"He hangeth the earth upon nothing."--_Job_ xxvi. 7.
[Page 98]
"Let a power be delegated to a finite spirit equal to the projection
of the most ponderous planet in its orbit, and, from an exhaustless
magazine, let this spirit select his grand central orb. Let him with
puissant arm locate it in space, and, obedient to his mandate, there
let it remain forever fixed. He proceeds to select his planetary
globes, which he is now required to marshal in their appropriate
order of distance from the sun. Heed well this distribution; for
should a single globe be misplaced, the divine harmony is destroyed
forever. Let us admit that finite intelligence may at length determine
the order of combination; the mighty host is arrayed in order.
These worlds, like fiery coursers, stand waiting the command to
fly. But, mighty spirit, heed well the grand step, ponder well
the direction in which thou wilt launch each wailing world; weigh
well the mighty impulse soon to be given, for out of the myriads
of directions, and the myriads of impulsive forces, there comes
but a single combination that will secure the perpetuity of your
complex scheme. In vain does the bewildered finite spirit attempt
to fathom this mighty depth. In vain does it seek to resolve the
stupendous problem. It turns away, and while endued with omnipotent
power, exclaims, 'Give to me infinite wisdom, or relieve me from
the impossible task!'"-0. M. MITCHEL, LL. D.
[Page 99]
VI.
_THE PLANETS, AS SEEN FROM SPACE_
If we were to go out into space a few millions of miles from either
pole of the sun, and were endowed with wonderful keenness of vision,
we should perceive certain facts, viz: That space is frightfully
dark except when we look directly at some luminous body. There is
no air to bend the light out of its course, no clouds or other
objects to reflect it in a thousand directions. Every star is a
brilliant point, even in perpetual sunshine. The cold is frightful
beyond the endurance of our bodies. There is no sound of voice in
the absence of air, and conversation by means of vocal organs being
impossible, it must be carried on by means of mind communication.
We see below an unrevolving point on the sun that marks its pole.
Ranged round in order are the various planets, each with its axis
pointing in very nearly the same direction. All planets, except
possibly Venus, and all moons except those of Uranus and Neptune,
present their equators to the sun. The direction of orbital and
axial revolution seen from above the North Pole would be opposite
to that of the hands of a watch.
[Illustration: Fig. 38.--Orbits and Comparative Sizes of the Planets.]
The speed of this orbital revolution must be proportioned to the
distance from the sun. The attraction of the sun varies inversely
as the square of the distance. [Page 100] It holds a planet with a
certain power; one twice as far off, with one-fourth that power.
This attraction must be counterbalanced by centrifugal force; great
force from great speed when attraction is great, and small from less
[Page 101] speed when attractive power is diminished by distance.
Hence Mercury must go 29.5 miles per second--seventy times as fast
as a rifle-ball that goes two-fifths of a mile in a second--or be
drawn into the sun; while Neptune, seventy-five times as far off,
and hence attracted only 1/5626 as much, must be slowed down to 3.4
miles a second to prevent its flying away from the feebler
attraction of the sun. The orbital velocity of the various planets
in miles per second is as follows:
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.
Conceive a large globe, to represent the sun, floating in a round
pond. The axis will be inclined 7-1/2° to the surface of the water,
one side of the equator be 7-1/2° below the surface, and the other
side the same distance above. Let the half-submerged earth sail
around the sun in an appropriate orbit. The surface of the water
will be the plane of the orbit, and the water that reaches out
to the shore, where the stars would be set, will be the plane of
the ecliptic. It is the plane of the earth's orbit extended to
the stars.
The orbits of all the planets do not lie in the same plane, but
are differently inclined to the plane of the ecliptic, or the plane
of the earth's orbit. Going out from the sun's equator, so as to
see all the orbits of the planets on the edge, we should see them
inclined to that of the earth, as in Fig. 40.
[Illustration: Fig. 40.--Inclination of the Planes of Orbits.]
If the earth, and Saturn, and Pallas were lying in [Page 107] the
same direction from the sun, and the outer bodies were to start in a
direct line for the sun, they would not collide with the earth on
their way; but Saturn would pass 4,000,000 and Pallas 50,000,000
miles over our heads. From this same cause we do not see Venus and
Mercury make a transit across the disk of the sun at every
revolution.
