Chapter IV: Outer Group of Planets

Jupiter.

248. _Orbit of Jupiter._--The orbit of Jupiter is inclined only a little over one degree to the ecliptic; and its eccentricity is only about half of that of Mars, being less than one-twentieth. The mean distance of Jupiter from the sun is about four hundred and eighty million miles; but, owing to the eccentricity of his orbit, his actual distance from the sun ranges from four hundred and fifty-seven to five hundred and three million miles.

249. _Distance of Jupiter from the Earth._--When Jupiter is in opposition, his mean distance from the earth is four hundred and eighty million miles _minus_ ninety-two million miles, or three hundred and eighty-eight million miles, and, when he is in conjunction, four hundred and eighty million miles _plus_ ninety-two million miles, or five hundred and seventy-two million miles. It will be seen that he is less than twice as far off in conjunction as in opposition, and that the ratio of his greatest to his least distance is very much less than in the case of Venus and Mars. This is owing to his very much greater distance from the sun. Owing to the eccentricities of the orbits of the earth and of Jupiter, the greatest and least distances of Jupiter from the earth vary somewhat from year to year.

250. _The Brightness and Apparent Size of Jupiter._--The apparent diameter of Jupiter varies from about fifty seconds to about thirty seconds. His apparent size at his extreme and mean distances from the earth is shown in Fig. 273.

Jupiter shines with a brilliant white light, which exceeds that of every other planet except Venus. The planet is, of course, brightest when near opposition.

251. _The Volume and Density of Jupiter._--Jupiter is the "giant planet" of our system, his mass largely exceeding that of all the other planets combined. His mean diameter is about eighty-five thousand miles; but the equatorial exceeds the polar diameter by five thousand miles. In volume he exceeds our earth about thirteen hundred times, but in mass only about two hundred and thirteen times. His specific gravity is, therefore, far less than that of the earth, and even less than that of water. The comparative size of Jupiter and the earth is shown in Fig. 274.

252. _The Sidereal and Synodical Periods of Jupiter._--It takes Jupiter nearly twelve years to make a _sidereal_ revolution, or a complete revolution around the sun, his orbital motion being at the rate of about eight miles a second. His _synodical_ period, or the time of his passage from opposition to opposition again, is three hundred and ninety-eight days.

253. _The Telescopic Aspect of Jupiter._--There are no really permanent markings on the disk of Jupiter; but his surface presents a very diversified appearance. The earlier telescopic observers descried dark belts across it, one north of the equator, and the other south of it. With the increase of telescopic power, it was seen that these bands were of a more complex structure than had been supposed, and consisted of stratified, cloud-like appearances, varying greatly in form and number. These change so rapidly, that the face of the planet rarely presents the same appearance on two successive nights. They are most strongly marked at some distance on each side of the planet's equator, and thus appear as two belts under a low magnifying power.

Both the outlines of the belts, and the color of portions of the planet, are subject to considerable changes. The equatorial regions, and the spaces between the belts generally, are often of a rosy tinge. This color is sometimes strongly marked, while at other times hardly a trace of it can be seen. A general telescopic view of Jupiter is given in Plate V.

254. _The Physical Constitution of Jupiter._--From the changeability of the belts, and of nearly all the visible features of Jupiter, it is clear that what we see on that planet is not the solid nucleus, but cloud-like formations, which cover the entire surface to a great depth. The planet appears to be covered with a deep and dense atmosphere, filled with thick masses of clouds and vapor. Until recently this cloud-laden atmosphere was supposed to be somewhat like that of our globe; but at present the physical constitution of Jupiter is believed to resemble that of the sun rather than that of the earth. Like the sun, he is brighter in the centre than near the edges, as is shown in the transits of the satellites over his disk. When the satellite first enters on the disk, it commonly seems like a bright spot on a dark background; but, as it approaches the centre, it appears like a dark spot on the bright surface of the planet. The centre is probably two or three times brighter than the edges. This may be, as in the case of the sun, because the light near the edge passes through a greater depth of atmosphere, and is diminished by absorption.

It has also been suspected that Jupiter shines partly by his own light, and not wholly by reflected sunlight. The planet cannot, however, emit any great amount of light; for, if it did, the satellites would shine by this light when they are in the shadow of the planet, whereas they totally disappear. It is possible that the brighter portions of the surface are from time to time slightly self-luminous.

Again: the interior of Jupiter seems to be the seat of an activity so enormous that it can be ascribed only to intense heat. Rapid movements are always occurring on his surface, often changing its aspect in a few hours. It is therefore probable that Jupiter is not yet covered by a solid crust, and that the fiery interior, whether liquid or gaseous, is surrounded by the dense vapors which cease to be luminous on rising into the higher and cooler regions of the atmosphere. Figs. 275 and 276 show the disk of Jupiter as it appeared in December, 1881.

255. _Rotation of Jupiter_.--Spots are sometimes visible which are much more permanent than the ordinary markings on the belts. The most remarkable of these is "the great red spot," which was first observed in July, 1878, and is still to be seen in February, 1882. It is shown just above the centre of the disk in Fig. 275. By watching these spots from day to day, the time of Jupiter's axial rotation has been found to be about nine hours and fifty minutes.

The axis of Jupiter deviates but slightly from a perpendicular to the plane of its orbit, as is shown in Fig. 277.

THE SATELLITES OF JUPITER.

