CELESTIAL TRAVELERS. 
183 
Although there are many points of similarity 
among the planets, each differs from all the rest. 
They are, indeed, a wonderful family. 
The baby Mercury skips around the sun in eighty- 
eight of our days, although its own day is longer than 
ours. ^Nevertheless, recent observations have led one 
astronomer to believe that its period of rotation coin¬ 
cides with its period of revolution; that it always 
turns the same side to the sun, as the moon does to 
the earth. Mercury is the most solid of all the 
planets. 
The earth is five and a half times as heavy as if it 
were made entirely of water. A cubic foot of the 
earth at its average density weighs 344 pounds, and 
one of Mercury weighs 403 pounds. 
Uranus is made of the lightest materials, only one- 
fifth as dense as the earth, which would be about the 
same as if it were made of water. 
The most interesting of the group is Jupiter. It 
is the giant planet. Its axis is but little inclined to 
its orbit and so it has a very slight change of seasons. 
It revolves in nearly the same plane as the earth. 
But the most wonderful part of Jupiter is the scen¬ 
ery which it presents through the telescope. At the 
equator we see a white belt, on each side a dark red 
zone, then alternately white and gray, which merge 
into blue at the poles. These belts are not always 
the same. They change in width and position, and 
occasionally almost disappear. Sometimes one seems 
to grow narrower, fiowing into the next one and mak¬ 
ing it wider. As we are looking, rifts form that 
