JUPITER WITHOUT HIS SATELLITES. 
255 
three revolutions of the fourth: but the approach is not so 
close as in the case of the third satellite. 
From the relation holding between the motions of the first 
three satellites it is impossible that all these bodies should be 
eclipsed at once ; but (as will be seen by fig. 1 ) at regular 
intervals all three are in the same straight line with the planet’s 
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 sun) is near the same line, these three satellites will 
be invisible, one or two being eclipsed, two or one (as the case 
maybe) being projected on Jupiter’s disc. Such a phenomenon 
is not unfrequently visible. 
That the fourth satellite may be hidden at the same time it 
must be nearly in a line with the other three. This relation 
is not often presented ; and, as already stated, the concurrence of 
this relation with the requisite configuration as respects the sun 
and earth, is an occurrence very seldom to be observed. 
A circumstance that tends to render the simultaneous dis- 
appearance of the four satellites more uncommon than it other- 
wise would be, is the fact that the fourth satellite is not 
necessarily eclipsed or occulted at each conjunction with Jupiter. 
It may pass above or below his disc or shadow. In fact this 
happens on an average in more than one-third of the revolutions 
of this satellite. This is ascribed by Sir J. Hersehel to the greater 
inclination of his orbit; but this is not the correct explanation. 
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 in- 
clination is always less than that of the others. The true reason 
why this satellite so often escapes eclipse, is its superior distance 
from Jupiter. 
It is commonly stated that the third satellite cannot possibly 
escape eclipse or occultation as it passes behind its primary, and 
must necessarily transit Jupiter’s disc when passing before the 
planet. I find, however, that it is just possible for the third 
satellite to pass clear of Jupiter’s disc in the latter case. A 
conjunction of many favourable circumstances is, however, 
required, and the phenomenon must be a very uncommon one 
— much more so, indeed, than that which forms the subject of 
the present paper. It is necessary that Jupiter should be in op- 
position when not far from perihelion, at which time it happens 
(and but for this the phenomenon could never take place) that 
j the earth is at nearly her greatest distance north of the plane 
of Jupiter’s orbit. The satellite’s orbit must have its maximum 
! inclination to Jupiter’s orbit, and the satellite must also be at 
its greatest distance from the last named plane. The other 
satellites must also be so situated that the third is at its maximum 
distance from Jupiter ; for it is noteworthy, that although the 
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