Between Mars and Jupiter is a distance of some 339,000,000 miles.  Subtract 35,000,000 miles next to Mars and 50,000,000 miles next to Jupiter, and there is left a zone 254,000,000 miles wide outside of which the asteroids never wander.  If any ever did, the attraction of Mars or Jupiter may have prevented their return.

Since the orbits of Mars and Jupiter show no sign of being affected by these bodies for a century past, it is probable that their number is limited, or at least that their combined mass does not approximate the size of a planet.  Professor Newcomb estimates that if all that are now discovered were put into one planet, it would not be over four hundred miles in diameter; and if a thousand more should exist, of the average size of those discovered since 1850, their addition would not increase the diameter to more than five hundred miles.

[Page 164] That all these bodies, which differ from each other in no respect except in brilliancy, can be noted and fixed so as not to be mistaken one for another, and instantly recognized though not seen for a dozen years, is one of the highest exemplifications of the accuracy of astronomical observation.

JUPITER.

The king of the gods; sign [Symbol], the bird of Jove.

DISTANCE FROM THE SUN, PERIHELION, 457,000,000 MILES; APHELION, 503,000,000 MILES.  DIAMETER, 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.