Part 15

267. _Physical Constitution of Saturn._--The physical constitution of Saturn seems to resemble that of Jupiter; but, being twice as far away, the planet cannot be so well studied. The farther an object is from the sun, the less it is illuminated; and, the farther it is from the earth, the smaller it appears: hence there is a double difficulty in examining the more distant planets. Under favorable circumstances, the surface of Saturn is seen to be diversified with very faint markings; and, with high telescopic powers, two or more very faint streaks, or belts, may be discerned parallel to its equator. These belts, like those of Jupiter, change their aspect from time to time; but they are so faint that the changes cannot be easily followed. It is only on rare occasions that the time of rotation can be determined from a study of the markings.

268. _Rotation of Saturn._--On the evening of Dec. 7, 1876, Professor Hall, who had been observing the satellites of Saturn with the great Washington telescope (18), saw a brilliant white spot near the equator of the planet. It seemed as if an immense eruption of incandescent matter had suddenly burst up from the interior. The spot gradually spread itself out into a long light streak, of which the brightest point was near the western end. It remained visible until January, when it became faint and ill-defined, and the planet was lost in the rays of the sun.

From all the observations on this spot, Professor Hall found the period of Saturn to be ten hours fourteen minutes, reckoning by the brightest part of the streak. Had the middle of the streak been taken, the time would have been less, because the bright matter seemed to be carried along in the direction of the planet's rotation. If this motion was due to a wind, the velocity of the current must have been between fifty and a hundred miles an hour. The axis of Saturn is inclined twenty-seven degrees from the perpendicular to its orbit.

269. _The Satellites of Saturn._--Saturn is accompanied by eight moons. Seven of these are shown in Fig. 287. The names of these satellites, in the order of their distances from the planet, are given in the accompanying table:--

Number. Name. Distance Sidereal Discoverer. from Period. Planet

1 Mimas 120,800 0 22 37 0.94 Herschel

2 Enceladus 155,000 1 8 53 1.37 Herschel

3 Tethys 191,900 1 21 18 1.88 Cassini

4 Dione 245,800 2 17 41 2.73 Cassini

5 Rhea 343,400 4 12 25 4.51 Cassini

6 Titan 796,100 15 22 41 15.94 Huyghens

7 Hyperion 963,300 21 7 7 21.29 Bond

8 Japetus 2,313,800 79 7 53 79.33 Cassini

The apparent brightness or visibility of these satellites follows the order of their discovery. The smallest telescope will show Titan, and one of very moderate size will show Japetus in the western part of its orbit. An instrument of four or five inches aperture will show Rhea, and perhaps Tethys and Dione; while seven or eight inches are required for Enceladus, even at its greatest elongation from the planet. Mimas can rarely be seen except at its greatest elongation, and then only with an aperture of twelve inches or more. Hyperion can be detected only with the most powerful telescopes, on account of its faintness and the difficulty of distinguishing it from minute stars.

_Japetus_, the outermost satellite, is remarkable for the fact, that while, in one part of its orbit, it is the brightest of the satellites except Titan, in the opposite part it is almost as faint as Hyperion, and can be seen only in large telescopes. When west of the planet, it is bright; when east of it, faint. This peculiarity has been accounted for by supposing that the satellite, like our moon, always presents the same face to the planet, and that one side of it is white and the other intensely black; but it is doubtful whether any known substance is so black as one side of the satellite must be to account for such extraordinary changes of brilliancy.

_Titan_, the largest of these satellites, is about the size of the largest satellite of Jupiter. The relative sizes of the satellites are shown in Fig. 288, and their orbits in Fig. 289.

Fig. 290 shows the transit of one of the satellites, and of its shadow, across the disk of the planet.

THE RINGS OF SATURN.

270. _General Appearance of the Rings._--Saturn is surrounded by a thin flat ring lying in the plane of its equator. This ring is probably less than a hundred miles thick. The part of it nearest Saturn reflects little sunlight to us; so that it has a dusky appearance, and is not easily seen, although it is not quite so dark as the sky seen between it and the planet. The outer edge of this dusky portion of the ring is at a distance from Saturn of between two and three times the earth's diameter. Outside of this dusky part of the ring is a much brighter portion, and outside of this another, which is somewhat fainter, but still so much brighter than the dusky part as to be easily seen. The width of the brighter parts of the ring is over three times the earth's diameter. To distinguish the parts, the outer one is called ring _A_, the middle one ring _B_, and the dusky one ring _C_. Between _A_ and _B_ is an apparently open space, nearly two thousand miles wide, which looks like a black line on the ring. Other divisions in the ring have been noticed at times; but this is the only one always seen with good telescopes at times when either side of the ring is in view from the earth. The general telescopic appearance of the ring is shown in Fig. 291.

