In illustration of the great diversity of the physical peculiarities
and probable condition of the planets, Sir John Herschel describes
the intensity of solar radiation as nearly seven times greater on
Mercury than on the earth, and on Uranus 330 times less; the proportion
between the two extremes being that of upwards of 2000 to 1. Let any
one figure to himself, (adds Sir John,) the condition of our globe
were the sun to be septupled, to say nothing of the greater ratio; or
were it diminished to a seventh, or to a 300th of its actual power!
Again, the intensity of gravity, or its efficacy in counteracting
muscular power and repressing animal activity, on Jupiter is nearly
two-and-a-half times that on the earth; on Mars not more than one-half;
on the moon one-sixth; and on the smaller planets probably not more
than one-twentieth; giving a scale of which the extremes are in the
proportion of sixty to one. Lastly, the density of Saturn hardly
exceeds one-eighth of the mean density of the earth, so that it must
consist of materials not much heavier than cork.
Jupiter is eleven times, Saturn ten times, Uranus five times, and
Neptune nearly six times, the diameter of our earth.
These four bodies revolve in space at such distances from the sun,
that if it were possible to start thence for each in succession,
and to travel at the railway speed of 33 miles per hour, the
traveller would reach
Jupiter in 1712 years
Saturn 3113 ”
Uranus 6226 ”
Neptune 9685 ”
If, therefore, a person had commenced his journey at the period of
the Christian era, he would now have to travel nearly 1300 years
before he would arrive at the planet Saturn; more than 4300 years
before he would reach Uranus; and no less than 7800 years before he
could reach the orbit of Neptune.
Yet the light which comes to us from these remote confines of the
solar system first issued from the sun, and is then reflected
from the surface of the planet. When the telescope is turned
towards Neptune, the observer’s eye sees the object by means of
light that issued from the sun eight hours before, and which
since then has passed nearly twice through that vast space which
railway speed would require almost a century of centuries to
accomplish.--_Bouvier’s Familiar Astronomy._