The Toledo chronicle. [volume] (Toledo, Tama County, Iowa) 1873-1924, 1874-11-05
Ere. l..

con junction. Thus, in Fig. I, suppose
+ F G represents the orbit of the earth,
A B What of Venus, and $ the sun. If
Venus is at v when the earth is at
i, it is in inferior conjunction. If it
is at A when the earth is at +, it is in
superior conjunction. But the orbit
OF Venus, as you see by the
figure, is not in the same plane with that
of the earth. Now if it were extended
until it met the earth's orbit, it would be
represented by the dotted line c d i,
and it would cross the earth's orbit at the
points + and G. These points, or the
corresponding places 4 and V, in the
real orbit of Venus, are called its nodes
Now, because Of this inclination of the
two orbits, the sun, Venus and the earth
will be in the same line only when Venus
is at, or near, one of It's nodes at the time
of conjunction. For if Venus is at b
when the earth is at I, it would be in in
ferior conjunction, because it is in that
part of its orbit which is most directly
between the earth and sun; but we
should see it in the direction of G. If.
however, it is at its node, Y, at the time
of conjunction, or when the earth is at
Il, we see ii in the same line as the sun,
and it then appears TO pass directly
across the suns disc This is what is
called a transit of Venus. Venus is
Opaque, like the earth, shining by the re-
Elected light of the sun; therefore the
bright side is toward the sun, aud at the
time of a transit it appears to us like Il
dark spot upon the suns bright surface
The transits of Venus happen only at
rare intervals, because it is seldom that
the three bodies are thus situated in ref
ercnce to each other. They occur in
pairs, eight years apart, and between the
pairs are 105 or else 122 years
The fact that they so rarely happen oc-
casions an interest in the transits but
this is by no means the only reason why
they arc SO carefully watched. Their
chief Importance lies in this: BY obscrv-
ing the path which the planet makes
across the sun we obtain data from
which the distance of the earth from the
sun can be calculated. The reldtjre dis.
tances of all the planets from the sun is
known; therefore, when the earth's dis
lance, expressed in miles, is obtained, we
have, as il were, a yard-stiek by which
the distances OF the other planets can he
measured To find the exact length of
this yard-stick has long been considered
the astronomers grandest problem, and
transit of Venus gives the most accu-
rate means of doing this
The last two transits were in 1161 and
1863. Previous to these the estimates
which had been made of the suns dis
tancc from the earth were very incorrect
The earliest estimate on record made it
about one-twcnteth Of its true distance;
and even at the time of these transits it
was too small by several million miles.
These transits were, however, watched
with great interest, the observations
made of them carefully compared, and
the distance computed to be about ninety
five million miles. Since then astrono
mers have calculated the suns distance
by several other methods, applying prin-
ciples which were not then known and,
although these methods are inferior to
that furnished by transit, yet, as the
different calculations very nearly agree,
it is supposed they are not far from cor
rect. They show the suns distance to
be a little over nincty-o million miles
The instruments which we now have
for measuring small angles and the
means for determining the latitude and
longitude of places are much superior to
those used hundred years ago, hence
the observations of the coming transit
will be much more exact, and will fur
nish means of testing the accuracy Of
previous calculations.
will now tell you something of how
the observations are taken and Of the
preparations which have been made for
this purpose.
The direct object is to obtain what is
called the suns parallaX. The parallax
OF an object is its apparent displacement