Fig. 292 shows the divisions of the rings as they were seen by Bond.

271. _Phases of Saturn's Ring._--The ring is inclined to the plane of the planet's orbit by an angle of twenty-seven degrees. The general aspect from the earth is nearly the same as from the sun. As the planet revolves around the sun, the axis and plane of the ring keep the same direction in space, just as the axis of the earth and the plane of the equator do.

When the planet is in one part of its orbit, we see the upper or northern side of the ring at an inclination of twenty-seven degrees, the greatest angle at which the ring can ever be seen. This phase of the ring is shown in Fig. 293.

When the planet has moved through a quarter of a revolution, the edge of the ring is turned towards the sun and the earth; and, owing to its extreme thinness, it is visible only in the most powerful telescopes as a fine line of light, stretching out on each side of the planet. This phase of the ring is shown in Fig. 294.

All the satellites, except Japetus, revolve very nearly in the plane of the ring: consequently, when the edge of the ring is turned towards the earth, the satellites seem to swing from one side of the planet to the other in a straight line, running along the thin edge of the ring like beads on a string. This phase affords the best opportunity of seeing the inner satellites, Mimas and Enceladus, which at other times are obscured by the brilliancy of the ring.

Fig. 295 shows a phase of the ring intermediate between the last two.

When the planet has moved ninety degrees farther, we again see the ring at an angle of twenty-seven degrees; but now it is the lower or southern side which is visible. When it has moved ninety degrees farther, the edge of the ring is again turned towards the earth and sun.

The successive phases of Saturn's ring during a complete revolution are shown in Fig. 296.

It will be seen that there are two opposite points of Saturn's orbit in which the rings are turned edgewise to us, and two points half-way between the former in which the ring is seen at its maximum inclination of about twenty-seven degrees. Since the planet performs a revolution in twenty-nine years and a half, these phases occur at average intervals of about seven years and four months.

272. _Disappearance of Saturn's Ring._--It will be seen from Fig. 297 that the plane of the ring may not be turned towards the sun and the earth at exactly the same time, and also that the earth may sometimes come on one side of the plane of the ring while the sun is shining on the other. In the figure, _E_, _E'_, _E''_, and _E'''_ is the orbit of the earth. When Saturn is at _S'_, or opposite, at _F_, the plane of the ring will pass through the sun, and then only the edge of the ring will be illumined. Were Saturn at _S_, and the earth at _E'_, the plane of the ring would pass through the earth. This would also be the case were the earth at _E'''_, and Saturn at _S''_. Were Saturn at _S_ or at _S''_, and the earth farther to the left or to the right, the sun would be shining on one side of the ring while we should be looking on the other. In all these cases the ring will disappear entirely in a telescope of ordinary power. With very powerful telescopes the ring will appear, in the first two cases, as a thin line of light (Fig. 298). It will be seen that all these cases of disappearance must take place when Saturn is in the parts of his orbit intercepted between the parallel lines _AC_ and _BD_. These lines are tangent to the earth's orbit, which they enclose, and are parallel to the plane of Saturn's ring. As Saturn passes away from these two lines on either side, the rings appear more and more open. When the dark side of the ring is in view, it appears as a black line crossing the planet; and on such occasions the sunlight reflected from the outer and inner edges of the rings _A_ and _B_ enables us to see traces of the ring on each side of Saturn, at least in places where two such reflections come nearly together. Fig. 299 illustrates this reflection from the edges at the divisions of the rings.

273. _Changes in Saturn's Ring._--The question whether changes are going on in the rings of Saturn is still unsettled. Some observers have believed that they saw additional divisions in the rings from time to time; but these may have been errors of vision, due partly to the shading which is known to exist on portions of the ring.

Professor Newcomb says, "As seen with the great Washington equatorial in the autumn of 1874, there was no great or sudden contrast between the inner or dark edge of the bright ring and the outer edge of the dusky ring. There was some suspicion that the one shaded into the other by insensible gradations. No one could for a moment suppose, as some observers have, that there was a separation between these two rings. All these considerations give rise to the question whether the dusky ring may not be growing at the expense of the inner bright ring."

Struve, in 1851, advanced the startling theory that the inner edge of the ring was gradually approaching the planet, the whole ring spreading inwards, and making the central opening smaller. The theory was based upon the descriptions and drawings of the rings by the astronomers of the seventeenth century, especially Huyghens, and the measures made by later astronomers up to 1851. This supposed change in the dimension of the ring is shown in Fig. 300.

274. _Constitution of Saturn's Ring._--The theory now generally held by astronomers is, that the ring is composed of a cloud of satellites too small to be separately seen in the telescope, and too close together to admit of visible intervals between them. The ring looks solid, because its parts are too small and too numerous to be seen singly. They are like the minute drops of water that make up clouds and fogs, which to our eyes seem like solid masses. In the dusky ring the particles may be so scattered that we can see through the cloud, the duskiness being due to the blending of light and darkness. Some believe, however, that the duskiness is caused by the darker color of the particles rather than by their being farther apart.

