National Republican., 1882-12-07

Kcplcr In 1881 first called attention Il
/ Ncnu5s transit across the sun, but he gdver
tisod it vt the wrong hour and she made tn,
passage when II was night in Europe the only
place where the astronomers were on the watcl
for her. Kepler announced the next transit
for 1861, but jeremiah Horrox figured it ou
that she would appear on the sun-surfact
again In 1859. Like q selfish curate he kepI
the knowledge all to himself, and, the day OF
the passage being Sunday he missed theeaply
portions of it; but his was the first recorded
observations of the a1i ,t-
TRANSIT or VENUS. t-HI
James Gregory in 1868 suggested the deter,
mining of the suns parallax by the transit oil
an inferior planet. HALEY, few years later
worked out Gregory suggestions and def1om-
strated how they could be utilized in 1851
and 1863, when he should be dead. They
were generally observed by astronomers, those
of Father He'll, of Germany being accepted
They were imperfect, however, in determin.
ing the suns parallax, consequently it left the
distance between the sun and earth unsettled
to the extent of half q million miles.
1t is difficult to explain the manner in
which
THE DISTANCE OF THE SUN
is determined by the transit of Venus in such
a way as to be understood by one unfamiliar
with the principals of geometry astronomy
and trigonometry. Somebody, whose name is
lost to the world, discovered, centuries ago
that the three angles of any triangle are equal
in degrees to the diameter of circle,
or $50 degrees. Therefore if the value
be known, the value of the third
may be ascertained by subtractng the
sum of the degree of the two from $50. The
value of an angle is the length in degrees of
that portion of circle contained between the
open points of the angles of q carpenters
compasses when placed inside circle. If the
length OF one side of u triangle and the values
of any two of its angles be known, the length,
of either of the two other sides may be deter
mined Again, if any object, as a house or a
tree, be viewed so that the whole object may
be seen, the line from the top and that from
the bottom of the object meet in the eye.
This is known as
THE VISUAL ANGLE,
and this angle decreases and increases in a
ratio directly proportional to the distance
from which the object is observed. That is,
if the visual angle of the house is ten degrees
when viewed from the distance of 100 yards,
it will be twenty degrees when seen from the
distance of fifty yards, and five degrees from
200 yards. This visual angle is called by
astronomers the horizontal parallax. If the
exact diameter of the sun were known its dis
tance could be quickly estimated by the prin-
ciples which give the increase and decrease of
the visual angle and if the observer knowing
the earth's diameter, could be transported to
the surface of the sun and obtain the visual
angle of our planet, the same problem would
be solved. -
The distances between the various stations
occupied by observers of the transit are known,
which is one side of the triangle. Tho path
traversed by Venus across the suns disk, as
viewed by observers in the Northern hemis-
phere will be different from that seen by, OBJ
servers in the Southern, hemisphere Both
paths will be parallel, but that seen blythe
Southern observers will be higher up oni the
suns face than that seen by their Northern
brethren, and The path farthest from ths,cen-
IF ofthe sun will be shortest. If the difference
between the time which it takesVenus to travel
over these paths be accurately determined
the differences between the parallaxes of Venus
and the sun will be known. Kepler's third
law of planetary motion demonstrates, that
the square Of the time of revolution of. each
planet around the sun is proportionate TO the
cube of its mean distance from that luminary.
If the period of Venus was ascertained yester+
day with accuracy the problem of deteymin-
ing the suns distance from the earth becomes
one of simple proportion. The task becomes
complicated, however, when we consider that
not only Venus, but the earth is constantly in
motion, the earth turning on its axis as well
as travelling in its orbit. Refraction, too binds
the sun to appear to be in different places
from those they really occupy.
TO GET THE SUMS PARALLAx
more perfect, great preparations were made to
observe the transit of 1874, but they were so
unsatisfactory that they were not made public
in either Europe or America. The observa-
tions taken by American astronomers were
more or less discussed among themselves and
these results privately made known to
European astronomers, but the latter
made such failure of their photog-
raphy, which was at such variance with
their telescope observations, that they
made neither public, and resolved to abandon
photography altogether, save in French OBJ
Servatories. The American photographs, how
ever, yielded excellent results, but they as
well as the telescope observations We're not
exact, and hence did not accomplish the de-
sired end. In the observations of 1874 there
arose difficulty known as the "black drop"
HALLEY's METHOD
consisted in observing the two internal "con-
facts" of Venus on the rim Of the sun. There
are four 6 contacts " in the transit The first
contact" occurs at the moment the advanced
edge of Venus comes between the observer
and the rim of the sun; the second contact" "
at the instant the rear edge of Venus passes
between the observer and the rim of the sun
the third and fourth Contacts" when the
advance and rear edges of Venus respectively
pass from between the observer and the rim
of the sun. AS in Halley's method, the OBJ
server had to see both contacts, and a cloud
might interfere with one, Delsle proposed
that observations be taken from different
points of the earth's surface, when, if two OBJ
servers observed together any one of the four
contacts, the problem might be solved. De-
isles method did not prove satisfactory in
1861, and in 1863 Halley's was again resorted
to only to be defeated by the black-drop,"
"THE BLACK DROP" e.
is explained by stating that just before the
first contact occurs Il black ligament appears
to connect the rim of the approaching planet
with the rim of the sun, thus rendering it
difficult to ascertain at just what instant the
contact occurs. The same trouble occurs in
the internal contacts, and when the planet is
leaving the sun the ligament, or "black
drop" connecting the two after the final
contact" Although HALEY claimed that
by the contact method the solar par
ailax could be obtained to within a
five-hundreth of its whole amount,
all transits so far have failed to give one hall
that accuracy. By the Delisle method, aided
by the perfection of American photography
(which can be defeated only by continuous
clouds, is not liable to misinterpretation, is so
free from error that II single negative is sus-
ceptible to an error of only about one half a
second of arc), as accurate result as can be
obtained, as a good observation of both inter-
nal contacts, by the taking of but few OF
them at any time during the transit.
AT THE NATIONAL OBSERVATORY
Gen. Hazen furnished better weather in
Washington than at many other stations for
the observations of the transit still the
weather was far from being satisfactory, At
the National Observatory there was an active