track or shadow Of the
transit, need to be wide apart, so as to give
long base-line Of measurement
Within few years, and notably in the
eclipse observations Of 1909, American as,rono-
mers have taken the lead in tho use of pho
Biography for celestial observations. BY stellar
photography maps have been obtained that
meet the precise tests Of micrometrical meas
urcment. The difficulties were enormous. In
taking such photographs the most dclieate
clockwork must carry tho pondcrous instru-
ments in direction contrary to the motion
of the earth during exposure. Vcry slight
atleie9sss VI HwpeialGs, y ..J aQ!hiqg Of