Recreations in Astronomy Part 12

The eye and mind of man have worked out a problem of marvellous difficulty in finding a true solution of the strange appearance of the rings. Galileo has the immortal honor of first having seen something peculiar about this planet. He wrote to the Duke of Tuscany, "When I view Saturn it seems _tricorps_. The central body seems the largest. The two others, situated, the one on the east, and the other on the west, seem to touch it. They are like two supporters, who help old Saturn on his way, and always remain at his side."

Looking a few years later, the rings having turned from view, he said, "It is possible that some demon mocked me;" and he refused to look any more.

Huyghens, in March, 1655, solved the problem of the triform appearance of Saturn. He saw them as handles on the two sides. In a year they had disappeared, and the planet was as round as it seemed to Galileo in 1612. He did not, however, despair; and in October, [Page 170]

1656, he was rewarded by seeing them appear again. He wrote of Saturn, "It is girdled by a thin plain ring, nowhere touching, inclined to the ecliptic."

Since that time discoveries have succeeded one another rapidly.

"We have seen by degrees a ring evolved out of a triform planet, and the great division of the ring and the irregularities on it brought to light. Enceladus, and coy Mimas, faintest of twinklers, are caught by Herschel's giant mirrors. And he, too, first of men, realizes the wonderful tenuity of the ring, along which he saw those satellites travelling like pearls strung on a silver thread.

Then Bond comes on the field, and furnishes evidence to show that we must multiply the number of separate rings we know not how many fold. And here we reach the golden age of Saturnian discovery, when Bond, with the giant refractor of Cambridge, and Dawes, with his 6-1/3-inch Munich gla.s.s, first beheld that wonderful dark semi-transparent ring, which still remains one of the wonders of our system. But the end is not yet: on the southern surface of the ring, ere summer fades into autumn, Otto Struve in turn comes upon the field, detects, as Dawes had previously done, a division even in the dark ring, and measures it, while it is invisible to La.s.sell's mirror--a proof, if one were needed, of the enormous superiority possessed by refractors in such inquiries. Then we approach 1861, when the ring plane again pa.s.ses through the earth, and Struve and Wray observe curious nebulous appearances."[*]

[Footnote *: Lockyer.]

Our opportunities for seeing Saturn vary greatly. As the earth at one part of its...o...b..t presents its south pole [Page 171] to the sun, then its equator, then the north pole, so Saturn; and we, in the direction of the sun, see the south side of the rings inclined at an angle of 27; next the edge of the rings, like a fine thread of light; then the north side at a similar inclination. On February 7th, 1878, Saturn was between Aquarius and Pisces, with the edge of the ring to the sun. In 1885, the planet being in Taurus, the south side of the rings will be seen at the greatest advantage. From 1881 till 1885 all circ.u.mstances will combine to give most favorable studies of Saturn. Meanwhile study the picture of it. The outer ring is narrow, dark, showing hints of another division, sometimes more evident than at others, as if it were in a state of flux. The inner, or second, ring is much brighter, especially on the outer edge, and shading off to the dusky edge next to the planet. There is no sign of division into a third dusky innermost ring, as was plainly seen by Bond. This, too, may be in a state of flux.

The markings of the planet are delicate, difficult of detection, and are not like those stark zebra stripes that are so often represented.

The distance between the planet and the second ring seems to be diminished one-half since 1657, and this ring has doubled its breadth in the same time. Some of this difference may be owing to our greater telescopic power, enabling us to see the ring closer to the planet; but in all probability the ring is closing in upon the central body, and will touch it by A.D. 2150. Thus the whole ring must ultimately fall upon the planet, instead of making a satellite.

We are anxious to learn the nature of such a ring. [Page 172]

Laplace mathematically demonstrated that it cannot be uniform and solid, and survive. Professor Peirce showed it could not be fluid, and continue. Then Professor Maxwell showed that it must be formed of clouds of satellites too small to be seen individually, and too near together for the s.p.a.ces to be discerned, unless, perhaps, we may except the inner dark ring, where they are not near enough to make it positively luminous. Indeed, there is some evidence that the meteoroids are far enough apart to make the ring partially transparent.

We look forward to the opportunities for observation in 1882 with the brightest hope that these difficult questions will be solved.

_Satellites of Saturn._

The first discovered satellite of Saturn seen by Huyghens was in 1655, and the last by the Bonds, father and son, of Cambridge, in 1848. These are eight in number, and are named:

Distant from Saturn's centre.

