THE LEADING MEMBERS OF OUR FAMILY--FIRST GROUP.
The chief distinction between stars and planets is, as before said,
that the stars shine entirely by their own light, while the planets
shine chiefly, if not entirely, by reflected light. The stars are suns;
mighty globes of glowing flame. The planets simply receive the light of
the sun, and shine with a brightness not their own.
A lamp shines by its own light; but a looking-glass, set in the sun’s
rays and flashing beams in all directions, shines by reflected light.
In a dark room it would be dark. If there were no sun to shine upon
Mars or Venus, we should see no brightness in them. The moon is like
the planets in this. She has only borrowed light to give, and none of
her own.
Any one of the planets removed to the distance of the nearest fixed
star, would be invisible to us. Reflected light will not shine nearly
so far as the direct light of a burning body. There may be thousands or
millions of planets circling round the stars--those great and distant
suns--just as our brother-planets circle round our sun. But it is
impossible for us to see them. The planets which we can see are close
neighbors, compared with the stars. I do not mean that they are near in
the sense in which we speak of nearness upon earth. They are only near
in comparison with what is so very much farther away.
For a while we must now leave alone all thought of the distant stars,
and try to gain a clear idea of the chief members of our own Family
Circle--that family circle of which the sun is the head, the center,
the source of life and warmth and light. There are two ways in which
astronomers group the planets of the Solar System. One way is to divide
them into the Inferior Planets, and the Superior Planets.
As the earth travels in her pathway round the sun, two planets travel
on their pathways round the sun nearer to him than ourselves. If the
pathway or orbit of our earth were pictured by a hoop laid upon the
table, with a ball in the center for the sun; then those two planets
would have two smaller hoops of different sizes _within_ ours; and the
rest would have larger hoops of different sizes _outside_ ours. The two
within are called inferior planets, and the rest outside are called
superior planets.
A round hoop would not make a good picture of an orbit. For the yearly
pathway of our earth is not in shape perfectly round, but slightly
oval; and the sun is not exactly in the center, but a little to one
side of the center. This is more or less the case with the orbits of
all the planets.
But the laying of the hoops upon the table would give no bad idea of
the way in which the orbits really lie in the heavens. The orbits of
all the chief planets do not slope and slant round the sun in all
manner of directions. They are placed almost in the same _plane_ as it
is called--or, as we might say, in the same _flat_. In these orbits
the planets all travel round in the same direction. One may overtake a
second on a neighboring orbit, and get ahead of him, but one planet
never goes back to meet another.
In speaking of the orbits, I do not mean that the planets have visible
marked pathways through the heavens, any more than a swallow has a
visible pathway through the sky, or a ship a marked pathway through the
sea. Yet each planet has his own orbit, and each planet so distinctly
keeps to his own, that astronomers can tell us precisely whereabouts in
the heavens any particular planet will be, at any particular time, long
years beforehand.
There is also another mode of grouping the planets, besides dividing
them into superior and inferior planets. By this other mode we find two
principal groups or quartets of planets, separated by a zone or belt of
a great many very small planets.
{ Mercury.
First Group { Venus.
{ Earth.
{ Mars.
The Asteroids or Planetoids.
{ Jupiter.
Second Group { Saturn.
{ Uranus.
{ Neptune.
The first four are small compared with the last four, though much
larger than any in the belt of tiny Asteroids.
It was believed at one time that a planet had been discovered nearer to
the sun than Mercury, and the name Vulcan was given to it. But no more
has been seen of Vulcan, and his existence is so doubtful that we must
not count him as a member of the family without further information.
Mercury is very much smaller than our Earth. The diameter of the earth
is eight thousand miles, but the diameter of Mercury is only about
three thousand miles--not even half that of the earth. Being so much
nearer to the sun than ourselves, the pulling of his attraction is much
greater, and this has to be balanced by greater speed, or Mercury would
soon fall down upon the sun. Our distance from the sun is ninety-three
millions of miles. Mercury’s distance is only about one-third of ours;
and instead of traveling, like the earth, at the rate of eighteen miles
each second, Mercury dashes headlong through space at the mad pace
of twenty-nine miles each second. It is a good thing the earth does
not follow his example, or she would soon break loose from the sun’s
control altogether.
The earth takes more than three hundred and sixty-five days, or twelve
months, to journey round the sun in her orbit. That is what we call
“the length of our year.” But Mercury’s year is only eighty-eight days,
or not quite three of our months. No wonder!--when his pathway is so
much shorter, and his speed so much greater than ours. So Mercury
has four years to one year on earth; and a person who had lived on
Mercury as long as five earthly years, would then be twenty years old.
The increased number of birthdays would scarcely be welcome in large
families, supposing we could pay a long visit there.
The sun, as seen from Mercury, looks about four and a half times as
large as from here; the heat and glare being increased in proportion.
No moon has ever been found, belonging to Mercury.
The planet Mercury is, like the earth and the moon, a globe of dark
matter which only shines and is visible by the illumination of the
solar light. Its motion round the central star, which brings it
sometimes between the sun and us, sometimes in an oblique direction,
sometimes at right angles, and shows us a part incessantly variable,
of its illuminated hemisphere, produces in its aspect, as seen in a
telescope, a succession of phases similar to those which the moon
presents to us. The cut represent the apparent variations of size and
the succession of phases, visible in the evening after sunset; when the
planet attains its most slender crescent, it is in the region of its
orbit nearest to the earth, and passes between the sun and us; then,
some weeks afterwards, it emerges from the solar rays and passes again
through the same series of phases in inverse order, as we see them by
reversing the figure.