Uranus.

275. _Orbit and Dimensions of Uranus._--Uranus, the smallest of the outer group of planets, has a diameter of nearly thirty-two thousand miles. It is a little less dense than Jupiter, and its mean distance from the sun is about seventeen hundred and seventy millions of miles. Its orbit has about the same eccentricity as that of Jupiter, and is inclined less than a degree to the ecliptic. Uranus makes a revolution around the sun in eighty-four years, moving at the rate of a little over four miles a second. It is visible to the naked eye as a star of the sixth magnitude.

As seen in a large telescope, the planet has a decidedly sea-green color; but no markings have with certainty been detected on its disk, so that nothing is really known with regard to its rotation. Fig. 301 shows the comparative size of Uranus and the earth.

276. _Discovery of Uranus._--This planet was discovered by Sir William Herschel in March, 1781. He was engaged at the time in examining the small stars of the constellation _Gemini_, or the Twins. He noticed that this object which had attracted his attention had an appreciable disk, and therefore could not be a star. He also perceived by its motion that it could not be a nebula; he therefore concluded that it was a comet, and announced his discovery as such. On attempting to compute its orbit, it was soon found that its motions could be accounted for only on the supposition that it was moving in a circular orbit at about twice the distance of Saturn from the sun. It was therefore recognized as a new planet, whose discovery nearly doubled the dimensions of the solar system as it was then known.

277. _The Name of the Planet._--Herschel, out of compliment to his patron, George III., proposed to call the new planet _Georgium Sidus_ (the Georgian Star); but this name found little favor. The name of _Herschel_ was proposed, and continued in use in England for a time, but did not meet with general approval. Various other names were suggested, and finally that of _Uranus_ was adopted.

278. _The Satellites of Uranus._--Uranus is accompanied by four satellites, whose orbits are shown in Fig. 302. These satellites are remarkable for the great inclination of their orbits to the plane of the planet's orbit, amounting to about eighty degrees, and for their _retrograde_ motion; that is, they move _from east to west_, instead of from west to east, as in the case of all the planets and of all the satellites previously discovered.

Neptune.

279. _Orbit and Dimensions of Neptune._--So far as known, Neptune is the most remote member of the solar system, its mean distance from the sun being twenty-seven hundred and seventy-five million miles. This distance is considerably less than twice that of Uranus. Neptune revolves around the sun in a period of a little less than a hundred and sixty-five years. Its orbit has but slight eccentricity, and is inclined less than two degrees to the ecliptic. This planet is considerably larger than Uranus, its diameter being nearly thirty-five thousand miles. It is somewhat less dense than Uranus. Neptune is invisible to the naked eye, and no telescope has revealed any markings on its disk: hence nothing is certainly known as to its rotation. Fig. 303 shows the comparative size of Neptune and the earth.

280. _The Discovery of Neptune._--The discovery of Neptune was made in 1846, and is justly regarded as one of the grandest triumphs of astronomy.

Soon after Uranus was discovered, certain irregularities in its motion were observed, which could not be explained. It is well known that the planets are all the while disturbing each other's motions, so that none of them describe perfect ellipses. These mutual disturbances are called _perturbations_. In the case of Uranus it was found, that, after making due allowance for the action of all the known planets, there were still certain perturbations in its course which had not been accounted for. This led astronomers to the suspicion that these might be caused by an unknown planet. Leverrier in France, and Adams in England, independently of each other, set themselves the difficult problem of computing the position and magnitude of a planet which would produce these perturbations. Both, by a most laborious computation, showed that the perturbations were such as would be produced by a planet revolving about the sun at about twice the distance of Uranus, and having a mass somewhat greater than that of this planet; and both pointed out the same part of the heavens as that in which the planet ought to be found at that time. Almost immediately after they had announced the conclusion to which they had arrived, the planet was found with the telescope. The astronomer who was searching for the planet at the suggestion of Leverrier was the first to recognize it: hence Leverrier has obtained the chief credit of the discovery.

The observed planet is proved to be nearer than the one predicted by Leverrier and Adams, and therefore of smaller magnitude.

281. _The Observed Planet not the Predicted One._--Professor Peirce always maintained that the planet found by observation was not the one whose existence had been predicted by Leverrier and Adams, though its action would completely explain all the irregularities in the motion of Uranus. His last statement on this point is as follows: "My position is, that there were _two possible planets_, either of which might have caused the observed irregular motions of Uranus. Each planet excluded the other; so that, if one was, the other was not. They coincided in direction from the earth at certain epochs, once in six hundred and fifty years. It was at one of these epochs that the prediction was made, and at no other time for six centuries could the prediction of the one planet have revealed the other. The observed planet was not the predicted one."

