[20] In a somewhat similar manner the term "crater," as applied to the ring-mountain formation on the moon, has evidently given a bias in favour of the volcanic theory as an explanation of that peculiar structure.

[21] Mr. Slipher's results (see note 2, page 213) were not then known.

CHAPTER XVIII

THE SUPERIOR PLANETS--_continued_

The planets, so far, have been divided into inferior and superior. Such a division, however, refers merely to the situation of their orbits with regard to that of our earth. There is, indeed, another manner in which they are often cla.s.sed, namely, according to size. On this principle they are divided into two groups; one group called the _Terrestrial Planets_, or those which have characteristics like our earth, and the other called the _Major Planets_, because they are all of very great size. The terrestrial planets are Mercury, Venus, the earth, and Mars.

The major planets are the remainder, namely, Jupiter, Saturn, Ura.n.u.s, and Neptune. As the earth's...o...b..t is the boundary which separates the inferior from the superior planets, so does the asteroidal belt divide the terrestrial from the major planets. We found the division into inferior and superior useful for emphasising the marked difference in aspect which those two cla.s.ses present as seen from our earth; the inferior planets showing phases like the moon when viewed in the telescope, whereas the superior planets do not. But the division into terrestrial and major planets is the more far-reaching cla.s.sification of the two, for it includes the whole number of planets, whereas the other arrangement necessarily excludes the earth. The members of each of these cla.s.ses have many definite characteristics in common. The terrestrial planets are all of them relatively small in size, comparatively near together, and have few or no satellites. They are, moreover, rather dense in structure. The major planets, on the other hand, are huge bodies, circulating at great distances from each other, and are, as a rule, provided with a number of satellites. With respect to structure, they may be fairly described as being loosely put together. Further, the markings on the surfaces of the terrestrial planets are permanent, whereas those on the major planets are continually shifting.

THE PLANET JUPITER

Jupiter is the greatest of the major planets. It has been justly called the "Giant" planet, for both in volume and in ma.s.s it exceeds all the other planets put together. When seen through the telescope it exhibits a surface plentifully covered with markings, the most remarkable being a series of broad parallel belts. The chief belt lies in the central parts of the planet, and is at present about 10,000 miles wide. It is bounded on either side by a reddish brown belt of about the same width. Bright spots also appear upon the surface of the planet, last for a while, and then disappear. The most notable of the latter is one known as the "Great Red Spot." This is situated a little beneath the southern red belt, and appeared for the first time about thirty years ago. It has undergone a good many changes in colour and brightness, and is still faintly visible. This spot is the most permanent marking which has yet been seen upon Jupiter. In general, the markings change so often that the surface which we see is evidently not solid, but of a fleeting nature akin to cloud (see Plate XIV., p. 230).

[Ill.u.s.tration: PLATE XIV. THE PLANET JUPITER

The Giant Planet as seen at 11.30 p.m., on the 11th of January, 1908, with a 12-1/2-inch reflecting telescope. The extensive oval marking in the upper portion of the disc is the "Great Red Spot." The South is at the top of the picture, the view being the _inverted_ one given by an astronomical telescope. From a drawing by the Rev. Theodore E.R.

Phillips, M.A., F.R.A.S., Director of the Jupiter Section of the British Astronomical a.s.sociation.

(Page 231)]

Observations of Jupiter's markings show that on an average the planet rotates on its axis in a period of about 9 hours 54 minutes. The mention here of _an average_ with reference to the rotation will, no doubt, recall to the reader's mind the similar case of the sun, the different portions of which rotate with different velocities. The parts of Jupiter which move quickest take 9 hours 50 minutes to go round, while those which move slowest take 9 hours 57 minutes. The middle portions rotate the fastest, a phenomenon which the reader will recollect was also the case with regard to the sun.

Jupiter is a very loosely packed body. Its density is on an average only about 1-1/2 times that of water, or about one-fourth the density of the earth; but its bulk is so great that the gravitation at that surface which we see is about 2-1/2 times what it is on the surface of the earth. In accordance, therefore, with the kinetic theory, we may expect the planet to retain an extensive layer of gases around it; and this is confirmed by the spectroscope, which gives evidence of the presence of a dense atmosphere.

