years ago, our own sun may have undergone some cataclysmic change
and this may, conceivably, have been brought about by disturbances
within the sun itself. Elements may have been so formed and
distributed within the interior of the sun that friction and internal
instability resulted and in time produced an upheaval of solar
elements with initial velocities so great that, possibly, through
electrical repulsion and light-pressure, portions of the ejected
streams were permanently detached from the sun and became the nuclei
of future planets. In some such way, it is conceivable, our own
planet Earth and the other members of our solar system may have been
brought into existence in the dim and distant past--many hundred
million years ago.
XIX
JUPITER AND HIS NINE MOONS
Jupiter shines by reflected sunlight with a brilliancy that usually
exceeds that of the brightest of the stars, Sirius. When seen during
the midnight hours the remarkable unflickering brightness of this
largest and most distinguished member of the solar system at once
serves to set it apart from the scintillating stars far beyond.
There is but one planet, Venus, that always surpasses Jupiter in
brilliancy, though Mars on the occasions of its close approaches
to the earth may equal or slightly surpass Jupiter in brightness.
As Venus never departs more than forty-eight degrees from the sun,
and so is never seen in the midnight hours, Jupiter usually shines
without a rival when visible at midnight. To one who has observed the
two planets together the silvery radiance and surpassing brilliancy
of Venus, due not to its size, but to its comparative nearness to
the earth, at once serves to distinguish it from the golden glow of
Jupiter.
Even the smallest telescopes of two- or three-inch aperture will show
the four historic moons of Jupiter which were the first celestial
objects to be discovered when Galileo turned his crude telescope to
the heavens in the year 1610.
The fact that these tiny points of light were actually revolving
around the great planet was soon detected by the famous astronomer
and we can imagine with what breathless interest he observed these
satellites of another world whose discovery dealt such a severe blow
to the old Ptolemaic theory that the earth was the center of the
universe. It was not until the great telescopes of modern times were
invented that the five additional moons of Jupiter were discovered.
The four satellites first observed by Galileo were fancifully named
Io, Europa, Ganymede and Callisto, in the order of their positions
outward from the planet, but these names are rarely used now, the
satellites being designated for convenience I, II, III and IV,
respectively. The first of the new satellites to be discovered was
Satellite V, which is the nearest to Jupiter of all the nine moons.
It is an extremely small body, not more than one hundred miles in
diameter, and to discover this tiny body as it skirted rapidly around
the great planet within sixty-seven thousand miles of its surface,
nearly lost in the glaring rays, was a difficult feat even for an
experienced observer. It was accomplished, however, by Prof. E.
E. Barnard with the great Lick refractor in 1892. Satellite V is
hopelessly beyond the reach of any but the greatest telescopes, as
are also the four satellites discovered since that date. In fact,
most of these tiny moons are observed photographically. Satellites VI
and VII were discovered photographically in 1905. They are both about
seven million miles from the planet and their paths loop through
one another; they are, moreover, highly inclined to each other at
an angle of nearly thirty degrees. When nearest together they are
separated by a distance of two million miles. Two more extremely
small bodies, known as Satellites VIII and IX, have been discovered
since then, one at Greenwich, England, in 1908, the other at the Lick
Observatory in 1914. These excessively faint bodies are the most
remote satellites of Jupiter and they are of particular interest
because they travel around the planet in a retrograde direction, or
from east to west, which is _opposite_ to the direction of revolution
prevailing in the solar system. The ninth and most distant satellite
of Saturn also retrogrades, or revolves in a clockwise rather than
a counter-clockwise direction around the planet. One explanation
given for this peculiarity of the outermost satellites of Jupiter
and Saturn is that this backward revolution around the planet is
more stable when the satellites are at great distances from the
primary, and the gravitational control that the planet exerts is
therefore weak. The moons of the planets are, of course, subject
to the attraction of the sun as well as to the attraction of the
controlling planet, and the greater the distance of the satellite
from the planet the stronger the pull exerted by the sun and the
weaker the bonds that bind the moon to the planet. Beyond a certain
limit it would be impossible for the planet to hold the satellite
against the sun's greater attraction and the satellite would leave
the planet to revolve directly around the sun, thereby becoming a
planet. It appears that as this danger limit is neared it is safer
for the satellite to "back" around the planet than to follow the
usual "west to east" direction of revolution. The eighth satellite
of Jupiter is more than fourteen million and the ninth more than
fifteen million miles from the parent planet and they require about
two years and three years, respectively, to complete one trip around
Jupiter. When we consider that Satellite V darts around the planet
in less than twelve hours at a distance of only sixty-seven thousand
miles from its surface we realise what tremendous differences exist
in the distances and periods of revolution of the nine moons. There
