Jupiter is nearly five times farther from the sun than we are. His
mean distance from that orb is four hundred and eighty-three millions
of miles. His orbit is not so eccentric as that of Mercury or of Mars,
but the eccentricity is sufficient to make his distance vary by as much
as forty-two millions of miles. His distance is five hundred and four
millions of miles when he is farthest from the sun, and four hundred
and sixty-two millions when he is nearest to it. On account of his
orbit being outside of ours, we are at times nearer to him and at
others farther from him than the sun ever is. At his best situation
when in opposition, we are three hundred and sixty-nine million miles
from him. This is more than ten times farther than we are from Mars
at that planet’s most favorable oppositions, and yet Jupiter is much
brighter at such times than Mars ever appears to be. At the times of
conjunction he is five hundred and ninety-six millions of miles from
us, but is still always brighter than a first-magnitude star like
Capella or Vega.
Although the distance of Jupiter from us varies thus two hundred and
twenty-seven million miles, there is never in him the marked difference
in brilliancy that we see in Mars. He is at all times so far away that
the variation in distance does not count for as much, though we can see
the effect of it plainly enough, even with the naked eye. Light, with
all its marvelous speed, consumes more than fifty-three minutes in its
journey from Jupiter to the earth when we are most widely separated
from him. When we are nearest to him light comes to us from the planet
in twenty minutes less time. At his average distance from the sun it
requires about forty-three minutes for light to pass from the sun to
Jupiter.
Notwithstanding the sun’s great power over Jupiter in shaping his
course, it does not give him much in return for his subserviency. So
far as light and brilliancy are concerned, it is to Jupiter a very
small sun indeed. To an observer on Jupiter the sun would not appear
to be more than one-fifth as large as it seems to us. The light it
furnishes to Jupiter is twenty-five times less than we receive; and if
the planet depended entirely upon the sun for heat, his temperature
would be more than two hundred degrees below zero, Fahrenheit. There
is every reason to believe that the little heat the sun gives to
this mighty planet does not count for much one way or the other at
the planet’s present stage of development. Jupiter does not need the
nourishing that the smaller terrestrial planets must have, or die. He
is probably almost a sun himself. We are not at all certain that the
planet is even so far cooled as to have a solid surface. If it has,
there is reason to think that the surface is at least red hot, and
gives to the planet a temperature higher than anything we have any
comprehension of. Jupiter’s atmosphere, too, is extremely thick and
dense, so that the planet is probably so protected that it gets very
little heat from the sun and loses very little of its own.
It is certain, however, that this great planet is not so much of a sun
as to shine by its own light. The light we receive, though it is very
brilliant, is reflected sunlight. This is shown by the fact that the
planet does not furnish light for its own satellites. When they pass
into its shadow the sunlight is shut off from them; and if they receive
any light from Jupiter, it is too dusky to be perceptible to us. That
the planet may have a red glow, though, is also suggested by the action
of the satellites. When they pass between us and Jupiter they sometimes
cast less of a shadow on his surface than would be expected, thus
indicating that the surface is not altogether dark, though it may only
dully glow rather than shine.