the planet's
surface.

In the passages of these attendant bodies behind the planet, into its
shadow, or across its face, respectively, it occasionally happens that
Galileo's four satellites all disappear from view, and the planet is
then seen for a while in the unusual condition of being apparently
without its customary attendants. An instance of this phenomenon took
place on the 3rd of October 1907. On that occasion, the satellites known
as I. and III. (_i.e._ Io and Ganymede) were eclipsed, that is to say,
obscured by passing into the planet's shadow; Satellite IV. (Callisto)
was occulted by the planet's disc; while Satellite II. (Europa), being
at the same moment in transit across the planet's face, was invisible
against that brilliant background. A number of instances of this kind of
occurrence are on record. Galileo, for example, noted one on the 15th of
March 1611, while Herschel observed another on the 23rd of May 1802.

It was indirectly to Jupiter's satellites that the world was first
indebted for its knowledge of the velocity of light. When the periods of
revolution of the satellites were originally determined, Jupiter
happened, at the time, to be at his nearest to us. From the periods thus
found tables were made for the prediction of the moments at which the
eclipses and other phenomena of the satellites should take place. As
Jupiter, in the course of his orbit, drew further away from the earth,
it was noticed that the disappearances of the satellites into the shadow
of the planet occurred regularly later than the time predicted. In the
year 1675, Roemer, a Danish astronomer, inferred from this, not that the
predictions were faulty, but that light did not travel instantaneously.
It appeared, in fact, to take longer to reach us, the greater the
distance it had to traverse. Thus, when the planet was far from the
earth, the last ray given out by the satellite, before its passage into
the shadow, took a longer time to cross the intervening space, than when
the planet was near. Modern experiments in physics have quite confirmed
this, and have proved for us that light does not travel across space in
the twinkling of an eye, as might hastily be supposed, but actually
moves, as has been already stated, at the rate of about 186,000 miles
per second.

THE PLANET SATURN

Seen in the telescope the planet Saturn is a wonderful and very
beautiful object. It is distinguished from all the other planets, in
fact from a