282. _Bode's Law Disproved._--The following table gives the distances of the planets according to Bode's law, their actual distances, and the error of the law in each case:--

Planet. Numbers of Actual Errors. Bode. Distances.

Mercury 0 + 4 = 4 3.9 0.1

Venus 3 + 4 = 7 7.2 0.2

Earth 6 + 4 = 10 10.0 0.0

Mars 12 + 4 = 16 15.2 0.8

Minor 24 + 4 = 28 20 to 35 planets

Jupiter 48 + 4 = 52 52.0 0.0

Saturn 96 + 4 = 100 95.4 4.6

Uranus 192 + 4 = 196 191.9 4.1

Neptune 384 + 4 = 388 300.6 87.4

It will be seen, that, before the discovery of Neptune, the agreement was so close as to indicate that this was an actual law of the distances; but the discovery of this planet completely disproved its existence.

283. _The Satellite of Neptune._--Neptune is accompanied by at least one moon, whose orbit is shown in Fig. 304. The orbit of this satellite is inclined about thirty degrees to the plane of the ecliptic, and the motion of the satellite is retrograde, or from east to west.

VII. COMETS AND METEORS.

I. COMETS.

General Phenomena of Comets.

284. _General Appearance of a Bright Comet._--Comets bright enough to be seen with the naked eye are composed of three parts, which run into each other by insensible gradations. These are the _nucleus_, the _coma_, and the _tail_.

The _nucleus_ is the bright centre of the comet, and appears to the eye as a star or planet.

The _coma_ is a nebulous mass surrounding the nucleus on all sides. Close to the nucleus it is almost as bright as the nucleus itself; but it gradually shades off in every direction. The nucleus and coma combined appear like a star shining through a small patch of fog; and these two together form what is called the _head_ of the comet.

The _tail_ is a continuation of the coma, and consists of a stream of milky light, growing wider and fainter as it recedes from the head, till the eye is unable to trace it.

The general appearance of one of the smaller of the brilliant comets is shown in Fig. 305.

285. _General Appearance of a Telescopic Comet._--The great majority of comets are too faint to be visible with the naked eye, and are called _telescopic_ comets. In these comets there seems to be a development of coma at the expense of nucleus and tail. In some cases the telescope fails to reveal any nucleus at all in one of these comets; at other times the nucleus is so faint and ill-defined as to be barely distinguishable. Fig. 306 shows a telescopic comet without any nucleus at all, and another with a slight condensation at the centre. In these comets it is generally impossible to distinguish the coma from the tail, the latter being either entirely invisible, as in Fig. 306, or else only an elongation of the coma, as shown in Fig. 307. Many comets appear simply as patches of foggy light of more or less irregular form.

286. _The Development of Telescopic Comets on their Approach to the Sun._--As a rule, all comets look nearly alike when they first come within the reach of the telescope. They appear at first as little foggy patches, without any tail, and often without any visible nucleus. As they approach the sun their peculiarities are rapidly developed. Fig. 308 shows such a comet as first seen, and the gradual development of its nucleus, head, and tail, as it approaches the sun.

If the comet is only a small one, the tail developed is small; but these small appendages have a great variety of form in different comets. Fig. 309 shows the singular form into which _Encke's_ comet was developed in 1871. Figs. 310 and 311 show other peculiar developments of telescopic comets.

287. _Development of Brilliant Comets on their Approach to the Sun._--Brilliant comets, as well as telescopic comets, appear nearly alike when they come into the view of the telescope; and it is only on their approach to the sun that their distinctive features are developed. Not only do these comets, when they first come into view, resemble each other, but they also bear a close resemblance to telescopic comets.

As the comet approaches the sun, bright vaporous jets, two or three in number, are emitted from the nucleus on the side of the sun and in the direction of the sun. These jets, though directed towards the sun, are soon more or less carried backward, as if repelled by the sun. Fig. 312 shows a succession of views of these jets as they were developed in the case of _Halley's_ comet in 1835.

The jets in this case seemed to have an oscillatory motion. At 1 and 2 they seemed to be attracted towards the sun, and in 3 to be repelled by him. In 4 and 5 they seemed to be again attracted, and in 6 to be repelled, but in a reverse direction to that in 3. In 7 they appeared to be again attracted. Bessel likened this oscillation of the jets to the vibration of a magnetic needle when presented to the pole of a magnet.

In the case of larger comets these luminous jets are surrounded by one or more envelops, which are thrown off in succession as the comet approaches the sun. The formation of these envelops was a conspicuous feature of _Donati's_ comet of 1858. A rough view of the jets and the surrounding envelops is given in Fig. 313. Fig. 314 gives a view of the envelops without the jets.