I. Mimas 119,725 miles.

II. Enceladus 153,630 "

III. Tethys 190,225 "

IV. Dione 243,670 "

V. Rhea 340,320 "

VI. t.i.tan 788,915 "

VII. Hyperion 954,160 "

VIII. j.a.petus 2,292,790 "

t.i.tan can be seen by almost any telescope; I., II., and III., only by the most powerful instrument. All except j.a.petus revolve nearly in the plane of the ring. Like the moons of Jupiter, they present remarkable and unaccountable variations of brilliancy. An inspection [Page 173] of the table reveals either an expectation that another moon will be discovered between V. and VI., and about three more between VII. and VIII., or that these gaps may be filled with groups of invisible asteroids, as the gap between Mars and Jupiter. This will become more evident by drawing Saturn, the rings, and orbits of the moons all as circles, on a scale of 10,000 miles to the inch.

Saturn will be in the centre, 70,000 miles in diameter; then a gap, decreasing twenty-nine miles a year to the first ring, of, say, 10,000 miles; a dark ring 9000 miles wide; next the brightest ring 18,300 miles wide; then a gap of 1750 miles; then the outer ring 10,000 miles wide; then the orbits of the satellites in order.

If the scenery of Jupiter is magnificent, that of Saturn must be sublime. If one could exist there, he might wander from the illuminated side of the rings, under their magnificent arches, to the darkened side, see the swift whirling moons; one of them presenting ten times the disk of the earth's moon, and so very near as to enable him to watch the advancing line of light that marks the lunar morning journeying round that orb.

URa.n.u.s.

Sign [Symbol]; the initial of Herschel, and sign of the world.

DISTANCE FROM THE SUN, 1,771,000,000 MILES. DIAMETER, 31,700 MILES.

AXIAL REVOLUTION UNKNOWN. ORBITAL, 84 YEARS. VELOCITY PER MINUTE, 252 MILES. MOONS, FOUR.

Ura.n.u.s was presented to the knowledge of man as an unexpected reward for honest work. It was first mistaken by its discoverer for a comet, a mere cloud of vapor; but it proved to be a world, and extended the [Page 174] boundaries of our solar system, in the moment of its discovery, as much as all investigation had done in all previous ages.

Sir William Herschel was engaged in mapping stars in 1781, when he first observed its sea-green disk. He proposed to call it _Georgium Sidus_, in honor of his king; but there were too many names of the G.o.ds in the sky to allow a mortal name to be placed among them. It was therefore called Ura.n.u.s, since, being the most distant body of our system, as was supposed, it might appropriately bear the name of the oldest G.o.d. Finding anything in G.o.d's realms of infinite riches ought not to lead men to regard that as final, but as a promise of more to follow.

This planet had been seen five times by Flamsteed before its character was determined--once nearly a century before--and eight times by Le Monnier. These names, which might easily have been a.s.sociated with a grand discovery, are a.s.sociated with careless observation.

Eyes were made not only to be kept open, but to have minds behind them to interpret their visions. Herschel thought he discovered six moons belonging to Ura.n.u.s, but subsequent investigation has limited the number to four. Two of these are seen with great difficulty by the most powerful telescopes.

If the plane of our moon's...o...b..t were tipped up to a greater inclination, revolving it on the line of nodes as an axis until it was turned 85, the moon, still continuing on its...o...b..t in that plane, would go over the poles instead of about the equator, and would go back to its old path when the plane was revolved 180; but its revolution would now be from east to west, or [Page 175]

retrograde. The plane of the moons of Ura.n.u.s has been thus inclined till it has pa.s.sed 10 beyond the pole, and the moons' motions are retrograde as regards other known celestial movements. How Ura.n.u.s itself revolves is not known. There are more worlds to conquer.

NEPTUNE.

G.o.d of the sea; sign [Symbol], his trident.

DISTANCE FROM THE SUN, 2,775,000,000 MILES. DIAMETER, 34,500 MILES.

VELOCITY PER MINUTE, 201.6 MILES. AXIAL REVOLUTION UNKNOWN. ORBITAL, 164.78 YEARS. ONE MOON.

Men sought for Neptune as the heroes sought the golden fleece.

The place of Ura.n.u.s had been mapped for nearly one hundred years by these accidental observations. On applying the law of universal gravitation, a slight discrepancy was found between its computed place and its observed place. This discrepancy was exceedingly slight. In 1830 it was only 20"; in 1840,190"; in 1884, 2'. Two stars that were 2' apart would appear as one to the keenest unaided eye, but such an error must not exist in astronomy. Years of work were given to its correction. Mr. John C. Adams, of Cambridge, England, finding that the attraction of a planet exterior to Ura.n.u.s would account for its irregularities, computed the place of such a hypothetical body with singular exactness in October, 1841; but neither he nor the royal astronomer Airy looked for it. Another opportunity for immortality was heedlessly neglected. Meanwhile, M. Leverrier, of Paris, was working at the same problem. In the summer of 1846 Leverrier announced the place of the exterior planet.