All things considered, it may be safely inferred that the interior of Jupiter is very hot, and that what we call its surface is not the actual body of the planet, but a voluminous layer of clouds and vapours driven upwards from the heated ma.s.s underneath. The planet was indeed formerly thought to be self-luminous; but this can hardly be the case, for those portions of the surface which happen to lie at any moment in the shadows cast by the satellites appear to be quite black. Again, when a satellite pa.s.ses into the great shadow cast by the planet it becomes entirely invisible, which would not be the case did the planet emit any perceptible light of its own.

In its revolutions around the sun, Jupiter is attended, so far as we know, by seven[22] satellites. Four of these were among the first celestial objects which Galileo discovered with his "optick tube," and he named them the "Medicean Stars" in honour of his patron, Cosmo de Medici. Being comparatively large bodies they might indeed just be seen with the naked eye, were it not for the overpowering glare of the planet.

It was only in quite recent times, namely, in 1892, that a fifth satellite was added to the system of Jupiter. This body, discovered by Professor E.E. Barnard, is very small. It circulates nearer to the planet than the innermost of Galileo's moons; and, on account of the glare, is a most difficult object to obtain a glimpse of, even in the best of telescopes. In December 1904 and January 1905 respectively, two more moons were added to the system, these being found by _photography_, by the American astronomer, Professor C.D. Perrine. Both the bodies in question revolve at a greater distance from the planet than the outermost of the older known satellites.

Galileo's moons, though the largest bodies of Jupiter's satellite system, are, as we have already pointed out, very small indeed when compared with the planet itself. The diameters of two of them, Europa and Io, are, however, about the same as that of our moon, while those of the other two, Callisto and Ganymede, are more than half as large again.

The recently discovered satellites are, on the other hand, insignificant; that found by Barnard, for example, being only about 100 miles in diameter.

Of the four original satellites Io is the nearest to Jupiter, and, seen from the planet, it would show a disc somewhat larger than that of our moon. The others would appear somewhat smaller. However, on account of the great distance of the sun, the entire light reflected to Jupiter by all the satellites should be very much less than what we get from our moon.

Barnard's satellite circles around Jupiter at a distance less than our moon is from us, and in a period of about 12 hours. Galileo's four satellites revolve in periods of about 2, 3-1/2, 7, and 16-1/2 days respectively, at distances lying roughly between a quarter of a million and one million miles. Perrine's two satellites are at a distance of about seven million miles, and take about nine months to complete their revolutions.

The larger satellites, when viewed in the telescope, exhibit certain defined markings; but the bodies are so far away from us, that only those details which are of great extent can be seen. The satellite Io, according to Professor Barnard, shows a darkish disc, with a broad white belt across its middle regions. Mr. Dougla.s.s, one of the observers at the Lowell Observatory, has noted upon Ganymede a number of markings somewhat resembling those seen on Mars, and he concludes, from their movement, that this satellite rotates on its axis in about seven days.

Professor Barnard, on the other hand, does not corroborate this, though he claims to have discovered bright polar caps on both Ganymede and Callisto.

In an earlier chapter we dealt at length with eclipses, occultations, and transits, and endeavoured to make clear the distinction between them. The system of Jupiter's satellites furnishes excellent examples of all these phenomena. The planet casts a very extensive shadow, and the satellites are constantly undergoing obscuration by pa.s.sing through it.

Such occurrences are plainly comparable to our lunar eclipses. Again, the satellites may, at one time, be occulted by the huge disc of the planet, and at another time seen in transit over its face. A fourth phenomenon is what is known as an _eclipse of the planet by a satellite_, which is the exact equivalent of what we style on the earth an eclipse of the sun. In this last case the shadow, cast by the satellite, appears as a round black spot in movement across the planet's surface.

In the pa.s.sages of these attendant bodies behind the planet, into its shadow, or across its face, respectively, it occasionally happens that Galileo's four satellites all disappear from view, and the planet is then seen for a while in the unusual condition of being apparently without its customary attendants. An instance of this phenomenon took place on the 3rd of October 1907. On that occasion, the satellites known as I. and III. (_i.e._ Io and Ganymede) were eclipsed, that is to say, obscured by pa.s.sing into the planet's shadow; Satellite IV. (Callisto) was occulted by the planet's disc; while Satellite II. (Europa), being at the same moment in transit across the planet's face, was invisible against that brilliant background. A number of instances of this kind of occurrence are on record. Galileo, for example, noted one on the 15th of March 1611, while Herschel observed another on the 23rd of May 1802.

