The Seattle post-intelligencer. [volume] (Seattle, Wash. Terr. [Wash.]) 1888-1914, 1892-12-11

the moons of Jupiter can not be seen with
the unaided eye excep. under conditions
that need not be again referred to- Were
objects of no greater briliiance quite aloof
from such bright orb rs one OF the great
planets, they would easily be discerned
without optical aid. In the ordinary
language of the astronomer, Jupiter's sat
elite would be reckoned as bright as star
OF the fourth magnitude.
Since the discovery of the satellites OF
Mars by Prof Asaph Hall at Washington,
in IS7T there has been no event In the
astronomical world which has assumed
the same interest as that OF which We are
now to speak. From one point of view it
might appear that the announcement OF
an addition OF another satellite to
Jupiter's system was no very significant
matter after all No doubt the new at
tendant OF the great planet Is certainly
very trifiing object as far as dimensions
are concerned. II is not nearly so large
as many of those minor planets, the
discoveries of which are constantly being
announced. How comes II that people
are talking about and thinking about Ju-
pliers fifth satellite, while there are
thousands, rather millions of stars
living un1osiced through space, and yet
any one of these stars is perhaps f0oo, 600
times as big as this little satellite, besides
being sun. which may presumably be
source of light and heat to planets
which circulate around it. What is
ment about the discovery of an object
which is probably among the most minute,
if not itself actually the most minute. of
all the objects of which our telescopes can
take cognizance? Why is it that an ap-
partition of great comet with a blazing
tail half across the sky would not have
half the interest for astronomers pos
sessed by this little stranger? Why
is il that sun spots, lunar craters,
the icecaps of Mars, and his newly dis
covered lakes, the revelations as to Venus
are for the moment forgotten or unheeded;
why IS every one so eager TO learn all
about this tiny little moon of Jupiter,
and on the tiptoe of expectation for every
further item of intelligence from the Lick
observatory?
Let me say at once that this extreme in
terest in the little object appears most
natural. / am glad TO share in il, for
there seem to be certain very good reasons
why such interest should be felt. In the
first place, Jupiter has always been a
favorite telescope object. The globe
itself is SO vast that the features
are on a sufficiently large scale to be dis
cernibie with comparatively small instru-
mental power. Thus it is that the cloud
belts on the great planet are familiar OBJ
jects TO every one who has ever used a tele-
scope. Then. too. the ever varying posi-
tions OF the four older satellites
make them a spectacle that 10 ALWAYS
ing an eclipse, in which satellite
pIunges into Jupiter's shadow and disap-
pears for q while. At another time We
note how the little object vanishes by oc-
cultation behind the body of the great
globe around which it revolves Then.
too, there are the singularly delicate and
beautiful phenomena of the transits of
the satellites in front of the planet. In
this case not only is the sate-
lite itself often to be traced in
the act of crossing, but, as a far more
striking manifestation, It's dark black
shadow is thrown on the brilliant globe
The movements of the satellites are so
rapid that the different phenomena We
have referred to are repeated frequently,
and many OF them can be discerned with
a comparatively small telescope. But the
Jovian system OF satellites has also a
claim on those astronomers who de-
vote themselves rather TO mathematical
research than to telescope observation.
Each of the moons is, of course, mainly
guided in its movement by the attraction
of the great globe itself. II there were
only single moon and if there were no
other interference. then the determination
of its movements would be com
parativeiy simple matter, and the
places occupied by the satellite at
every date could be predicted with COM
ptete confidence. In no one of the planets,
however, can so simple a condition of
things as We have supposed be realized. 1t
is no doubt true that the earth is attended
by but single moon, but then the move
ment of the earth's companion is rem
dered highly complex by the fact that the
attraction OF the sun constantly tends To
make it swerve from the simple elliptic
path which il would otherwise pursue.
But the movement of our moon, however
embarrassing a problem it may present to
mathematician, is nevertheless SIMPLE
ity itself III comparison with the move
amt trh srh hos ha 4 or
Juniter's moons. In the case of
each of these little bodies there is
not only force exerted by the sun with
the effect of disturbing the satellites mo-
ton, bat each one of the four globes
attracts each one of the others and Is in
turn attracted by it. The consequence of
this is that the movements of Jupiter's
satellites form one of the most trouble
some problems which the mathematical
astronomer has ever been called to solve
1t presents peculiar features of difficulty
arising from the exceptional character of
the Jovian system; but these very diff
culties, so far from deterring mathemati-
clans from the study, have in some cases
acted as stimulus. A considerable
part of Laplace's famous book. the
'Mecanlque Celeste" Is devoted to
the study OF the system of Jupiter's
satellites. He has contrived certain ana
iytical methods for encountering the many
points that arise. and he has succeeded in
explaining some of the most remarkable
dynamicai features of the system MANY
other mathematicians have also essayed
the task of & thorough calculation of
the problem. Indeed, on more than
one occasion the question has been
propounded by the Academy of
Sciences at Paris. and a considerable rriza
has been offered tor a satisfactory discus
sion. The test OF the com pleteness of such
q theory would be sought IN the precision
with which II would enable the movements
of the satellites 10 lee predicted. No doubt
good deal has been done in this
way. our Nautical .1lptdnGek, for
instance, announces with all need
fui details. the various eclipses
aud OCCUPATIONS Of the satellites as we'll
as their transiis and the movements OF the
shadows across the disc. Ii is quite pos
sihle, even with our present knowledge of
the subject, 10 predict such phenomena
for some years in advance The
accuracy with which these indications
can be made is amply sufficient for
the ordinary purposes for which fhev are
required. But III such Investigations
the requirements OF science demand a
much closer degree of approximation be-
tween what - observed and what is caicu-
lated thAn i possible in the present state
of our know ledge Of Juplter's satellites
There - much work yet 10 be done before
the movements OF This system are reduced
10 satisfactory order.