is also great disparity in the sizes of the various moons. The five
moons discovered in modern times are all excessively faint and
extremely small. The diameter of the largest of these, Satellite
V, is less than one hundred miles. On the other hand, the four
historic moons of Jupiter are of planetary dimensions. The smallest,
Satellite II, is slightly larger than our own moon, while the
largest, Satellite III, has a diameter, according to measurements
made with the 40-inch Yerkes refractor in 1916, of three thousand
nine hundred and eight miles, which is only four hundred miles less
than the diameter of Mars. The periods of revolution of these four
satellites range from one day and eighteen hours for the nearest,
which is about two hundred and sixty-one thousand miles from the
center of Jupiter, to sixteen days and sixteen and one-half hours for
the most distant, which is more than one million one hundred and
sixty thousand miles from the planet. These four moons are so near
to the great planet that they are continually dipping into his huge
shadow and experiencing an eclipse of the sun which, owing to the
nearness and great size of Jupiter, lasts for two or three hours. At
times of eclipse the moon suddenly disappears from the observer's
view, though it may be considerably to one side of the planet. Its
reappearance later on is just as sudden, or it may pass out of the
shadow while hidden from us behind the disk of the planet, in which
case its reappearance is invisible from the earth. The occultations
of the satellites, or, in other words, their disappearance behind
the planet's disk, are also interesting phenomena to observe, as
are their "transits" across the disk of the planet as the satellite
passes in front of it. Not only the satellite itself but its shadow
as well can be seen, a small black dot passing over the surface of
Jupiter. The satellite is totally eclipsing the sun for this small
dark portion of the planet's disk. Two satellites and their shadows
are frequently seen crossing the face of the planet at the same
time. It is possible to observe all the phenomena of the satellite's
transits and shadows, eclipses and occultations with very small
telescopes. From observations of the eclipses of Jupiter's satellites
the important discovery of the finite velocity of light was first
made as far back as the year 1675.
Faint surface markings have been made out at certain times on the
largest of the four satellites, Satellite III, and also on Satellite
I. Observations of the markings on the former seem to indicate that
it always keeps the same face turned toward Jupiter as does our own
moon toward the earth.
There are also reasons for believing that the equatorial regions of
Satellite I are light colored and the polar regions dark. There is
the possibility that forms of life may exist on these satellites of
Jupiter, though they are more likely barren, lifeless worlds, such as
Mercury and the moon. Their great distance from the earth, never less
than three hundred and sixty-eight million miles, makes observations
of their surface markings very difficult.
How beautiful beyond description must the heavens appear as viewed
from the satellites of Jupiter! Viewed from the distance of Io, or
Satellite I, the mighty planet Jupiter presents a spectacle such
as the eye of man has never been privileged to behold. The huge
flattened globe, ninety thousand miles in equatorial diameter, equal
in mass to _three hundred planets such as our own_ and in volume to
nearly _fourteen hundred_, fills a space in the heavens nearly twenty
degrees in extent as viewed from this satellite. Fifteen hundred of
our own full moons would hardly fill the same space. Whirling on its
axis with frightful speed in a period of less than ten hours, the
huge ball glides rapidly but majestically onward through the sky. A
far distant sun shrunk to but one-fifth the diameter of the full moon
throws light and shade across the rapidly changing surface of the
planet, rich in the reds, browns and yellows and all the gorgeous
shades and tints of its dense, seething, gaseous envelope. The phases
of the moon on a greatly enlarged scale rapidly succeed each other
on Jupiter as it is viewed from the satellite in all positions with
reference to the sun. The cause of the belts of Jupiter, that lie
parallel to the planet's equator and are constantly changing in
number, width and shade, as well as the nature of all the peculiar
splashes of color and intensely white flecks that come and go in
the dense atmosphere of the planet would not be such a mystery to
us were it possible to view the great planet from the distance of
Satellite I, which is about as far from the surface of Jupiter as
the moon is from the earth. It is uncertain whether the planet is
entirely gaseous throughout or has a central core of solid or liquid
matter. Its density is only one and one-quarter times that of water
and slightly less than that of the sun, showing that it is composed
largely, if not entirely, of matter in a gaseous state. Jupiter is a
world as different from our own as it is possible to imagine. There
is no visible surface crust and there are no permanent markings.
Different spots on the planet's disk give different periods of
rotation showing that it is atmospheric phenomena that we observe.
All is constant flux and change on Jupiter. Dense vapors arise
from a highly heated interior and spread out into belts parallel
to the equator in the direction of the planet's rotation. From its
nearest satellite all the interesting changes of color and form that
constantly take place in the atmosphere of this great globe could be
observed in great detail. The high percentage of light and heat that
Jupiter reflects from the sun to its nearer satellites makes it a
secondary sun to them of tremendous size though feeble strength.
