red satellites are, on the other hand,
insignificant; that found by Barnard, for example, being only about 100
miles in diameter.

Of the four original satellites Io is the nearest to Jupiter, and, seen
from the planet, it would show a disc somewhat larger than that of our
moon. The others would appear somewhat smaller. However, on account of
the great distance of the sun, the entire light reflected to Jupiter by
all the satellites should be very much less than what we get from our
moon.

Barnard's satellite circles around Jupiter at a distance less than our
moon is from us, and in a period of about 12 hours. Galileo's four
satellites revolve in periods of about 2, 3-1/2, 7, and 16-1/2 days
respectively, at distances lying roughly between a quarter of a million
and one million miles. Perrine's two satellites are at a distance of
about seven million miles, and take about nine months to complete their
revolutions.

The larger satellites, when viewed in the telescope, exhibit certain
defined markings; but the bodies are so far away from us, that only
those details which are of great extent can be seen. The satellite Io,
according to Professor Barnard, shows a darkish disc, with a broad white
belt across its middle regions. Mr. Douglass, one of the observers at
the Lowell Observatory, has noted upon Ganymede a number of markings
somewhat resembling those seen on Mars, and he concludes, from their
movement, that this satellite rotates on its axis in about seven days.
Professor Barnard, on the other hand, does not corroborate this, though
he claims to have discovered bright polar caps on both Ganymede and
Callisto.

In an earlier chapter we dealt at length with eclipses, occultations,
and transits, and endeavoured to make clear the distinction between
them. The system of Jupiter's satellites furnishes excellent examples of
all these phenomena. The planet casts a very extensive shadow, and the
satellites are constantly undergoing obscuration by passing through it.
Such occurrences are plainly comparable to our lunar eclipses. Again,
the satellites may, at one time, be occulted by the huge disc of the
planet, and at another time seen in transit over its face. A fourth
phenomenon is what is known as an _eclipse of the planet by a
satellite_, which is the exact equivalent of what we style on the earth
an eclipse of the sun. In this last case the shadow, cast by the
satellite, appears as a round black spot in movement across