It was indirectly to Jupiter's satellites that the world was first indebted for its knowledge of the velocity of light. When the periods of revolution of the satellites were originally determined, Jupiter happened, at the time, to be at his nearest to us. From the periods thus found tables were made for the prediction of the moments at which the eclipses and other phenomena of the satellites should take place. As Jupiter, in the course of his...o...b..t, drew further away from the earth, it was noticed that the disappearances of the satellites into the shadow of the planet occurred regularly later than the time predicted. In the year 1675, Roemer, a Danish astronomer, inferred from this, not that the predictions were faulty, but that light did not travel instantaneously.

It appeared, in fact, to take longer to reach us, the greater the distance it had to traverse. Thus, when the planet was far from the earth, the last ray given out by the satellite, before its pa.s.sage into the shadow, took a longer time to cross the intervening s.p.a.ce, than when the planet was near. Modern experiments in physics have quite confirmed this, and have proved for us that light does not travel across s.p.a.ce in the twinkling of an eye, as might hastily be supposed, but actually moves, as has been already stated, at the rate of about 186,000 miles per second.

THE PLANET SATURN

Seen in the telescope the planet Saturn is a wonderful and very beautiful object. It is distinguished from all the other planets, in fact from all known celestial bodies, through being girt around its equator by what looks like a broad, flat ring of exceeding thinness.

This, however, upon closer examination, is found to be actually composed of three concentric rings. The outermost of these is nearly of the same brightness as the body of the planet itself. The ring which comes immediately within it is also bright, and is separated from the outer one all the way round by a relatively narrow s.p.a.ce, known as "Ca.s.sini's division," because it was discovered by the celebrated French astronomer, J.D. Ca.s.sini, in the year 1675. Inside the second ring, and merging insensibly into it, is a third one, known as the "c.r.a.pe ring,"

because it is darker in hue than the others and partly transparent, the body of Saturn being visible through it. The inner boundary of this third and last ring does not adjoin the planet, but is everywhere separated from it by a definite s.p.a.ce. This ring was discovered _independently_[23] in 1850 by Bond in America and Dawes in England.

[Ill.u.s.tration: PLATE XV. THE PLANET SATURN

From a drawing made by Professor Barnard with the Great Lick Telescope.

The black band fringing the outer ring, where it crosses the disc, is portion of the _shadow which the rings cast upon the planet_. The black wedge-shaped mark, where the rings disappear behind the disc at the left-hand side, is portion of the _shadow which the planet casts upon the rings_.

(Page 237)]

As distinguished from the c.r.a.pe ring, the bright rings must have a considerable closeness of texture; for the shadow of the planet may be seen projected upon them, and their shadows in turn projected upon the surface of the planet (see Plate XV., p. 236).

According to Professor Barnard, the entire breadth of the ring system, that is to say, from one side to the other of the outer ring, is 172,310 miles, or somewhat more than double the planet's diameter.

In the varying views which we get of Saturn, the system of the rings is presented to us at very different angles. Sometimes we are enabled to gaze upon its broad expanse; at other times, however, its thin edge is turned exactly towards us, an occurrence which takes place after intervals of about fifteen years. When this happened in 1892 the rings are said to have disappeared entirely from view in the great Lick telescope. We thus get an idea of their small degree of thickness, which would appear to be only about 50 miles. The last time the system of rings was exactly edgewise to the earth was on the 3rd of October 1907.

The question of the composition of these rings has given rise to a good deal of speculation. It was formerly supposed that they were either solid or liquid, but in 1857 it was proved by Clerk Maxwell that a structure of this kind would not be able to stand. He showed, however, that they could be fully explained by supposing them to consist of an immense number of separate solid particles, or, as one might otherwise put it, extremely small satellites, circling in dense swarms around the middle portions of the planet. It is therefore believed that we have here the materials ready for the formation of a satellite or satellites; but that the powerful gravitative action, arising through the planet's being so near at hand, is too great ever to allow these materials to aggregate themselves into a solid ma.s.s. There is, as a matter of fact, a minimum distance from the body of any planet within which it can be shown that a satellite will be unable to form on account of gravitational stress. This is known as "Roche's limit," from the name of a French astronomer who specially investigated the question.