As seen from Satellite I the other three major moons of Jupiter
present all the phases of our own moon in rapid succession, due to
their constantly changing positions with reference to the sun. The
five small moons, discovered in modern times, are so minute that they
are simply star-like points of light even when viewed from the other
moons of Jupiter.
To keep track of the rapidly changing positions and various phases
of the moons of Jupiter as seen from any one of them, as well as
the rapid _apparent_ motion of the planet through the sky due to
the revolutions of the satellite around the planet, would be a
troublesome task for an astronomer stationed on one of these far
distant worlds. It would be a common sight to see in the sky at one
time the huge planet, the far-distant, shrunken sun, and one, two
or three moons. Seen from the moons of Jupiter the constellations
would appear as they do to us on earth, for such a slight change in
position as five hundred million miles, more or less, is trivial
when one is looking at the stars. Observations of the stars from the
nearest moon of Jupiter would be attended with great difficulties at
times, since reflected sunlight from a body nearly twenty degrees in
diameter would be extremely troublesome, especially were the phases
of the planets near that of the full moon. We know how the presence
of our own moon in the heavens at the full dims the brightness of
the stars so that only the brightest stars are seen. Even as viewed
from the fourth or most distant of the major satellites the planet
subtends an angle of nearly five degrees. Occultations of the stars
are many and frequent as the huge planet globe glides swiftly through
the heavens. Many a moonlight night appears almost as day owing to
the presence of the enormous, brilliantly reflecting ball of light
and at times two or three moons in addition. Only the brightest stars
could possibly be seen under such circumstances. When, however, the
small worlds pass into the shadow of the great mother planet and not
only the light of the sun but also the reflected light of Jupiter
disappears for many minutes, the stars shine forth in all their glory
there as here. At such times some of the larger moons would usually
be seen shining by the reflected light of the far distant sun. Saturn
also would be visible as a magnificent star, but beautiful Venus and
ruddy Mars would fail to appear. Tiny bodies, mere specks of light at
this distance, they would be lost to view in the glare of the sun.
XX
THE RINGS AND MOONS OF SATURN
Nearly everyone has felt at some time or other a strong desire to
gaze at some of the beauties and wonders of the heavens through
a telescope and the one object that all of us wish to see, if,
perchance, this desire is to be gratified is Saturn, whose unusual
ring system has, so far as we know, no counterpart in the sky.
All the planets in the solar system with the exception of the two
innermost, Mercury and Venus, are attended by satellites, but Saturn,
alone, has in addition to a family of nine moons, three distinct
rings of great dimensions which are composed of swarms of minute
particles revolving around the planet.
Why Saturn should be the only planet to possess such a system of
rings has never been explained in an entirely satisfactory manner.
There is an interesting law known as "Roche's Law," however, named
from its investigator, that states that no satellite of a planet can
exist intact with 2.44 times the radius of the planet. This limit is
spoken of as "Roche's Limit" and applying it to the planet Saturn we
find that the rings of Saturn fall within this limit. It does not
necessarily follow from this that the minute particles of which the
rings are composed are the shattered remains of one small satellite,
but rather that they are the material from which a satellite might
have been formed were it not so close to the planet. Within "Roche's
Limit" the mutual attraction of the various particles for each other
that would tend eventually to gather them into one body is overcome
by tidal forces that arise from such close proximity to the huge
planet. The stress and strain of such forces is so great that no
grouping of particles can take place. This explains, possibly, why
the rings continue to exist in their present condition. The total
quantity of matter in the rings is known to be very small, for
it does not disturb the motions of any of the nearer and smaller
satellites, though tiny Mimas, six hundred miles in diameter, is only
thirty-one thousand miles beyond the outer edge of the outer ring.
An interesting observation was made a few years ago of the passage
of the rings of the planet between us and a star. Though the light
of the star was diminished to one-fourth of its normal brightness
when the rings passed before it, at no time was its light entirely
eclipsed by any of the particles. It was computed that if the
diameters of any of the individual particles had amounted to as much
as three or four miles the star would have been temporarily eclipsed.
An upper limit for the size of the moonlets was thus obtained. The
average diameter of the particles is probably much less than three
miles.