The conclusion was in striking coincidence with that of Mr. [Page 176] Clark. Mr. Challis commenced to search for the planet near the indicated place, and actually saw and mapped the star August 4th, 1846, but did not recognize its planetary character. Dr. Galle, of Berlin, on the 23d of September, 1846, found an object with a planetary disk not plotted on the map of stars. It was the sought-for world. It would seem easy to find a world seventy-six times as large as the earth, and easy to recognize it when seen. The fact that it could be discovered only by such care conveys an overwhelming idea of the distance where it moves.

[Ill.u.s.tration: Fig. 66.--Perturbation of Ura.n.u.s.]

The effect of these perturbations by an exterior planet is understood from Fig. 66. Ura.n.u.s and Neptune were in conjunction, as shown, in 1822. But in 1820 it had been found that Ura.n.u.s was too far from the sun, and too much accelerated. Since 1800, Neptune, in his...o...b..t from F to E, had been hastening Ura.n.u.s in his...o...b..t D from C to B, and also drawing it farther from the sun. After 1822, Neptune, in pa.s.sing from E to D, had been r.e.t.a.r.ding Ura.n.u.s in his...o...b..t from B to A.

We have seen it is easy to miss immortality. There is still another instance. Lalande saw Neptune on May 8th and 10th, 1795, noted that it had moved a little, and that the observations did not agree; but, supposing the first was wrong, carelessly missed the glory of once more doubling the bounds of the empire of the sun.

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It is time to pause and review our knowledge of this system. The first view reveals a moon and earth endowed with a force of inertia going on in s.p.a.ce in straight lines; but an invisible elastic cord of attraction holds them together, just counterbalancing this tendency to fly apart, and hence they circle round their centre of gravity.

The revolving earth turns every part of its surface to the moon in each twenty-four hours. By an axial revolution in the same time that the moon goes round the earth, the moon holds the same point of its surface constantly toward the earth. If we were to add one, two, four, eight moons at appropriate distances, the result would be the same. There is, however, another attractive influence--that of the sun. The sun attracts both earth and moon, but their nearer affection for each other keeps them from going apart. They both, revolving on their axes and around their centre of gravity, sweep in a vastly wider curve around the sun. Add as many moons as has Jupiter or Saturn, the result is the same--an orderly carrying of worlds through s.p.a.ce.

There lies the unsupported sun in the centre, nearer to infinity in all its capacities and intensities of force than our minds can measure, filling the whole dome to where the stars are set with light, heat, and power. It holds five small worlds--Vulcan, Mercury, Venus, Earth, and Mars--within a s.p.a.ce whose radius it would require a locomotive half a thousand years to traverse. It next holds some indeterminate number of asteroids, and the great Jupiter, equal in volume to 13,000 earths. It holds Saturn, Ura.n.u.s, and Neptune, and all their variously related satellites and rings. The two thoughts that overwhelm us are distance and power. The period of [Page 178]

man's whole history is not sufficient for an express train to traverse half the distance to Neptune. Thought wearies and fails in seeking to grasp such distances; it can scarcely comprehend one million miles, and here are thousands of them. Even the wings of imagination grow weary and droop. When we stand on that outermost of planets, the very last sentinel of the outposts of the king, the very sun grown dim and small in the distance, we have taken only one step of the infinite distance to the stars. They have not changed their relative position--they have not grown brighter by our approach. Neptune carries us round a vast circle about the centre of the dome of stars, but we seem no nearer its sides. In visiting planets, we have been only visiting next-door neighbors in the streets of a seaport town. We know that there are similar neighbors about Sirius and Arcturus, but a vast sea rolls between. As we said, we stand with the outermost sentinel; but into the great void beyond the king of day sends his comets as scouts, and they fly thousands of years without for one instant missing the steady grasp of the power of the sun. It is nearer almightiness than we are able to think.

If we cannot solve the problems of the present existence of worlds, how little can we expect to fathom the unsoundable depths of their creation and development through ages measureless to man! Yet the very difficulty provokes the most ambitious thought. We toil at the problem because it has been hitherto unsolvable. Every error we make, and discover to be such, helps toward the final solution.

Every earnest thinker who climbs the shining worlds as steps to a higher thought is trying to solve the problem G.o.d has given us to do.

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IX.

THE NEBULAR HYPOTHESIS.

"And the earth was without form, and void; and darkness was upon the face of the deep."--_Genesis_ i. 2.

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