New-York tribune., 1874-04-24
OBSERVING THE TRANSIT OF VENUS
BY PROF SIMON STwcoND

OBSERVING THE TRANSIT or VENUS
BY PROF. SIMON SEwcoMB.
This was description by the astronomer in
charge OF the Washington Observatory OF tho proceed
ngs this country for the great astronomical event of
next December The first steps In respect TO the oh.
servations on the Transit OF Venus we're taken
in this city four years The plan adopted
divided itself into three parts the nature
OF the observations the stations be chosen, aud tho
organization OF the patties OF observers. 1he flDd,
mental idea o; the methods adopted then stated.
The two classes OF determinations were. the
hand observations OF interior exterior contact
ON the other measurements between the oenters
the Photographic method.
Hitherto the visual method. however employed Ka.
proved very uncertain IL Is now about 200 sincs
HALEY observed transit OF Venus the Island Of 81
not now be accepted. The difficulty Is in knowinx
tine exact moment Of the ingrcss or egress OF tha
planet from the suns disk. shc approach OF the two
points OF light when the planet cuts the disk ot sun
Is matter not only OF the most delicate observation s
point In which intrinsic uncertainty IA involved
Even at the last transit OF Mercury Observers differed
enormously IN noting the time OF dsappeamnce Of ito
dark line of sky which separated that planet from the
DIFFICULTIES IN OBSERVING TRANSIT.
The best way to ascertain the conditions Of this
phenomenon is by the observation OF the transit of an
artificial Venus artificial This done.
rather than year ago. at the high point visible
from the Observatory. Tho artificial and planet
placed on Wldors building, at distance OF $3.50
10 atmospheric vapors and softenngs Of tho
outline. The apparent size CT tho artificial
aud planet those that will he presented by tho
real bodies the time of the transit 1t found that
the sun looks little larger and the planet lutte
smaller than their true magnitudes. When the light OF
the is close 70 the planet the space between is
parently filed by hgament known as the black drop.
t is commonly supposed that the moment OF disappear
Of the black drop Is the moment or contact; but tho
uncertainty thus occasioned Is OF serious moment TIA
not due merely TO bad atmosphere; t Is an absolute
effect independent Of this cause In obseryng the ar-
fcsl Venus the same difficulty Ia more or less encoun
tered. Even in airer atmosphere than We hope for 40
the time OF the transit mast expect this phenomenon
In moving toward the edge OF the SUM- before reaching
in, there Is again a similar source OF error. cloud
seems pass through tho thread Of light between tho
planet and the edge OF the sun the bright line grows
the moment OF true contact. No ordinary observer can
fix this time with accuracy. Another method
Biography but the difficulty here is that the photograph
Is dependent upon the comparative aetino power OF tao
thread Of light ard every photographer knows that the
slightest haze Will make difference IN the Impression
on the sensitive plate The light thrown on one plate
without any corresponding difference In the facts
think shall not attempt the photographlng 0f tho
IL was supposed that the OF Venus into tho
suu's atmosphere could be observed by means Of tho
sDectroscope with great accuracy; but COMMITTEES ia
Germany, including Zow lner and Awers, came IN tho
conclusion that this after all one Of the most un'
certain methods, and 1t was finally gwen up eatrey
It has been found however that the moment when tho
planet first makes notch on the sun is well defined
sion, and the experiments Of stribner OF 81.
Petersburg agree with ours on the artificial Eenus 10 tbls
particular. Another method OF determining THIS problem
consists observing the distances OF the ceners OF tho
two bodies. The Germans thought OF doing TINS oy
ber OF these instruments is mpractcable, as they arc
cunDrous aud expensive. believe there Is not ho"-
ometer IN this country.
STATIONS AND ORGANIZATION.
Let us now consider the Of stations. Suppose
for instance had four stations two northern and
two southern. II these divided into two classes OF
observations and at the north and and Is at The
SOUTH as A is not comparable with Is, tho failure OF
either at the north aud az the south, or AZ me
north and at the south would render all the four OBJ
seryatons vaueless Therefore, all the observations
tl all llni
ectng stations has been their meteOi0Ogy, tno
question at issue oeing theis ,l,, ma1l"V hon,
years circulars were sent to American Consul.
IN almost every part OF the world where the rsns,s
those points III November ana December, and every
other source OF similar information was utilized. We
mad thought OF selecting from number OF others Hurds
Islands in the Southern Indian ocean as being ono
biies good weather easily TO be mad EsPeci,Iy
favorable this view IL Pelng III emma ana Niadl'o
favorable HSkodadi was very Objectionable in respect
to weather; Yokohama was just as bad Nagasaki wa"
the only satisfactory station in the southern hems
In Tasmania New-zeaand is nearly ss favorable. But
from ali the other proposed Southern stations tho
accounts were very bad notably at the proposed sa0h
at hurds Islands the almost uniform report was clouds.
rain, tempests, and snOw the chances OF observation
there dd not exceed tgotenhs this station was there
fore given up. The most favorable station left at the
South Kerguelen Island though somewhat neghbor-
party Will also be landed practicable, at Crolsete In
stead OF sending four parties to each hemisphere we
equaliZe the chances as to weather. nope I" got
complete results from two parties ia each hemisphere
To each there detailed two officers from to
Observatory two from the Coast Survey, one from the
army, from the navy and two civilians. Pros Han
and Harkness 20 10 Hobart Town. Among the civilians
are the celebrated astronomers, PrOs Watson OF Anu
ArbOr and Peters OF CUton, NJ Y The constitution OF
each party Is such that In OF disability on the part
Of It's chief the second officer can take his place ach
party Will have three photographers chief photo
grapher who must have been OF long experience IN the
business; "a assistant that mas had practice,
second assistant trained only for the OCCASION
Nearly all the second assistants positions nave Dceu
filled by students or graduates Of various schools and
colleges throughout the country The par
ties for southern station Will sa We expect about
June These all ready the photographers are 10
De in ul practice here week The northern parties
will 20 later and all together. The Navy departments
has furnished ship the swstara 10 go TO the southern
stations The long1tudes Of the stations VIII be deter
mined by oceutations wherever telegraph communities
is impracticabe; but already there Is such CON-
muuicatIon between vladivostok and Hobart Town.
Arrangements made with the Governments Tor ex-
changing longitude Signals and the prospect OF the ex-
tension Of cables TO NewZeaand and other points gives
fair hope that tnere will be only few points where tue
method Of oecultatiOns will be the sole resort.