The thickness of the ring system is not over fifty or one hundred
miles, but its total diameter is one hundred and seventy-two thousand
miles. There are, in all, three concentric rings. The faint inner
ring, known as the "crape" ring, is invisible in a telescope under
four inches in aperture. The width of this inner ring is eleven
thousand miles. Just beyond the crape ring is the chief, bright
ring, eighteen thousand miles in width. It shades gradually in
brightness from its juncture with the crape ring to its most luminous
portion at its outer edge, which is separated from the third or
outer ring by a gap two thousand two hundred miles in width, known
as Cassini's Division. The third or outer ring is eleven thousand
miles wide and is less bright than the central ring. The inner edge
of the inner ring is but six thousand miles above the surface of the
planet. On account of the curvature of the planet the ring system
is invisible from the north and south poles of Saturn. As in the
case of the satellites of a planet the inner particles of the rings
revolve around the planet more rapidly than the outer particles. The
innermost particles of the crape ring require but five hours for one
journey around Saturn while the outermost particles of the outer ring
require one hundred and thirty-seven hours, or nearly six days to
complete one revolution.
In addition to the gap in the rings known as Cassini's Division
several other fainter divisions exist. If a group of moonlets were
to revolve around the planet in the positions marked by these gaps
their periods of revolution would be commensurable with the periods
of several of the satellites of Saturn. As a result the attraction
exerted on such particles by these satellites would gradually disturb
their motion in such a way as to draw them away from these positions.
It is owing, therefore, to the attraction of the satellites of Saturn
for the moonlets that these gaps in the rings exist.
As a result of the disturbances produced in the motion of the
moonlets by the satellites of Saturn collisions are bound to occur
occasionally among the various particles. When two particles collide
the period of revolution of one or both of them is reduced and as a
result collisions tend to bring the moonlets gradually closer and
closer to the surface of the planet. The dusky inner ring, it is
believed, may consist largely of particles whose periods have been
continually shortened by collisions.
Saturn may, therefore, lose its ring system in the course of time
through its gradually being drawn down upon the planet by collisions
of the various particles until all of the material is finally swept
up by the planet. Such a change would probably require millions of
years, however, as collisions are probably, on the whole, infrequent.
It is possible that the ring system of Saturn may have been much more
extensive in the past than it is now and other members of our solar
system may have had such appendages in the far distant past.
The appearance of the rings of Saturn as viewed from our planet
changes periodically as a result of the revolution of the earth
and Saturn around the sun, which places them in constantly changing
positions with reference to each other. The rings lie in the plane of
Saturn's equator, which is inclined twenty-seven degrees to its orbit
and twenty-eight degrees to the Earth's orbit.
Since the position of the equator remains parallel to itself while
the planet is journeying around the sun it happens that half the time
the earth is elevated above the plane of the rings and the remainder
of the time it lies below the plane of the rings. Twice in the period
of Saturn's revolution around the sun, which occupies nearly thirty
years, the earth lies directly in the plane of the rings and at this
time the rings entirely disappear from view for a short time. Mid-way
between the two dates of disappearance the rings are tilted at their
widest angle with reference to the earth and they are then seen to
the best advantage. As the date of their disappearance approaches
they appear more and more like a line of light extending to either
side of the planet's equator. Even in the most powerful telescope
the rings entirely disappear from view for a few hours at the time
the earth lies exactly in the same plane. It is at this time that
the ball of the planet is best seen. Its flattening at the poles,
which is nearly ten per cent. of its equatorial diameter then gives
it a decidedly oval appearance. Ordinarily one of the hemispheres
of Saturn is partly or entirely concealed by the rings so that the
oblate form is not so noticeable. It was the change in the tilt and
visibility of the rings that so perplexed Galileo when he attempted
to make out the nature of these appendages of Saturn with his
crude telescope of insufficient magnifying power. So great was his
bewilderment when the rings finally disappeared that he cried out in
despair that Saturn must have swallowed his children, according to
the legend. He finally became so exasperated with the results of his
observations that he gave up observing the planet. The true nature of
these appendages of Saturn remained a mystery until Huygens solved
the problem in 1655, some time after the death of Galileo.
In addition to the rings, Saturn has nine satellites named, in the
order of their distance outward from the planet, Mimas, Enceladus,
Tethys, Dione, Rhea, Titan, Hyperion, Iapetus and Phoebe. The
last-mentioned satellite was discovered by W. H. Pickering in 1899.
It aroused great interest at the time because it was the first
satellite to be discovered with "retrograde" motion in its orbit. Two
satellites of Jupiter since discovered revolve in the same direction
around their primary.
The satellites of Saturn are approximate to those of Jupiter in
size and exactly equal them in number. The largest, Titan, is three
thousand miles in diameter and can be easily seen with the smallest
telescopes. With a four-inch telescope five of the satellites can
be readily found, though they are not as interesting to observe as
the satellites of Jupiter because they are far more distant from
the earth. The time they require to make one journey around Saturn
varies from nearly twenty-three hours for Mimas, the nearest, to
approximately five hundred and twenty-four days for Phoebe, the most
distant.