[Illustration: Fig. 41.--Inclination of Orbits of Venus and Earth.
Nodal Line, D B.]
Fig. 41 shows a view of the orbits of the earth and Venus seen
not from the edge but from a position somewhat above. The point E,
where Venus crosses the plane of the earth's orbit, is called the
ascending node. If the earth were at B when Venus is at E, Venus
would be seen on the disk of the sun, making a transit. The same
would be true if the earth were at D, and Venus at the descending
node F.
This general view of the flying spheres is full of interest. [Page
108] While quivering themselves with thunderous noises, all is
silent about them; earthquakes may be struggling on their surfaces,
but there is no hint of contention in the quiet of space. They are
too distant from one another to exchange signals, except, perhaps,
the fleet of asteroids that sail the azure between Mars and Jupiter.
Some of these come near together, continuing to fill each other's
sky for days with brightness, then one gradually draws ahead. They
have all phases for each other--crescent, half, full, and gibbous.
These hundreds of bodies fill the realm where they are with
inexhaustible variety. Beyond are vast spaces--cold, dark, void of
matter, but full of power. Occasionally a little spark of light
looms up rapidly into a world so huge that a thousand of our earths
could not occupy its vast bulk. It swings its four or eight moons
with perfect skill and infinite strength; but they go by and leave
the silence unbroken, the darkness unlighted for years.
Nevertheless, every part of space is full of power. Nowhere in its
wide orbit can a world find a place; at no time in its eons of
flight can it find an instant when the sun does not hold it in
safety and life.
_The Outlook from the Earth._
If we come in from our wanderings in space and take an outlook from
the earth, we shall observe certain movements, easily interpreted
now that we know the system, but nearly inexplicable to men who
naturally supposed that the earth was the largest, most stable,
and central body in the universe.
We see, first of all, sun, moon, and stars rise in the east, mount
the heavens, and set in the west. As I [Page 109] revolve in my
pivoted study-chair, and see all sides of the room--library, maps,
photographs, telescope, and windows--I have no suspicion that it is
the room that whirls; but looking out of a car-window in a depot at
another car, one cannot tell which is moving, whether it be his car
or the other. In regard to the world, we have come to feel its
whirl. We have noticed the pyramids of Egypt lifted to hide the sun;
the mountains of Hymettus hurled down, so as to disclose the moon
that was behind them to the watchers on the Acropolis; and the
mighty mountains of Moab removed to reveal the stars of the east.
Train the telescope on any star; it must be moved frequently, or the
world will roll the instrument away from the object. Suspend a
cannon-ball by a fine wire at the equator; set it vibrating north
and south, and it swings all day in precisely the same direction.
But suspend it directly over the north pole, and set it swinging
toward Washington; in six hours after it is swinging toward Rome, in
Italy; in twelve hours, toward Siam, in Asia; in nineteen hours,
toward the Sandwich Islands; and in twenty-four, toward Washington
again, not because it has changed the plane of its vibration, but
because the earth has whirled beneath it, and the torsion of the
wire has not been sufficient to compel the plane of the original
direction to change with the turning of the earth. The law of
inertia keeps it moving in the same direction. The same experimental
proof of revolution is shown in a proportional degree at any point
between the pole and the equator.
But the watchers on the Acropolis do not get turned over so as to
see the moon at the same time every night. [Page 110] We turn down
our eastern horizon, but we do not find fair Luna at the same moment
we did the night before. We are obliged to roll on for some thirty
to fifty minutes longer before we find the moon. It must be going in
the same direction, and it takes us longer to get round to it than
if if it were always in the same spot; so we notice a star near the
moon one night--it is 13° west of the moon the next night. The moon
is going around the earth from west to east, and if it goes 13° in
one day, it will take a little more than twenty-seven days to go the
entire circle of 360°.
[Illustration: Fig. 42.--Showing the Sun's Movement among the Stars.]
[Page 111]
In our outlook we soon observe that we do not by our revolution
come to see the same stars rise at the same hour every night. Orion
and the Pleiades, our familiar friends in the winter heavens, are
gone from the summer sky. Have they fled, or are we turned from
them? This is easily understood from Fig. 42.
