s, and in fact rather more--especially if it be covered with clouds.
One reason of the peculiar brilliancy of Venus is that she is a very
cloudy planet.[8] Seen from the moon the earth would look exactly as the
moon does to us, only a little brighter and sixteen times as big (four
times the diameter).

[Illustration: FIG. 42.--Galileo's method of estimating the height of
lunar mountain.

_AB'BC_ is the illuminated half of the moon. _SA_ is a solar ray just
catching the peak of the mountain _M_. Then by geometry, as _MN_ is to
_MA_, so is _MA_ to _MB'_; whence the height of the mountain, _MN_, can
be determined. The earth and spectator are supposed to be somewhere in
the direction _BA_ produced, _i.e._ towards the top of the page.]

Galileo made a very good estimate of the height of lunar mountains,
of which many are five miles high and some as much as seven. He did
this simply by measuring from the half-moon's straight edge the
distance at which their peaks caught the rising or setting sun. The
above simple diagram shows that as this distance is to the diameter
of the moon, so is the height of the sun-tipped mountain to the
aforesaid distance.

Wherever Galileo turned his telescope new stars appeared. The Milky Way,
which had so puzzled the ancients, was found to be composed of stars.
Stars that appeared single to the eye were some of them found to be
double; and at intervals were found hazy nebulous wisps, some of which
seemed to be star clusters, while others seemed only a fleecy cloud.

[Illustration: FIG. 43.--Some clusters and nebulae.]

[Illustration: FIG. 44.--Jupiter's satellites, showing the stages of
their discovery.]

Now we come to his most brilliant, at least his most sensational,
discovery. Examining Jupiter minutely on January 7, 1610, he noticed
three little stars near it, which he noted down as fixing its then
position. On the following night Jupiter had moved to the other side of
the three stars. This was natural enough, but was it moving the right
way? On examination it appeared not. Was it possible the tables were
wrong? The next evening was cloudy, and he had to curb his feverish
impatience. On the 10th there were only two, and those on the other
side. On the 11th two again, but one bigger than the other. On the 12th
the three re-appeared, and on the 13th there were four. No more
appeared.

Jupiter then had moons like the earth, four of them in fact, and t