Evening star. [volume] (Washington, D.C.) 1854-1972, 1882-12-06
VENUS AND THE sun.
Something About the Transit that Dc
CURED To-day.


THE IMPORTANCE OF THE PHENOMENON TO As
TRoNoHEEs-wHAT SCIENTISTS EXPECT TO
LEARN FROM 1.-THE WAY THE DISTANCE TO
THE Sun Is NEUTRALITY-SUSPICIOUS. OF ANCIENT
AsTROSOxyRs-THE WAY THE TRANsITs OF
VENTS ARE OBSERVED.
The determination Of the distance of the sun
from the earth is one of the capital problems of
astronomy. Any school boy if asked how far
the sun is from the earth will answer without
hesitation %, eco miles. but an astronomer.
who is expected to be strictly correct VIII prob
ably give no answer at all or only pretend TO
give an approximate figure In the history of
astronomy several methods are recorded as hay
ing bcca applied by Ingenious Iuataemat!cians
to ascertain the distance of the sun ArjstarcAils
attempted 10 solve the problem By measuring
the angle between the sun and the moon when
the latter appeared half Illuminated Ptoiemy
attempted to figure it out by geometrcal cal
culation founded on the principle that the more
distant the sun the narrower will be the shadow
OF the earth al the distance of tie moon. Uuyr-
heus in the seventeenth century hit upon the
plan of measuring the apparent magnitude
OF the earth as seen from the sun.
assuming that the magnitude of the
earth was mean between that of Venus and
Mars, and by this method calculated that the
sun was SOGoct0 of miles from the earth-a
result nearer the truth than that obtained by
any OF his predecessors. In the same century
astronomers turned their attention TO the method
OF obtaining the solar parallax by first deter
mining that OF one of the planets near the earth
anu Inen easily calculating that of the sun.
through the known proportions of the solar sys-
TEN. Another method Of modern times
when promises very accurate results, is that of
determining the velocity of light. and then
measuring the time it requires for light TO pass
across the plane of the earth's orbit. The most
celebrated method of determining the solar
paraiax. however, and that from which as
tronomers expect to obtain the most reliable re-
suits is that afforded by the transts of Venus
over the face of the sun. To-day astronomers
In nearly every part of the globe where the
weather permits are making observations of the
Transit The great mass of figures they will Oh
tain will afford material with which mathenia-
ticians will busy themselves for months ard
years, the most refined calculations being
required to obtain the magical number
which will be the ''open sesame to at least a
portion OF the secrets of the heavens.
THE ASTRONOMICAL yARD-srICK
IL Is not mere curiosity to know the
dist ance of the only that has led all the civ
ilized powers of the earth to devote large sums
to the observations made to-day. Astronomers
would care little to know the distance of the
cuh diOHc III IL were HOT THAT IillI kHOWie
gives them standard measurement BY which
81 celestial spaces can he fathomed. In the
10th century kepler laid down the laws of plan
etary motion by which the relative distances of
the members of the solar system were deter
mined. Thus it has been known for instance
that Mars is more than One-aHd-a-haif times as
1ar from the sun the earth. and that Jupiter
is more than five times as far from the earth as
the SUM- Now. if anyone could determine the
distance between the parfh mr. an pt Earr Iau
and the sun IL would require but very simple
operation in arithmetic to find the distances of
all the other planetary bodies. This unit of
celestial measurement. it is hoped. will be very
accurately determined by the observations made
to-day.
WHAT PARALLAX Is.
In astronomical terms the observations TODAY
are made for the purpose of determining the
solar parallax- Parallax is defin ed i'm a general
way as the difference between the directions of
body as seen from two different points. Other
conditions being equal. the more distant the
body the less this difference or the less the par
aax- If two men at different points, the dis
lance between the points being known observe
an object, such as a lamp-post at street corner
the lines of vision together the line connecting
the two points will form triangle of which the
lamp-post will form one of tine points. Now by
determining the angle formed by the line of Wis
lon at the lamp-post. since the base-line of the
tanee OF the lamp-pOst from the observers can
be determined. The degree of the angle at the
1amp-pOst would be its parallax. Such is the
problem offered for solution in order To deter
mine the distance of the sun. only the
angle or parallay is so minute
that the calculations are liable to errors from
many sources In measuring such minute par
allaxes it is doubtful whether the position of the
body on the celestial sphere can be determined
with the necessary accuracy. and in such case
resort is had to relative paralaX. By this is
meant the difference between the parallaXes of
two bodies lying nearly in the same direction.
When the planet Venus is between the earth
and the sun. As 1t was to-day. as to be seen
projected on the suns face the apparent dis
fance of the planet from the center or from the
edge or limb of the sun. may be found with con-
siderable accuracy. so unusual facilities are Of
Tered for obtaining the relative paraax. This
distance Is different as from different parts
OF the earths surface at the same moment ow-
Ing to the effect of parallaX. Observers at dit
ferent stations will see Venus projected on diff
ferent parts Of the suns face. BY comparing
the observations taken at stations situated in
different parts of the earth. the final result will
be obtained. These observations will show the
difference between the parallax of Venus and
that of the sun. The ratio of the paralayes OF
the two bodies being taken from the proportions
OF the system, the parallaXes of both from this
or relative paraliax may be determined.
The last transit of Venus occurred in Decem-
Der, 1874 That transit was visible from no
point on the American continent.
THE REGION OF VISIBILITY
OF the transit OF to-day was quite different from
that OF 1874 1t included the whole American
continent. except some portions in or near the
Arctic circle. The beginning was visible over a
large part of Africa, and the end over most of
the Pacific ocean The best methods of max
Ing and recording the observations of the transit
has been subject of discussion with astrono-
mers ever since the science has been advanced
Tar enough to predict the dates of transits The
method followed for long time has been to
observe and note the time and other circum-
stances attending the external and internal con-
facts The external contact occurs when the
planet appears just outside. but touching the
disk of the sun. The internal contact occurs
when the planet is just inside the edge of the
disk.
PHOTOGRAPHIC OBSERVATIONS.
The development OF the art of photography
suggested new method of observation when
preparations were being made to observe the
transit of 1874. Photography was used by every
European nation In those observations, but with
ont success The American observers alone OBJ