The Seattle post-intelligencer. [volume] (Seattle, Wash. Terr. [Wash.]) 1888-1914, 1892-12-11
JUpITER'S NEW MOON
Prof. Barnard's Discovery a
Very Important One.
RESULTS THAT WILL FOLLOW.
Sir Robert Ball Discusses tho Jovian
system and the Probable Condi-
ton of the Fifth Satellite.

has never been the object of so much
interest as il is at the present moment. It
will be remembered that among the first
fruits of the new instrument for looking
at objects which were a long way off, was
the great discovery of the system of which
Jupiter was the center. The four satellites
lie just on the dividing line between OBJ
jects which can be seen with the unaided
eye, and objects which require optical
assistance 10 make them visible. 1t seems
to be certain that there have been indi-
yidual. gifted with rare powers of vision
who under exceptionally favorable cir
cumstances have been able to discern one
or other of the satellites of Jupiter with
out optical aid. There is certain testi-
mony which seems TO show that long be-
fore the invention Of Galileo's tube for
studying the heavens, one or two of these
satellites had been seen by the Chinese.
But it would be futile to say that these
giimpses of the moons of Jupiter
really amounted to any anticipa-
ton of the great discovery of
Galileo. Such mere casual observations
never thoroughly demonstrated the exist
ence of the little bodies, still less would they
yield such a volume of accurate knowledge
as would enable US to- determine their
movements. so as to say when they would
again be likely to be visible No astrono-
mer ever seriously entertained the notion
that there was such a system of attend
ants revolving around Jupiter until their
existence had been demonstrated once and
for all by the telescope of Calico.
There can be no doubt that the moons of
Jupiter are in themselves quite bright
enough to be ordinarily seen by the I'M
aided eye were it not for a single circum-
stance. They lie too close to the great
planet. At first sight it might seem that
the very fact that they were placed in the
brilliant illumination which Jupiter ra-
dates should rather tend to make them
more easily discerned. The nearer an OBJ
ject is held to n source of light the bet
ter it can generally be seen. Does it
not therefore appear to be some
what paradoxical to say that the reason
we are generally unable to see the moons
of Jupiter with the unaided eye is because
they lie so close to a lustrous globe? Ought
not that TO be the very reason why they
should be seen with all the greater facil-
ity2 This is point which may require
few words of explanation. because il is in
timately connected with the recent great
discovery of which it is our special inten-
ton to treat.
We hold book near the candle when
we want to read, because under the cir
curastances supposed the only light avail
able comes from the candle. The type
has no other illumination, and the nearer
it is to the candle the brighter the
printing appears. But this is not at
all analogous to the case of Jupiter
and his satellites. We cannot think
of Jupiter as the candle, and
satellite as q page of the book. If such
were indeed the anology then the nearer
the satellite lay to the body of the planet
the more brightly would it be illuminated,
and under certain circumstances the more
easily would it be seen. The very oppo-
site is, however, VIGOROUSLY the case. 1t
will be instructive to see wherein the diff
ference lies.
1t is, of course, quite true that Jupiter is
q brilliant source of light, but in this re-
spect the light that emanates from the
planet is of very different character from
the light which radiates from the sun.
Jupiter is not sun-like object diffusing
radiance around in virtue of his
own intrinsic brilliancy. If the sat
elites revolving around the planet
were really in the condition of
the planets themselves as they revolve
around the sun, then the closer the sateI-
lite was to the planet the greater would be
the illumination 1t receives. But of
course this is not all the case. Although
we have excellent reasons for believing
that Jupiter is in truth highly
heated globe, yet We are perfectly
assured that the temperature falls
far short of that which would be
required if the great planet were to dis
pense it's beams around in the same man.
ner as a miniature sun. Indeed, there are
facts connected with the satellites them
selves which render it perfectly clear that
Jupiter. so far from possessing a sun-like
radiance, is absolutely devoid of any
light whatever. 1t shines
merely rs the earth itself shines, or as the
moon shines, or the planet Mars or Venus
or Saturn, by the sunbeams which fall
upon it, As to the light which illumines
the satellites of Jupiter, it also can only
come from the sun. We cannot, indeed
say that the light-radiating power of a
certain area on Jupiter is the same
as that from an equal area on
one OF his moons. There are GER
tainly intrinsic differences between the
material constitution of the great planet
anu OF MS satellites which prevent us
from affirming that they are at all times
equally capable of rejecting light Some
portions are winter than Others, and there
fore return larger fraction of the sun
light which falls upon them. STILL
however, We may for the purpose of the
present argument remember that as both
Jupiter and the moons are illuminated by
the same sun. they are both sufficiently
nearly OF the same brightness. 1t there
fore follows that there would be no gain of
luster To the satellite in being near Jupi-
ter. Note, then, the difference between
what would have happened If Jupiter were
and what does actually happen
when Jupiter is merely q planet. In the
former case there would be distinct ac-
cession of brightness TO the satellite the
closer it made its approach. In the latter
case there would be no variation of bright
ness at all.
To follow the argument a step further
we must think of what takes place in the
eye of the astronomer who is observing
formed of the great globe. The image
is extremely minute, but, owing chiefly
to- the disturbances OF our atmos-
phere. the genuine Image Is sur
rounded with region in which the
nerves of the retina are more or less al
fected by the light, which ought, ii al
sources of disturbance could be excluded.
10 be entirely concentrated within the
image itself OF course, vision of other
objects on this affected part OF the retina
viii be corresp ondingly impaired. IL the
image OF a satellite fall upon IL. then
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