The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1869-09-15
POPULAR SCIENCE.
rhe Transit of Venus.

various parts OF the world to observe the
transit of Venus across the suns disc in7S7l,
and thus aid in the solution of problem in
which all humanity is indirectly Interested.
The problem which has to be solved, which
tbe Astronomer Royal has we'll called ''the
poblest problem in astronomy" is the suns
distance from the earth, and it may be well
no show at the outset why and how it is thus
attacked It is well known that, to find the
distance of any object on the earth from us,
it is not at all necessary to walk over and
actually measure mechanically the interval; it
suffices to mark out a much smaller distance,
called base line, and then from either end
of this base line to observe the angle between
the distant object and the other end of the
base Only one condition is necessary; and,
for instrumental reasons, the base line must
be Of appreciable length with regard to the
distance of the object. Such mode
OF measurement even may De
applied to the moon, which is
roughly quarter of million miles off, the
observations being made, say, at GreenWlcn
and the Cape of Good Hope, since the dis
tance between those places-the base line-ls
appreciable when compared with the moon
distance. But when we come to the sun the
case is different If we could place two OBJ
servers on the equator, one in Longitude
deg and the other in longitude $50 deg,
we should then have the largest diameter OF
the planet as the base line; but, compared
with the suns distance, 1900 miles (the earths
largest diameter, and consequently the great
est distance between any two places on lt) is
instrumentally HjJ-our base line is inappre-
ciable-and this, the most obvious and direct
method, therefore fails
1t is generally supposed that HALEY was the
astronomer who first pointed out the flank
attack on the suns distance rendered possible
by the transits of Venus over the suns disc;
but this is not the case. The suggestion is
due to James Gregory, who suggested in 1868
that observations of Venus or Mercury, when
they come between us and the sun, and are
seen to pass over his disc, may give us the re-
quired information. An attempt to explain
this will require little attention. The
method is really founded on one Of Kepler's
laws, by which mankind became acquainted
with the reJdtjCe distances of the planets from
the sun long before they could determine their
absolute distances. The thing to be done, there
fore, is to measure the distance of the nearest
planet from u.s. and then something like rule
Of-three sum tells us the distance sought
a. e. the suns distance from us. Now the
planet which in its journey round the sun
comes nearest to us is Venus, and she comes,
as we now know near enough to us to allow
us to apply the base line method, as in the
case of the moon, were it not for the unfor-
tunate circumstance that, as her path lies
within ours, when she is nearest to us she is
between us and the sun, and consequently
has her non-illuminated side towards us, so
that she is generally invisible at such times.
But not always, for sometimes she comes eX-
actly between us and the sun, and appears as
black dot on the suns face; that is, we have
transit of Venus over the sun.
Now let us see what happens, and let us
regard the sun as a screen on which the
planet is visible. In the first place. an OBJ
server at the centre of the earth would see
the planet travelling in a straight line over
some part of the disc. An observer at the
North Pole would see the planets path pro-
jected lower down on the sun; similarly an
observer at the South Pole would see the path
projected higher up. In fact, as seen from
the North and South Poles, the paths of the
planet over the sun would be separated by a
certain interval.
Now suppose the sun to be exactly as far
from Venus on one side as the earth is on the
other, it is evident that the apparent interval
between the two paths would represent on
the sun distance exactly equal to that be-
tween the two observers; but we know, to
start with, that the distances of Venus from
the earth and sun are as 2S to ~ nearly so
that the interval between the two paths will
always bear this relation to the distance be-
tween the two stations on the earth from
which they are observed. If it were possible
at the same moment of time to photograph
the planet on the sun from two distant sta
tions such as we have imagined, the problem
would be at once solved, and in this way.
We could determine the length of the line
as seen at Venus, which Joins the two stations
on the earth at which the observations are
made; we could then increase this in the ratio
of 2S to ~, to find the exact separation of the
black dots representing Venus on the photo
graphs. Hence we could determine the size
of the sun, and hence its distance. But in
practice the thing is not so easy, the amount
of separation of the apparent paths of the
planet over our screen-the sun-can only be
laboriously determined from their length, be-
cause simultaneous observations are out of
the question; and as the difference in the
lengths of the paths--that is, the time the
planet takes to travel over the SUMS thus
the point of inquiry, it is necessary to make
this difference as great as possible to give
accuracy to the result. From this require
ment comes the necessity of choosing the
stations at which the transit is to be observed.
most carefully bearing in mind at the outset
that the earth is rotating globe-a conside-
ration which complicates the matter to tre
mendous extent. Hence it was that Captain
Cook went to Otaheite in 1863 on the occa
sion of the last transit, that Father He'll OBJ
served in Lapland that Mr.. Green observed
at King Georges Island in the South Seas
and SO on.
We will give the results of the last, anC
state briefly the work that has been done il
the interval, in order to give an idea o.
the extraordinary interest which centres il
the observations of the one in 1874 for OBJ
servations of which the arrangements vii
require to be begun at once. If we take VI
EsFerpuson'S Astronomy " an admirable hoo