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Page 30
Mercury 29.55 | Jupiter 8.06
Venus 21.61 | Saturn 5.95
Earth 18.38 | Uranus 4.20
Mars 14.99 | Neptune 3.36
Hence, while the 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 each side of the
equator. Every place has its twelve hours' darkness and light at
each revolution. But at C--the axis of the earth always preserving
the same direction--the northern pole is shrouded in continual
gloom. Every place [Page 105] north of the equator gets more
darkness than light, and hence winter.
The varying inclination of the axes of the different planets gives
a wonderful variety to their seasons. The sun is always nearly
over the equator of Jupiter, and every place has nearly its five
hours day and five hours night. The seasons of Earth, Mars, and
Saturn are so much alike, except in length, that no comment is
necessary. The ice-fields at either pole of Mars are observed to
enlarge and contract, according as it is winter or summer there.
Saturn's seasons are each seven and a half years long. The alternate
darkness and light at the poles is fifteen years long.
But the seasons of Venus present the greatest anomaly, if its assigned
inclination of axis (75�) can be relied on as correct, which is
doubtful. Its tropic zone extends nearly to the pole, and at the
same time the winter at the other pole reaches the equator. The
short period of this planet causes it to present the south pole to
the sun only one hundred and twelve days after it has been scorching
the one at the north. This gives two winters, springs, summers, and
autumns to the equator in two hundred and twenty-five days.
If each whirling world should leave behind it a trail of light to
mark its orbit, and our perceptions of form were sufficiently acute,
we should see that these curves of light are not exact circles, but
a little flattened into an ellipse, with the sun always in one
of the foci. Hence each planet is nearer to the sun at one part
of its orbit than another; that point is called the perihelion,
and the farthest point aphelion. This eccentricity of orbit, or
distance of the sun from the centre, is very small. [Page 106] In
the case of Venus it is only .007 of the whole, and in no instance
is it more than .2, viz., that of Mercury. This makes the sun appear
twice as large, bright, and hot as seen and felt on Mercury at its
perihelion than at its aphelion. The earth is 3,236,000 miles nearer
to the sun in our winter than summer. Hence the summer in the
southern hemisphere is more intolerable than in the northern. But
this eccentricity is steadily diminishing at a uniform rate, by
reason of the perturbing influence of the other planets. In the case
of some other planets it is steadily increasing, and, if it were to
go on a sufficient time, might cause frightful extremes of
temperature; but Lalande has shown that there are limits at which it
is said, "Thus far shalt thou go, and no farther." Then a
compensative diminution will follow.
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