, 1877-08-25


!" seu.
!s tho same thing as saying that straight lines
drawn from thu centric Of any one Of these stars
difference between tho angular distances Of
Mars from star, as seen from anr two points
on the earth'e surface, will, therefore, bo prac
t,eally equal to the angie which Is torn1ed BY
those two points. The distance In miles be-
tween the two places Of observation can be cn1-
culaicd, when wo know their longlsu1cs
moth Triangle, in which we know The magnitude
Of the base, and of lta opposite angle. The
altltudesIand thu other angles are easily cal
cu1atcd; and VIC thus find tho aetual distance
tion. It ia already known that, taking the
earth's mean distance from tho sun as unity,
her octual distance at the tlno Of opposition
Mars from tho earth will b. 0.a78, or the mcan
distance Of tho earth from the san wlll bc 2.05
times that Of Mars from ua at tho dato Of oppo-
BE- o. 13Q, 100 miles, tho other distance would be
O., ,000, tnb nearly.
Tho observations may bcrctluced 50 as to give
the angle at tho planet, which is formed by o
line to tho carth's centre, and tangent to tlur
equatorial suriace. This angie Ia the equatorial
horizontal pdrallax. In tic present tnstaueo It's
yaluc Is about 23.4 seconds OF nrr. II - evident
riai rattius' Il tdo shortest siae ot rehteansiel
triangle, whence, BY trtgonometrY,,tll cosecant
Of this anglo, multiplied Into tho eatth's cqua-
torial radius, glvcs the distance Of the planet
from tho earth.
Tho transit Of Venus In December, 1874, was
observed for the satne purpose, and the obserya-
ttonH nra being reduced on tho same principle NJ
that TO Lc employed pow: The chlei dli1crenso
hnrkTfnuuH. NEfrtt TN fhN Afen1nrA-
ment Of Venus whi1c In this tho es'essial pault
will hc taken a. tho background OIL which to
measure tbs displacemcns u Mars put
which operated 10 disadvantage, in tue transit,
MS compared with the opposition. The solar
9utltne Is bclleyed 70 DO continuously changing;
though relatively small boing great enough q
catse considerable difference III the results OBJ
taincd BY measuring tho angular distances. Of
tho planet from tho edge OF the solar disc. Aud
dil1cuIt TO determine the instants Of
staSf j. comparatively steady; % ths oatilna @ q
II. sfs n sssyS "a s @ S.s tes snsy as Of
walton + MUCH greater In tlse case Of MARI than
io that Ut Venus. Tho similur opposition Of
Mars. in 1852, enabled astronomers 70 correct
in the. last century,, aud it + not Improbable
that ths present opposition will give results less
discordnnt, aud In0re Trustworthy, than those
OF tne tfatait ot tErcQ 5eara ago.
asAprINd rMa SURFACE or MARS.
Tie opposition will be utilized for another
important purposem;to imp out tho surface Of
tho planet Mars. We have already maps which
were drawn during former periTcal opposlt!ona,
chiefly those Of 1S:0 and 1883, showing the lead
possible that the improved Instruments Of The
prcstnt day will add largely to cur knowlcdge
Of ths minor details Of arcoEraphy, and may
even lead to important mud!acations In our
3fFAk with rAu n fh@ NH atrnt cOn4ftnnU nj
Ills surface,
The angular diameter Of Mars nt the tino ot
| opposition will bo 2d8 seconds; about cqual to
tho anglo rub tended BY y circle one foot fn
observer. A clear, ssl1l atmosphere will permit
| this disc 10 be 'maRuiacd up to much larger
arcs than that which tho full moon prestpts TO
nsxta The psGss, s4 p4 9s sy s ss
, at tho Washlu9ton Observatory. The announce
llent has already been made Id TIE TRIBUNE
that 10 aad discovered ono, aud poas!blya scc-
Tha Official report OF tho Superintendent Of tho
Observatory contains the following:
III s r. sbss s: a., ~
md which DO supposed Was seen for tho dFst tino
faet discovered Aaa. II as il hours and ID m!a:
data 10. Z shs % psnets y %otioas Il /: I'd % e
secoFn'ac tho ob]Act - \ satellite. It Was, how.
h: is,a. Il % istS IT, III 1ot 81
81 angus. ThIs ~ s4slsto ''as an appasent distance
IL magnltae Prot lis,t eatlnss<s II tho thirteenth
ea tss.ts.nth TIE p'1s q s otbt b.: new
~ sstes eases
hour i apps.1s 70 b. quite .s Silaht :. she iii
one, and IT. olongation hAs nearly Tho saus angle.
% .sa IL
: periodic time Ass not yet known, The following