256. _Jupiter's Four Moons._--Jupiter is accompanied by four moons, as shown in Fig. 278. The diameters of these moons range from about twenty-two hundred to thirty-seven hundred miles. The second from the planet is the smallest, and the third the largest. The smallest is about the size of our moon; the largest considerably exceeds Mercury, and almost rivals Mars, in bulk. The sizes of these moons, compared with those of the earth and its moon, are shown in Fig. 279.

The names of these satellites, in the order of their distance from the planet, are _Io_, _Europa_, _Ganymede_, and _Callisto_. Their times of revolution range from about a day and three-fourths up to about sixteen days and a half. Their orbits are shown in Fig. 280.

257. _The Variability of Jupiter's Satellites._--Remarkable variations in the light of these moons have led to the supposition that violent changes are taking place on their surfaces. It was formerly believed, that, like our moon, they always present the same face to the planet, and that the changes in their brilliancy are due to differences in the luminosity of parts of their surface which are successively turned towards us during a revolution; but careful measurements of their light show that this hypothesis does not account for the changes, which are sometimes very sudden. The satellites are too distant for examination of their surfaces with the telescope: hence it is impossible to give any certain explanation of these phenomena.

258. _Eclipses of Jupiter's Satellites._--Jupiter, like the earth, casts a shadow away from the sun, as shown in Fig. 281; and, whenever one of his moons passes into this shadow, it becomes eclipsed. On the other hand, whenever one of the moons throws its shadow on Jupiter, the sun is eclipsed to that part of the planet which lies within the shadow.

To the inhabitants of Jupiter (if there are any, and if they can see through the clouds) these eclipses must be very familiar affairs; for in consequence of the small inclinations of the orbits of the satellites to the planet's equator, and the small inclination of the latter to the plane of Jupiter's orbit, all the satellites, except the most distant one, are eclipsed in every revolution. A spectator on Jupiter might therefore witness during the planetary year forty-five hundred eclipses of the moons, and about the same number of the sun.

259. _Transits of Jupiter's Satellites._--Whenever one of Jupiter's moons passes in front of the planet, it is said to make a _transit_ across his disk. When a moon is making a transit, it presents its bright hemisphere towards the earth, as will be seen from Fig. 282: hence it is usually seen as a bright spot on the planet's disk; though sometimes, on the brighter central portions of the disk, it appears dark.

It will be seen from Fig. 282 that the shadow of a moon does not fall upon the part of the planet's disk that is covered by the moon: hence we may observe the transit of both the moon and its shadow. The shadow appears as a small black spot, which will precede or follow the moon according to the position of the earth in its orbit. Fig. 283 shows two moons of Jupiter in transit.

260. _Occultations of Jupiter's Satellites._--The eclipse of a moon of Jupiter must be carefully distinguished from the _occultation_ of a moon by the planet. In the case of an eclipse, the moon ceases to be visible, because the mass of Jupiter is interposed between the sun and the moon, which ceases to be luminous, because the sun's light is cut off; but, in the case of an occupation, the moon gets into such a position that the body of Jupiter is interposed between it and the earth, thus rendering the moon invisible to us. The third satellite, _m''_ (Fig. 282), is invisible from the earth _E_, having become _occulted_ when it passed behind the planet's disk; but it will not be _eclipsed_ until it passes into the shadow of Jupiter.

261. _Jupiter without Satellites._--It occasionally happens that every one of Jupiter's satellites will disappear at the same time, either by being eclipsed or occulted, or by being in transit. In this event, Jupiter will appear without satellites. This occurred on the 21st of August, 1867. The position of Jupiter's satellites at this time is shown in Fig. 284.

Saturn.

THE PLANET AND HIS MOONS.

262. _The Orbit of Saturn._--The orbit of Saturn is rather more eccentric than that of Jupiter, its eccentricity being somewhat more than one-twentieth. Its inclination to the ecliptic is about two degrees and a half. The mean distance of Saturn from the sun is about eight hundred and eighty million miles. It is about a hundred million miles nearer the sun at perihelion than at aphelion.

263. _Distance of Saturn from the Earth._--The mean distance of Saturn from the earth at opposition is eight hundred and eighty million miles _minus_ ninety-two million miles, or seven hundred and eighty-eight million; and at conjunction, eight hundred and eighty million miles _plus_ ninety-two million, or nine hundred and seventy-two million. Owing to the eccentricity of the orbit of Saturn, his distance from the earth at opposition and at conjunction varies by about a hundred million miles at different times; but he is so immensely far away, that this is only a small fraction of his mean distance.

264. _Apparent Size and Brightness of Saturn._--The apparent diameter of Saturn varies from about twenty seconds to about fourteen seconds. His apparent size at his extreme and mean distances from the earth is shown in Fig. 285.

The planet generally shines with the brilliancy of a moderate first-magnitude star, and with a dingy, reddish light, as if seen through a smoky atmosphere.

265. _Volume and Density of Saturn._--The real diameter of Saturn is about seventy thousand miles, and its volume over seven hundred times that of the earth. The comparative size of the earth and Saturn is shown in Fig. 286. This planet is a little more than half as dense as Jupiter.

266. _The Sidereal and Synodical Periods of Saturn._--Saturn makes a complete revolution round the sun in a period of about twenty-nine years and a half, moving in his orbit at the rate of about six miles a second. The planet passes from opposition to opposition again in a period of three hundred and seventy-eight days, or thirteen days over a year.

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.