The daily gazette., 1874-10-15

tvestaentions. The method by which tho
transit OF Venus is used to determine tha
| distance OF the sun is not essentially dis
| similar from that which is familiar in tb.
operations OF surveying. To observers
mile apart from each othc.. the size of a
distant object appears different, and the
aiHerence affords the means of calculat-
ing its relative distance from them. Nv
Venus, when passing between the sun
ana the earth, describes an apparent path
across the suns disc. This path appears
Longer or shorter according to the position
OF the observer on the earth surface. and
If observations are taken by persons post
edat northern and southern stations or at
eastern and.western. .statines respectively
the difference in the length or time OF
transit affords. materials for calculating
the relative distance of the sun as com
pared with the earth's diameter. Rough
ly speaking, the question whether the
difference should be observed between
stations north and south or east and west
occasioned the great controversy last year
between the advocates of Halley's and
Dehsle's methods; and the quarrel has
been settled in way which might have
suggested itself to the controversialists
beforehand d-namely, by arranging to take
observations in both ways.
Such is the main principle of these obser-
excessive deiicacy. The ObseI vations alreaty
made vary only by decims's Of a second III
the critical angle, and yet the results depen-
Gent on them as respects the suns distance
vary by as much as four millions OF miles
From 1853 ti' about 1862 the suns mean
distance wns considered TO BE- ninety hve
and hall million miles, end as late as 1886
the Astronomer Royal said there was no pro.
liability OF an error o! hslf million miles
IG IA h,.k
tions, partly astronomical and partly con-
nected with the velocity OF light threw
boudt upon the old calculation aud it Is now
thought that nj iuety-one m'llou miles and q
hs'f is nearer the true mean dis 'ance. For
the confirmation Of these independent re-
searches we are now about 10 appeal to the
minutest observations by astronomers Of the
moments OF contact of q little black spot
with the suns disc. Even hero We have t.
distinguish between real f1d apparent con-
tact, and for the last hundred yeers the
controversy hes depended upon th,s distiuc.
ton. What slight matters may mar the
most elaborate propF -ations was III lustrated
In 1861. Three astronomers were watching
for the moinsnt OF what is called "egress."
One OF them. on observing it, exclaimed,
Now, aud all three invo.untarily put down
the same time. The effect was 10 transform
what what would Otherwise have been
shteO-hoto oboCtNattOu lulO u hlu2lc one. nat
the chief Circumstance which renders the
observations SO CT iiticai remains to be stated
II u only at average intervals of twice in
lZ2 years that Venus crosses the sun in the
manner described; end these two occasions
are nearly eight years apart. The lrs pair
of trsusits occurred IN 1891 and 1869,
Upon The observations then made all sub.
sequeut calculation have been founded, and
1t is not until this year that We have had an
opportunity OF testing their correctness. As
they were the first made, ii was impossible
they should be free from error. lu The CEN
tury which has since elapsed, the danger to
l be avoided aud the points to be watched
| have been indicated by numbers OF acute
| caiculators end practicgl astronomers, aud
| the observers this time will go 10 their work
| thoroughly forewaIned and ioreafmcd In
maDse advances have since bet n made in the
j construction OF Astronomical instruments,
and photography has added au Invaluable
means OF recording obaervationS. 1t would
| be discteditable to science il such advanta-
ges were not turned to account; and for the
same reason as We have just explained, II
| the present opportunity be- missed, no other
l will occur until June IN the two Years 2Uo4
and 2Ol2. 1t depends on our enterprise and
| skill Ill the present year wheiher iour gcu-
| Grafton OF astronomers shall be destitute OF
the evidence which the transit OF Venus can
| afford VI the suns distance. Tue couse-
quence. OF couIse, Is That every nation whica
pretands TO cont butte 10 civilized progres.
Is lifting out one or more expeditions OF oh-
Iservatiou. Besides ourselves, the French,
Germans, Americaus, Russisus, Duich, ant
| Italians are to have share Ia the waIk. II
Ia, OF course, only at particular portions Of
1 the earths surface that observations can b.
made with advantage, aud these are scitter-
ed from Siberia 10 the Antartic regions, and
from Egypt TO the Sandwich Islands. Tne
Germans are said TO be sending five expedl
tions the Russians, whose extending terri
tory offers peculiar facilities, will have. 1t il
reported, as many as twcnty-sly stations,
the Italians three, and the Dutch one. ILo
Expeditions from the United States ore, 81
l course, on su extensive scale. Eight stitl n,
are named AS hay ny been St leetci by lhsm.
: and their observations wilt he yeIy yasi7i
/ in character