New-York tribune., 1874-12-05

/ transit Two observers are StaIlOneu db points vi
the earth A. r, nearly onequarter of its whole cir
conference from each other one of them being in a
position where he VIII see Venus the horizon.
| From each observer a line is drawn touching the
edge OF Venus c. where the lines cross. and thence
extending to the edge of the sun, D. These lines
4 L and d, represent the view taken by OBJ
servers of Venus at the beginning of the transit d
and T being points the suns edge where these
observers would respectively see the edge of Venus
when approaching the sun, first touch it.
The space between observers, one-quarter Of
the earths surface, say 5000 miles. calculated
the earths diameter ,the line d B being extended
TO the earths center in the diagram). Half the
earth's diameter is 1856 miles. Let Us suppose that
it is found, on comparing observations after the
transit. that the time by the clock when one of
these observers first saw the edge of Venus touching
the edge of the sun was five and thrcequarter min
utes later than the time when the other observer
saw this contact Ii would be safe to infer that the
shadow of the transit If choose to speak of III
moved 1856 miles in 8. minutes, or 6SS miles in
minute. This is the gain of the motion Of Venus
over the motion OF the earth, while bOth are going
round the sun; and this gain is measured on the or
bit of the earth. Now reyertiDg TO the proportions
named in our symbol Of two wheels. this gain Of We
nus is Gye-eighths Of the earth's motion. If five
cighths are CSs, eight eghths 11O0. Therefore,
the earth moves in its orbit around the sun at the
rate of lOG miles in minute; this is G600O miles
per hour 900 miles per day and 5181600 miles
per year The semi-dameter of circle of 100000
miles is % 900 900 miles, and the latter is therefore
the distance of the sun from the earth.
But let us see what would happen if the observers
made mistake Of quarter Of a minute as to the
time when the edge of Venus touched the edge OF
the SUM- Suppose the difference Of the time as OBJ
served was only 81 instead of - minutes. Then the
gain If Venus would have been calculated as 10
miles instead of 6S8; the speed Of the earth ll5O
miles per minute instead of 11O0 the distance Oh the
earth from the sun, 9G0O00O0 instead OF 9@,ooO'VOO
miles In other words error in observing, OF the
sixteenth part of a minute OF time, Will make an
error of 100000 miles IN computing the suns DIS
tance
This method of calculating the suns distance
from observations on the transit of Venus Is known
| 90 the Deljsle method Certain allowances have to
he made in the computations, and 1t may be worth
while 70 allude 10 some of them If the observers
were separated By many degrees Of Latitude, so that
+bav would see the lines traced by Venus on the sun
very wide apart as represented in the diagram, the
curvature of the suns edge might make difference
in the time when and the other saw the transit
| since they would 1t on different parts OF the
| curve and one would see 1t sooner than the Other
| This difficulty might be measurably avoided py
| statoning the observers more nearly on the same
lines of latitude. Allowance must be made Tor the
revolution of the earth on It's axis; this would be
| less than 100 miles in the 8. minutes II observers
were stationed at the equator, and much less III a'
trdes far north or south. Besides these and other
matters susceptible of calculation there are certain
nractieal difficulties which have thus been summed
| nm by Mr. Proctor reported Ia TRIBUNE Extra
| Nos -. =.
tno~ tine absolute moment because one observer If as
far away & a. Thev cannot Communicate and. there
,oro the only way they can compare their te u Nv
SnonS the true time their station But IN 0rder 10
hOw tie true time 1t is necessary that the Longitude
shouId he known. Suppose In Ensanu CERTAIN vNrWi &
hanDened al O'Clock in the afternoon lhen you Wi WIZ
H IL in New-York earlier by i've hours IL would
certainly be difficult to be accurate wisnan 'h ,Ie on NJ
e,s nnst know the time OF their stations WITHIN
second or two and while this obstacle might DC sur
moah'u vet il would be very difficult TO doit In desc
iata Mace or island on the earths suriaco "u '""' "
| the difficulty OF Delisles metnou'
THE DIRECT AND PHOTOGRAPHIC METHODS,
IN an editorial article, entitled Venus and Pho
| - y TIE TRIBUNE of Dec. II, 1872. referred to
one ~ the methods which will be employed curing
,he transit in the following words:
~ =, ,,Bcrlt IN give In a few words even an approx
| =. no,,on of the process OF calculation oS "nich
nservatops on the transit OF Venus are made aVaape
%, measuring our distance from the Sun'
s sbaII make the attempt II person seh'ea ~ ioou
within few yards Of IN open "inao" 'no1. aa,
1,r and level with his eyes sheet OF paper. and
80 anon ,t from each eye line toward some one point
nudIng on the opposite side OF the street say at
- 80 -. ~, mo fact his results would not De UNLIKE tho'o
when attempting to obtain by direct Oh
srvat,op the distance of the SUM- He would nave sno
cstanos between his eyes the base OF rHnee ou'
the two lines upon his sheet OF paper no
,a., xouId aow toward each other 80 signify 'ha' no
o,a e,a Great difficulty estimating their ratc OF ap
ob mr. in other words. In measuring the angie "n''n
me side. Of his triangle form with rs b1se 3D. 'aoasa
astronomers may take the whole diameter OF 'o s,s %
as baseliue, aud observers on opposite "acS ., sn@
gobe may furnish their angular medsareuio""s u
| drawn toward the sun those hnes Will 30 'o1"N