When the observer on the earth at A looks into the midnight sky
he sees the stars at E; but as the earth passes on to B, he sees
those stars at E three minutes sooner every night; and at midnight
the stars at F are over his head. Thus in a year, by going around
the sun, we have every star of the celestial dome in our midnight
sky. We see also how the sun appears among the successive
constellations. When we are at A, we see the sun among the stars
at G; but as we move toward B, the sun appears to move toward H.
If we had observed the sun rise on the 20th of August, 1876, we
should have seen it rise a little before Regulus, and a little
south of it, in such a relation as circle 1 is to the star in Fig.
43. By sunset the earth had moved enough to make the sun appear
to be at circle 2, and by the next morning at circle 3, at which
time Regulus would rise before the sun. Thus the earth's motion
seems to make the sun traverse a regular circle among the stars
once a year: but it is not the sun that moves.
[Illustration: Fig. 43.]
There are certain stars that have such irregular, uncertain, vagarious
ways that they were called vagabonds, or planets, by the early
astronomers. Here is the path of Jupiter in the year 1866 (Fig.
44). These bodies go forward for awhile, then stop, start aside,
then retrograde, [Page 112] and go on again. Some are never seen far
from the sun, and others in all parts of the ecliptic.
[Illustration: Fig. 44.]
First see them as they stand to-day, as in Fig. 45. The observer
stands on the earth at A. It has rolled over so far that he cannot
see the sun; it has set. But Venus is still in sight; Jupiter is
45° behind Venus, and Saturn is seen 90° farther east. When A has
rolled a little farther, if he is awake, he will see Mars before
he sees the sun; or, in common language, Venus will set after,
and Mars rise before the sun. All these bodies at near and far
distances seem set in the starry dome, as the different stars seem
in Fig. 42, p. 110.
[Illustration: Fig. 45. Showing Position of Planets.]
The mysterious movements of advance and retreat are rendered
intelligible by Fig. 46. The planet Mercury is at A, and, seen from
the earth, B is located at _a_, [Page 113] on the background of the
stars it seems to be among. It remains apparently stationary at _a_
for some time, because approaching the earth in nearly a straight
line. Passing D to C, it appears to retrograde among the stars to
_c_; remains apparently stationary for some time, then, in passing
from C to E and A, appears to pass back among the stars to _a_. The
progress of the earth, meanwhile, although it greatly retards the
apparent motion from A to C, greatly hastens it from C to A.
[Illustration: Fig. 46.--Apparent Movements of an Inferior Planet.]
It is also apparent that Mercury and Venus, seen from the earth,
can never appear far from the sun. They must be just behind the
sun as evening stars, or just before it as heralds of the morning.
Venus is never more than 47° from the sun, and Mercury never more
than 30°; indeed, it keeps so near the sun that very few people
have ever seen the brilliant sparkler. Observe how much larger the
planet appears near the earth in conjunction at D than in opposition
at E. Observe also what phases it must present, and how transits
sometimes take place.
[Page 114]
The movement of a superior planet, one whose orbit is exterior
to the earth, is clear from Fig. 47. When the earth is at A and
Mars at B, it will appear among the stars at C. When the earth is
at D, Mars having moved more slowly to E, will have retrograded
to F. It remains there while the earth passes on, in a line nearly
straight, from Mars to G; then, as the earth begins to curve around
the sun, Mars will appear to retraverse the distance from F to
C, and beyond. The farther the superior planet is from the earth
the less will be the retrograde movement.
[Illustration: Fig. 47.--Illustrating Movements of a Superior Planet.]
The reader should draw the orbits in proportion, and, remembering
the relative speed of each planet, note the movement of each in
different parts of their orbits.
To account for these most simple movements, the earlier astronomers
invented the most complex and impossible machinery. They thought the
earth the centre, and that the sun, moon, and stars were carried
about it, as stoves around a person to warm him. They thought these
strange movements of the planets were accomplished by mounting them
on subsidiary eccentric wheels in the revolving crystal sphere.
All that was [Page 115] needed to give them a right conception was a
sinking of their world and themselves to an appropriate proportion,
and an enlargement of their vision, to take in from an exalted
stand-point a view of the simplicity of the perfect plan.