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

I cult to observe one of the large satellites
| when in the act of transit across the
/ planets disc, so that We can hardly be
surprised that the fransits of an object
/ which is sucn an extremely small fraction
/ of their size should not be perceived. OF
| course, there is q notable difference be-
! tween the case of a transit of q satellite
| over its primary and that of planet, like
/ Venus or Mercury in front OF the suns
| disc. In the latter case the planet appears
as black spot against the brilliant back
| ground. In fact, it may be remembered
| that an unsuccessful search for an intra-
| Mercurial planet has actually been con-
| Quoted IN the manner thus suggested by
| seeing if il could not be observed during
| the progress of the transit. But the case
| is very different when satellite of Jupi-
| ter transits over the face of the planet.
| The iustre of the satellite. arising as it
| does from sunbeams only, is equal to the
| lustre of the face of the planet, except in
| SO far as inequalities in the intrinsic re-
| fiecting powers of the two bodies may
| suffice TO cause difference. The shadow
| of the new satellite on the globe of the
| planet. though no doubt it would be an
nevertheless be intensely black in
COMPARISON with the surrounding sur
face, and therefore it might be
expected that it ought to be compare
lively easy to see when sufficient optical
power was available. 1t must, however.
be observed that the diameter of this
shadow is considerably less than the tenth
of a second. and, therefore, far too minute
to be recognizable as a dark spot. AS
there is but little variation in Jupiter's
distance. it will be almost equally we'll
displayed at every opposition, ii not to
observatories in the British islands, then
to observatories elsewhere. Thus, for in
stance. if the opposition happened to be in
June, as will sometimes occur, then,
though the planet will be very low down
for observers in our ATTITUDES, yet it will
be very favorably placed for astronomers
in the Southern hemisphere. Thus we
may hope that we shall speedily accumu-
late considerable quantity of observa-
tons relative to the new object.
To realize all that is implied by this dis
covery of an additional moon to the four
previously known members of Jupiter's
system it will be necessary to refer to an
other point. Every one who knows any
thing of astronomy is aware that the dis
tances of the several satellites from the
center of the planet and the periodic times
IN which they revolve are connected by
definite relation. This is, of course, an
immediate inference from kepler's famous
law. 1t may be worth while to refer to
this, as it leads to the recognition of GER
tain important facts in connection with
the new discovery. Let us consider, for
instance, the innermost of the four we'll
known satellites of Jupiter. 1t revolyes
round its primary in period which, &C
cording to the best determination, may be
taken as day. Is hours, q minutes. 81 Sec
onds. We may regard this orbit as circular
and the distance of the satellite from the
center Of Jupiter as $4,000 miles.
In like manner the outermost of
the four satellites revolves around
Jupiter in period of l6 days, Is
hours, 81 minutes, IL seconds. and sthe
Of 10 I
There is certain relation between the
four magnitudes have named, which is
expressed by saying that the squares of
the times are proportional to the cubes of
the distances. As this law depends upon
gravitation, it must be obeyed by any new
satellite, and here we can foresee that the
Barnard moon, whatever else it may do
must at all events revolve in an orbit under
such conditions that the cube of its radius
bears to the square of the periodic time
exactly the same relation as in the case of
each of the other satellites. The applica-
ton of this condition has proved suffici-
emily satisfactory. AS We are at present
informed, the distance of the new satellite
is 1i2. 400 miles, and the period now as
signed is ii hours, 30 minutes. These two
quantities do not exactly conform to the
required conditions, but the discrepancy
is inconsiderable: for the distance stated.
the time should have been somewhat
more; for the time stated, the distance
should be somewhat less. 1t is not sur
prising that these early announcements
should lack the precision that will be ex-
pected when more time has been given to
the investigation of the little object
In estimating the distance of a satellite
from its primary the most natural unit of
measurement to adopt is not to be ex-
pressed in miles or in thousands of miles,
1t should rather be given in terms OF the
equatorial radius of the planet. The sense
of proportion is gratitied in this way of
looking at the matter. This is specially
| ad vantageous In the case of Jupiter's
moons, and we shall proceed to illustrate
| it by pointing out the movements that
would be appropriate for moons placed at
| d iferent distances from the center of
Jupiter. The critical case of a moon
which was as close as possible to the sur
| face OF the planet so as just to Craze it is
| one of peculiar interest. The moon
| SO circumstanced would have to hurry
| round its vast circle in something
| less than three hours. If its pace fell short
| of that so demanded the body would fall
/ into the planet. Were it greater. then the
| body would fly off into a different path
altogether. Ii a satellite were situated at
| distance from Jupiter exactly equal to
| the radius of the planet at it's equator,
| then the time of revolution would be lust
a little more than eight hours. With
| every increase in the distance there is a
! liiCTSdSe IN the period. /
need not follow the matter into anv
| further detail beyond stating that if the
distance of the satellite We're ten times the
radius of the planet, then the periodic
| time would be about ninety-two hours.
There is however, one special case of so
much interest that it must not be passed
over We have hitherto said nothing as
TO the rotation of Jupiter on its axis
! Were Jupiter rigid body throughout its
| mass, and could il contain neither oceans
nor an atmosphere, then the speed at
| which the planet rotates would have no
| significance so far AZ movements of the
| satellites were concerned. But. of course.
the suppositlon just made is anything but
| correct with regard to the constitution of
| Jupiter. 1t is doubtful if there are anv
| parts OF its vast globe which could be de-
| scribed as rigid, while it is certain that it
| Is enveloped by prodigious atmosphere.
| In other words. Jupiter is composed of
| materials which are liable TO tidal infiu-
| ence. This being so. the speed of Jupiter's
| rotation on its axis is very import
ant element in the consideration
| lltes. We may take the period of
the rotation at hours, 55l{ minutes.
| This would be- the periodic time of a
| satellite which was situated at distance
above the surface of the planet which was
about one and q quarter times as much as
| has radius; more accurately. this magni-
! tude would be 2.2TS from the center Of
I lNNfter tho tyOit ha+Nr -,