he earth makes one revolution in its year, Mercury
has made over four revolutions, or passed through four years; the
slower Neptune has made only 1/164 of one revolution.

The time of axial revolution which determines the length of the
day varies with different planets. The periods of the four planets
nearest the sun vary only half an hour from that of the earth,
while the enormous bodies of Jupiter and Saturn revolve in ten
and ten and a quarter hours respectively. This high rate of speed,
and its resultant, centrifugal force, has aided in preventing these
bodies from becoming as dense as they would otherwise be--Jupiter
being only 0.24 as dense as the earth, and Saturn only 0.13. This
extremely rapid revolution produces a great flattening at the poles.
If Jupiter should rotate four times more rapidly than it does, it
could not be held together compactly. As it is, the polar diameter
is five thousand miles less than the equatorial: the difference
in diameters produced by the [Page 102] same cause on the earth,
owing to the slower motion and smaller mass, being only twenty-six
miles. The effect of this will be more specifically treated
hereafter.

The difference in the size of the planets is very noticeable. If
we represent the sun by a gilded globe two feet in diameter, we
must represent Vulcan and Mercury by mustard-seeds; Venus, by a
pea; Earth, by another; Mars, by one-half the size; Asteroids, by
the motes in a sunbeam; Jupiter, by a small-sized orange; Saturn,
by a smaller one; Uranus, by a cherry; and Neptune, by one a little
larger.

Apply the principle that attraction is in proportion to the mass,
and a man who weighs one hundred and fifty pounds on the earth
weighs three hundred and ninety-six on Jupiter, and only fifty-eight
on Mars; while on the Asteroids he could play with bowlders for
marbles, hurl hills like Milton's angels, leap into the fifth-story
windows with ease, tumble over precipices without harm, and go
around the little worlds in seven jumps.

[Illustration: Fig. 39.--Orbit of Earth, showing Parallelism of
Axis and Seasons.]

The seasons of a planet are caused by the inclination of its axis
to the plane of its orbit. In Fig. 39 the rotating earth is seen
at A, with its northern pole turning in constant sunlight, and
its southern pole in constant darkness; everywhere south of the
equator is more darkness than day, and hence winter. Passing on
to B, the world is seen illuminated equally on