[Illustration: PHASES OF MERCURY BEFORE INFERIOR CONJUNCTION--EVENING
STAR.]
Venus, the second inferior planet, is nearly the same size as our
earth. Seen from the earth, she is one of the most brilliant and
beautiful of all the planets. Her speed is three miles a second faster
than ours, and her distance from the sun is about two-thirds that of
our own; so that the orbit of Venus lies half-way between the orbit of
Mercury and the orbit of the earth. The day of Venus is about half an
hour shorter than ours. Her year is nearly two hundred and twenty-five
days, or seven and a half of our months. One or two astronomers have
fancied that they caught glimpses of a moon near Venus; but this is
still quite doubtful, and indeed it is believed to have been a mistake.
Venus and Mercury are only visible as morning and evening planets.
Venus, being farther from the sun, does not go before and follow after
him quite so closely as Mercury, and she is therefore the longer within
sight.
When Venus, traveling on her orbit, comes just between the sun and us,
her dark side is turned towards the earth, and we can catch no glimpse
of her. When she reaches that part of her orbit which is farthest from
us, quite on the other side of the sun, her great distance from us
makes her light seem less. But about half-way round on either side, she
shows exceeding brilliancy, and that is the best view we can get of her.
Seen through a telescope, Venus undergoes phases like those of Mercury,
or of our moon. That is to say, we really have “new Venus,” “quarter
Venus,” “half Venus,” “full Venus,” and so on.
Of all the luminaries in the heavens, the sun and moon excepted, the
planet Venus is the most conspicuous and splendid. She appears like
a brilliant lamp amid the lesser orbs of night, and alternately
anticipates the morning dawn, and ushers in the evening twilight.
When she is to the westward of the sun, in winter, she cheers our
mornings with her vivid light, and is a prelude to the near approach
of the break of day and the rising sun. When she is eastward of that
luminary, her light bursts upon us after sunset, before any of the
other radiant orbs of heaven make their appearance; and she discharges,
in some measure, the functions of the absent moon. The brilliancy of
this planet has been noticed in all ages, and has been frequently
the subject of description and admiration both by shepherds and by
poets. The Greek poets distinguished it by the name of Phosphor,
“light-bringer,” when it rose before the sun, and Hesperus, “the west,”
when it appeared in the evening after the sun retired. It is now
generally distinguished by the name of the Morning and Evening Star.
Next to the orbit of Venus comes the orbit of our own Earth, the third
planet of the first group.
Mars, the fourth of the inner quartet, but the first of the superior
planets, is a good deal smaller than Venus or the earth. The name Mars,
from the heathen god of war, was given on account of his fiery reddish
color. Mars is better placed than Venus for being observed from earth.
When he is at the nearest point of his orbit to us, we see him full in
the blaze of sunlight; whereas Venus, at her nearest point, turns her
bright face away.
The length of the day of Mars--or, in other words, the time he takes
to turn upon his axis--is only forty minutes longer than that of
earth. Mars’ journey round the sun is completed in the course of six
hundred and eighty-seven days, not much less than two of our years.
His distance from the sun is about one hundred and forty millions of
miles, and his speed is fourteen miles a second. We shall find, with
the increasing distance of each planet, that the slower pace balances
the lessened amount of the sun’s attraction.
Passing on from Mars, the last of the first group of planets, we reach
the belt of Asteroids, sometimes called Planetoids, Minor Planets, or
Telescopic Planets. They are so tiny that Mercury is a giant compared
with the largest among them.
The zone of space containing all these little planets is more than a
hundred millions of miles broad. Their orbits do not lie flat in almost
the same plane, but slant about variously in a very entangled fashion.
If a neat model were made of this zone, with a slender piece of wire to
represent each orbit, it would be found impossible to lift up one wire
without pulling up all the rest with it. Those asteroids lying nearest
to the sun take about three of our years to travel round him, and those
lying farthest take about six of our years.
New members of the group are very often found. The number of asteroids
now known amounts to about three hundred and twenty. Ceres is estimated
by Professor Barnard, of the Yerkes Observatory, Chicago University, to
be six hundred miles in diameter. Vesta is about two hundred and fifty
miles in diameter. Meta, on the other hand, is less than seventy-five
miles. There are some smaller ones that do not measure twenty miles in
diameter, and it is probable that there are many which are so small as
to be absolutely invisible to the best telescopes, and which measure
only a few hundred rods in diameter. Twenty thousand Vestas would be
needed to make one globe equal to our earth in size.
Are they _worlds_? Why not? Is not a drop of water, shown in the
microscope, peopled with a multitude of various beings? Does not a
stone in a meadow hide a world of swarming insects? Is not the leaf
of a plant a world for the species which inhabit and prey upon it?
Doubtless among the multitude of small planets there are those which
must remain desert and sterile, because the conditions of life (of any
kind) are not found united. But we can not doubt that on the majority
the ever-active forces of nature have produced, as in our world,
creations appropriate to these minute planets. Let us repeat, moreover,
that for nature there is neither great nor little. And there is no
necessity to flatter ourselves with a supreme disdain for these little
worlds; for in reality the inhabitants of Jupiter would have more right
to despise us than we have to despise Vesta, Ceres, Pallas, or Juno.
The disparity is greater between Jupiter and the earth than between the
earth and these planets. A world of two, three, or four hundred miles
in diameter is still a continent worthy to satisfy the ambition of a
Xerxes or a Tamerlane.