clipsed in
Jupiter's _shadow_; (iii.) they reappear from behind his _disc_. On the
other hand, _after_ opposition, (i.) the shadows travel _behind_ the
satellites in transiting the disc; (ii.) the satellites are occulted by
the _disc_; (iii.) they reappear from eclipse in Jupiter's _shadow_.

Conjunctions of the satellites are common phenomena, and may be waited
for by the observer who sees the chance. An eclipse of one satellite by
the shadow of another is not a common phenomenon; in fact, I have never
heard of such an eclipse being seen. That a satellite should be quite
extinguished by another's shadow is a phenomenon not absolutely
impossible, but which cannot happen save at long intervals.

The shadows are not _black spots_ as is erroneously stated in nearly all
popular works on astronomy. The shadow of the fourth, for instance, is
nearly all penumbra, the really black part being quite minute by
comparison. The shadow of the third has a considerable penumbra, and
even that of the first is not wholly black. These penumbras may not be
perceptible, but they affect the appearance of the shadows. For
instance, the shadow of the fourth is perceptibly larger but less black
than that of the third, though the third is the larger satellite.

In transit the first satellite moves fastest, the fourth slowest, the
others in their order. The shadow moves just as fast (appreciably) as
the satellite it belongs to. Sometimes the shadow of the satellite may
be seen to overtake (apparently) the disc of another. In such a case the
shadow does not pass over the disc, but the disc conceals the shadow.
This is explained by the fact that the shadow, if visible throughout its
length, would be a line reaching slantwise from the satellite it belongs
to, and the end of the shadow (that is, the point where it meets the
disc) is _not_ the point where the shadow crosses the orbit of any inner
satellite. Thus the latter may be interposed between the end of the
shadow--the only part of the shadow really visible--and the eye; but the
end of the shadow _cannot_ be interposed between the satellite and the
eye. If a satellite _on the disc_ were eclipsed by another satellite,
the black spot thus formed would be in another place from the black spot
on the planet's body. I mention all this because, simple as the question
may seem, I have known careful observers to make mistakes on this
subject. A shadow is seen crossing the disc and overtaking, apparently