Evening star. [volume] (Washington, D.C.) 1854-1972, 1882-03-04
THE TRANSIT OF VENUS.
The Phenomenon to Occur Next De.
CenHber.

MEASURING THE DISTANCE BETWEEN Us AND
THE sUN-How THE SOLAR PARALLAx Is
FOUND BIZ NOTING THE TRANSIT OF VENUS
THE APPARATUS TO BE USED BY THE AMERI-
CAN OBSERVERS.
Any school boy ifasked how far the earth Is from
the sun will answer promptly, though if the
same question were propounded to a learned as
tronomer he would probably decline to make
oath to any statement. Not that the question
has not been determined with sufficient accuracy
for all ordinary purposes, but there are discrep-
ancies OF few thousand miles or 80 in the fig
ureS OF different mathematcians which become
quite important in the nice calculations of
science. Since the remOtest historic times. the
great astronomers have given attention to the
problem Probably the earliest attempt recorded
TO determine the relative distance of the sun
was that Of Arstarchus. who fiourished the third
century before Christ. His method was to meas
ure the angle between the sun and the moon
when the latter appeared half Illuminated. He
announced as the result of his observations that
the sun was twenty times as far as the moon
ratio which modern observations has proven to
be twenty times too small PtOlemy subsequent
ly calculated from the apparent diameter of the
earth 8 shadow, cast upon the moon, that the
suns distance from the earth was 1910 radii
OF the earth, and his figures were accepted as
accurate during la centuries and until the mis-
takes of PtOlemy were exposed by COper-
nlcus and his disciples Still no estimate that
could be proved to be approximately correct
was substituted for Plotemy's figures until the
latter part of the 18th century, though Huy-
Shens, century before. by ingenious guess
work. arrived at very nearly the same result
which has been approved by modern science.
He assumed, from the symmetrical proportions
OF the solar system. that the magnitude of the
earth would be somewhere near the average of
the magnitudes of Venus and Mars From this
aSSnmNtIOD q fn orr fhov
ntude OF the earth, as it would appear to an OBJ
server on the sum- and then calculated the angle
which the sem-diameter Of the supposed
earth would subtend from the sun, which would
be the solar parallax.
THE MODERN METHOD
OF solving the problem has been to obtain the
solar parallax, or, in other words. the angle at
the sun, subtended by semi-diameter Of the
earth. Parallax, in general, is defined as the
difference in the direction of body as seen from
two different points. A familiar illustration of
the effect of parallax. Is seen in the case of a
passenger on railway train who observes that
objects near the train pass by swiftly, while
those at great distance seem to move very
slowly. BY careful observations taken at two
different points, whose distance from each other
Is known. the angle at the object of observation
and the observer has but to solve simple prob
lem ID trigonometry to discover the distance of
the object
The distance of the sun, however. is so great
that 1t has been found to be impossible to meas
ure its minute parallaX by any such simpe
means as that described so resort is had to what
is known as relative parallax, the difference be-
tween the paraax of some body lying nearly in
the same direction as the sun and that Of the
sun being determined.
THE PLANET VENUS,
by habit of periodically crossing the face Of the
sun. furnishes the astronomers with lust the
conditions they require for determining the solar
paralax. HALEY, prior to the transit of 1861.
announced that observations of the phenomenon
made from distant points Of the earth could be
used In determining the distance of the sun. In
consequence OF the parallax Of Venus two OBJ
servers at distant points Of the earth's surface
Will see her describe slightly different paths It
is Dy the distance between these paths that the
parallaX has hitherto been determined the pro-
portlonal distances between the sun and Venus
and the sun and the earth being we'll known.
through kepler's third law. Since that time
various values have been ascribed to the
solar parallax. ranging from 8 to 10 seconds of
angular measure. The results of the Obserya-
tions OF 1863 were not worked out completely
ior half century, and then Encke figured out
the value of the parallax to be 857l sec
onds. The corresponding distance of the
sun Is $4,000 miles, number which is gen
erally adopted in text books on ASTRONOMY The
reduction OF the observations of the transit OF
1874 would appear to slightly enlarge the paral-
lax. and hence reduce the distance The value
obtained by the French and American astrono-
mers is S.SS seconds
Since 1861, therefore,
THE REGULARLY RECURRING TRANsrrs
OF venus have been observed with much inter-
est by astronomers. In the present century
thousands of dollars. and the labor of many
eminent scientists. have been devoted to the
determination OF the solar parallax by the OBJ
seryatiOns of the transit of Venus. In 1874
costly expeditions were sent to different parts
OF the globe, not only by our government. but
Dy the olHerent governments of Europe. The
phenomenon Will occur again ON the 9Th of
December next. and then Venus, swinging
around her predestined and endless course. viii
not cross the face of the sun again for I2lL
years. The astronomers Of the world recO1-
nizing the fact that
THE COMING TRANSIT
will be the last that any one now living will
nave opportunity to observe, are already astir,
and laying plans to ensure the most satisfactory
results for their labors.
Congress has been asked to appropriate
$500 to defray the expenses Of expeditions to
be sent out from Washington to observe the
transit 1t is proposed by the commission. who
nave general charge Of the work to send at
least four expeditions to the southern hem
isphere-One to New Zealand, one to the south
ern extremity Of Africa. and two to South Amer
1ca-thOugh the details will not be arranged nm
I'll Congress has made the needed appropria-
lions. 1t is hoped by the members of the com
mission that Congress will make the appropria-
ton early enough to permit the expeditions to
De thoroughly equipped and organized and
started on their journeys in good season. AS
the whole transit will be visible in the eastern
hall OF the United States, the observations to
be taken in the northern hemisphere will not
require so much enterprise or expense as those
TO be taken in the south. suggestion. which
meets with considerable favor, has been made
that the work be entrusted to the different Cb-
seryatOries OF our country.
THE APPARATUS
used in 1874 will be used in the coming obserya-
tions. The most improved and delicate appll-
ances that science could devise were then
employed to note the different stages of the
transit. Over century had elapsed since the
last transit Of Venus, and during that century
great advances were made in the art Of observing
and the application Of scientific methods. By
the art OF photography the light of the sun itself
was enlisted in the work aud made an indeIibIe
record OF every phase Of the phenomenon The
United States then sent out three expeditions
10 stations In the northern hemisphere and
hVe to stations in the southern hemisphere
mpfaOrOIRUfp9t t,o;r