The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1867-08-02
AN ASTRONOMICAL PHENOMENON.
Jupiter without HIS Satellites In August.

In the July number of the London Popular
Science ReUiel0. Mr.. Richard A. Proctor, author
OF "Saturn ard its Systems" directs attention to
a phenomenon which is TO occur this year. On
the 2lst of August the planet Jupiter will
appear, in telescopes OF moderate power, to be
unaccOmpaned tor the space of one hour and
three-quarters by the satellites usually seen in
attendance upon him. This phenomenon has
been so seldom observed that considerable inte-
rest is attached to it. MolyneuX, on November
II, 1851 20. S.), Sir w. Herschei, on May 20,
1902, Wallis, on April TO- 1926, and Dawes and
Gnesbach, on September 81 1s43, are the only
observers who have hitherto seen it.
Mr.. Proctor says:-"It cannot be doubted,
however, that if the weather be favorable the
number of observers who have seen the pheno-
menon will be very largely increased before
midnight, August q. 1t will not be wholly as
a matter of curiosity that observations will be
made on that night The record of phenomena
presented by Jupter's satellites Is regular part
of observatory work. and is very necessary
for the Improvement of the theory of their
motions-an important astronomical subject.
A special value is attached to the record
Of phenomena separated by a small in
terVal of time, so that the observations
made are fairly comparable, inter Sc, free from
the errors arising from variations in clock rates,
instrumental changes, and the like. Now, on
the evening of August 81 there will be eight
phenomena visible within six hours, viz. the
dlsappearances and reappearances of four satel-
lites. To observers suitably armed there will,
indeed, be NO less than thirteen phenomena
visible within the above-named interval; since
of the entrances and exits of the shadows of
three satellites, six phenomena in ALL five will
be observable with good telescopes" He adds:-
"From the relations holding between the mo-
tons Of the first three satellites, it is impossible
that all these bodies should be ecipsed at once;
but at regular intervals all three are in the same
straight line with the planets centre. If this
happen when the sun (and therefore the earth
which, with reference to Jupiter, may always be
considered to be close to the SUM is near the
same line, these three satellites will be inyisi-
BILE, one or two being eclipsed, two or one (as
the case may be- being projected on Jupiter's
disc. Such phenomenon is not unfrequently
visible.
'That the fourth satellite may be hidden at the
same time, 1t must be nearly in a line with the
other three. This relation is not often pre-
sented; and, as already stated, the concurrence
of this relation with the requisite cOn6guratlon
as respects the snn and earth is an occurrence
very seldom to be observed.
"a circumstance that tends to render the
simultaneous disappearance of the four satel-
htes more uncommon than it otherwise would
be, is the tact that the fourth satellite is not
necessarily eclipsed or occuted at each con-
junction with Jupiter. 1t may pass above or
below his disc or shadow. In fact this han
pens on an average in more than One-third of
the revolutions OF this satellite. This is ascrbed
by Sir j. Herschel to the greater inclination of
his orbit but this is not the correct explana-
ton. In fact, the inclination of the fourth
satellite is at present less than that OF any of the
others, and the mean value of its inclination Is
always less than that OF the others. The true
reason why lhls SATELLITE 50 OIten escapes eclipse
is its superior distance from Jupiter.
at is commonly stated that the third satellite
cannot possibly escape eclipse or occultation as
| it passes behind its primary, and must neces
sarily transit Jupiter's disc when passing be-
fore the planet. / find, however that it is lust
possible for the third satellite to pass clear of
| JupitPrs disc in the latter case. A conjunction
| of many favorable circumstances is, however,
| required, and the phenomenon must be very
uncommon One-muCh more so, indeed, than
that which forms the subject of the present paper
1.-THE following facts Will suffice to enable the
| telescopist satisfactorily to observe the pheno-
menon of August 81. The planet rises at half
| past z, almost at the same moment that the sun
| sets At 7'44 Green Wch meantime, the shadow
of the third satellite itself passes on to the disc
and the satellite itself passes on to tue cisc at
| 8'l4. The first phenomenon will not be obserVa.
| ble as the sun will not be low enough beneath
| the horizon nor Jupiter high enough above the
| horizon. Neither will the second phenomenon
nor the entry Of the fourth satellites shadow on
| the disc, which occurs at Sl7, be easily seen
| The other eleven phenomena VIII be readily
seen, however At 9'l9 the second satellite will
disappear in the shadow OF the planet. at 9,28
| the fourth satellite will enter on Jupiteris disc
At 9'57 the shadow of the trst satellite will make
| It's appearance, followed in seven minutes by the
entry OF the satellite itself on Jupiter's disc Al
this time (l0'4) Jupiter will be without satellites
in telescopes of moderate power, but large tele9
scopes will exhibit three satellites on his disc
together with their three shadows. AT II ,99 the
shadow of the third satellite passes off the disc
at IL 40 the satellite itself. At 1918 the second
satellite reappears from behind Jupiter. nj IS.Ie
the shadow OF the first satellite passes off the
disc, the satellite itself seven minutes Iator
Lastly, at 1859 the shadow of the fonTtb sataI
lite and at l04 the fourth satellite itself nassn
off Jupiter's disc" ''' passes
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