The daily dispatch. [volume] (Richmond [Va.]) 1850-1884, 1869-09-17
The Transit of Venus.

sent ont 10 various parts of the world to
observe the transit of Venus across the
SUMS disc in 1874, and thus aid in the solu-
tion Of q problem in which all humanity is
indirectly interested.
The problem which has to be solved.
which the Astroncmer-Royal has well called
"the noblest problem in astronomy" is
the SUMS distance from the earth and it
NAV be we'll to show at the outset why and
how it is thus attacked. 1t 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 mcchani-
cGllv the interval it sufices to mark out n
1nuch smaller distance. called q 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 lnStru-
mental reasons, the base line must be Of ap-
probable length with regard to the distance
Of the object. Such q mode of measure
mcnt even may be applied to the moon, which
is roughly a quarter of a million miles of, the
observations being made (say) at Green
wich and the Cape Of Good Hope, since the
distance between those places-the base
line-is appreciate when compared with
the moons distance. But when we come to
the sun the case is different. If we could
place two observers on the equator, one in
longitude 0 degree, and the other in lonGi-
tudc $50 degrees, we should then have the
largest diameter of the planet as the base
line: but. compared with the suns dis
lance, 1900 miles (thc carth's largest dia-
meter, and consequently the greatest dis
lance between my two places on it, Is in
strumentally nil--our base line is inappre-
~ciable-and this, the most obvious and di-
rect 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 We
nus or Mercury, when they come between
lis and the sun, and are seen to pass over
his disc, may give US the required lnforma-
ton. An attempt to explain this will re-
quire n little attention. The method is
really founded on one of Kepler's laws, by
which mankind became acquainted with
the ~reldtire distances Of the planets from
the sun long before they could deter
mine their absolute distances. The thing
10 DE uOlp. tIltTtlOic, 10 " hiSisulS liz
distance Of the nearest planet from
us, and then something like q rule-of-
three sum tells us the distance sought
I. 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,
rs we now know, near enough to us to allow
us to apply the base-linc method, az in the
case of the moon, were it not for the unfor-
&-.- lin. NJ h1, ,n41 1.n.
within ours, whom 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 exactly between us and the sun, and
appears as a black dot on the suns face
that is, We have a transit Of Venus over
the sun.
Now let us see what happens, and let us
regard the sun as n 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 nart Of tlc disc. An observer at the
North Pole would see the planets path pro-
iected lower down on the sun similarly, m
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 re-
present On the sun a distance exactly equal
TO that between the two observers; but We
know. to start with, that the distance 0!
Venus from the earth and sun are rs % to
II, nearly, so that the interva between the
two paths will always bear this relation TO
the distance between 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 stations such as we have
imagined, the problem would be at once
solved, and in this way: We could deter
mine the length Of the line, as seen at Venus,
which joins the two stations on the earth
ut which the observations are made we
could then increase this in the ratio Of IS
to -, 10 find the exact separation OF the
black dots representing Venus OH 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 screcn-the
sun-can only be laboriously determined
from their length, because simultaneous
observations arc 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 re-
suit. From this requirement 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 II roting globe-a consideration which
complicates the matter to a tremendous eX-
tent. Hence it was that Captain Cook went
to Otaheite in 1869 on the occasion of the
last transit, that Father He'll observed 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, and
state briefly the work that had been done
in the interval in order to give ar idea Of
the extraordinary interest which centres
in the observations Of the one in 1874 for
observations Of which the arrangements
will require to be begun at once. If We
take up . Ferguson's Astronomy" an ad-
mirable book written about the middle Of
the last century, we shall find it roundly
stated that the carth's distance from the
sun is 50, 600, 900 miles. The first transit
OF Venus lin 1863 brought this up to
IS, l73, 900 miles-a number so near the one
obtained il the next transit (that of 1862
that till quite recently some foul play was
suspected in the observations made in the
last-named year to render the results
similar. Fortunately, however, we are now