extend, in the form of
a very elongated cone, to a distance far beyond the orbit of the outer
satellite. The second satellite is immersed in this shadow, and
consequently eclipsed. The eclipse of a satellite must not be attributed
to the intervention of the body of Jupiter between the satellite and the
earth. Such an occurrence is called an occultation, and the third
satellite is shown in this condition. The second and the third
satellites are thus alike invisible, but the cause of the invisibility
is quite different in the two cases. The eclipse is much the more
striking phenomenon of the two, because the satellite, at the moment it
plunges into the darkness, may be still at some apparent distance from
the edge of the planet, and is thus seen up to the moment of the
eclipse. In an occultation the satellite in passing behind the planet
is, at the time of disappearance, close to the planet's bright edge,
and the extinction of the light from the small body cannot be observed
with the same impressiveness as the occurrence of an eclipse.

A satellite also assumes another remarkable situation when in the course
of transit over the face of the planet. The satellite itself is not
always very easy to see in such circumstances, but the beautiful shadow
which it casts forms a sharp black spot on the brilliant orb: the
satellite will, indeed, frequently cast a visible shadow when it passes
between the planet and the sun, even though it be not actually at the
moment in front of the planet, as it is seen from the earth.

The periods in which the four principal moons of Jupiter revolve around
their primary are respectively, 1 day 18 hrs. 27 min. 34 secs. for the
first; 3 days 13 hrs. 13 min. 42 secs., for the second; 7 days 3 hrs. 42
min. 33 secs, for the third; and 16 days 16 hrs. 32 min. 11 secs. for
the fourth. We thus observe that the periods of Jupiter's satellites are
decidedly briefer than that of our moon. Even the satellite most distant
from the great planet requires for each revolution less than two-thirds
of an ordinary lunar month. The innermost of these bodies, revolving as
it does in less than two days, presents a striking series of ceaseless
and rapid changes, and it becomes eclipsed during every revolution. The
distance from the centre of Jupiter to the orbit of the innermost of
these four attendants is a quarter of a million miles, while the radius
of the outermost is a little more than a million miles. The second o