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.