IAMETER, EQUATORIAL, 87,500 MILES; POLAR, 82,500
MILES. VOLUME, 1300 EARTHS. MASS, 213 EARTHS. AXIAL REVOLUTION, 9H.
55M 20S. ORBITAL REVOLUTION, 11 YEARS 317 DAYS. VELOCITY, 483.6
MILES PER MINUTE.

[Illustration: Fig. 63.--Jupiter as seen by the great Washington
Telescope. Drawn by Mr. Holden.]

Jupiter rightly wears the name of the "giant planet." His orbit
is more nearly circular than most smaller planets. He could not
turn short corners with facility. We know little of his surface.
His spots and belts are [Page 165] changeable as clouds, which they
probably are. Some spots may be slightly self-luminous, but not the
part of the planet we see. It is covered with an enormous depth of
atmosphere. Since the markings in the belts move about one hundred
miles a day, the Jovian tempests are probably not violent. It is,
however, a singular and unaccountable fact, as remarked by Arago,
that its trade-winds move in an opposite direction from ours.
Jupiter receives only one twenty-seventh as much light and heat from
the sun as the earth receives. Its lighter density, being about that
of water, indicates that it still has internal heat of its own.
Indeed, it is likely that this planet has not yet cooled so as to
have any solid crust, and if its dense vapors could be deposited on
the surface, its appearance might be more suggestive of the sun than
of the earth.

_Satellites of Jupiter._

In one respect Jupiter seems like a minor sun--he is royally attended
by a group of planets: we call them moons. This system is a favorite
object of study to everyone possessing a telescope. Indeed, I have
known a man who could see these moons with the naked eye, and give
their various positions without mistake. Galileo first revealed
them to ordinary men. We see their orbits so nearly on the edge
that the moons seem to be sliding back and forth across and behind
the disk, and to varying distances on either side. Fig. 64 is the
representation of their appearance at successive observations in
November, 1878. Their motion is so swift, and the means of comparison
by one another and the planet so excellent, that they can be seen
to change their places, [Page 166] be occulted, emerge from shadow,
and eclipse the planet, in an hour's watching.

[Illustration: Fig. 64.--_a._ Various Positions of Jupiter's Moons;
_b._ Greatest Elongation of each Satellite.]

ELEMENTS OF JUPITER'S SATELLITES.
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