There thus appears to be a certain degree of a.n.a.logy between Saturn's rings and the asteroids. Empty s.p.a.ces, too, exist in the asteroidal zone, the relative position of one of which bears a striking resemblance to that of "Ca.s.sini's division." It is suggested, indeed, that this division had its origin in gravitational disturbances produced by the attraction of the larger satellites, just as the empty s.p.a.ces in the asteroidal zone are supposed to be the result of perturbations caused by the Giant Planet hard by.

It has long been understood that the system of the rings must be rotating around Saturn, for if they were not in motion his intense gravitational attraction would quickly tear them in pieces. This was at length proved to be the fact by the late Professor Keeler, Director of the Lick Observatory, who from spectroscopic observations found that those portions of the rings situated near to the planet rotated faster than those farther from it. This directly supports the view that the rings are composed of satellites; for, as we have already seen, the nearer a satellite is to its primary the faster it will revolve. On the other hand, were the rings solid, their outer portions would move the fastest; as we have seen takes place in the body of the earth, for example. The ma.s.s of the ring system, however, must be exceedingly small, for it does not appear to produce any disturbances in the movements of Saturn's satellites. From the kinetic theory, therefore, one would not expect to find any atmosphere on the rings, and the absence of it is duly shown by spectroscopic observations.

The diameter of Saturn, roughly speaking, is about one-fifth less than that of Jupiter. The planet is very flattened at the poles, this flattening being quite noticeable in a good telescope. For instance, the diameter across the equator is about 76,470 miles, while from pole to pole it is much less, namely, 69,770.

The surface of Saturn bears a strong resemblance to that of Jupiter. Its markings, though not so well defined, are of the same belt-like description; and from observation of them it appears that the planet rotates _on an average_ in a little over ten hours. The rotation is in fact of the same peculiar kind as that of the sun and Jupiter; but the difference of speed at which the various portions of Saturn go round are even more marked than in the case of the Giant Planet. The density of Saturn is less than that of Jupiter; so that it must be largely in a condition of vapour, and in all probability at a still earlier stage of planetary evolution.

Up to the present we know of as many as ten satellites circling around Saturn, which is more than any other planet of the solar system can lay claim to. Two of these, however, are very recent discoveries; one, Phoebe, having been found by photography in August 1898, and the other, Themis, in 1904, also by the same means. For both of these we are indebted to Professor W.H. Pickering. Themis is said to be _the faintest object in the solar system_. It cannot be _seen_, even with the largest telescope in existence; a fact which should hardly fail to impress upon one the great advantage the photographic plate possesses in these researches over the human eye.

The most important of the whole Saturnian family of satellites are the two known as t.i.tan and j.a.petus. These were discovered respectively by Huyghens in 1655 and by Ca.s.sini in 1671. j.a.petus is about the same size as our moon; while the diameter of t.i.tan, the largest of the satellites, is about half as much again. t.i.tan takes about sixteen days to revolve around Saturn, while j.a.petus takes more than two months and a half. The former is about three-quarters of a million miles distant from the planet, and the latter about two and a quarter millions. To Sir William Herschel we are indebted for the discovery of two more satellites, one of which he found on the evening that he used his celebrated 40-foot telescope for the first time. The ninth satellite, Phoebe, one of the two discovered by Professor Pickering, is perhaps the most remarkable body in the solar system, for all the other known members of that system perform their revolutions in one fixed direction, whereas this satellite revolves in the _contrary_ direction.

In consequence of the great distance of Saturn, the sun, as seen from the planet, would appear so small that it would scarcely show any disc.

The planet, indeed, only receives from the sun about one-ninetieth of the heat and light which the earth receives. Owing to this diminished intensity of illumination, the combined light reflected to Saturn by the whole of its satellites must be very small.

With the sole exception of Jupiter, not one of the planets circulating nearer to the sun could be seen from Saturn, as they would be entirely lost in the solar glare. For an observer upon Saturn, Jupiter would, therefore, fill much the same position as Venus does for us, regularly displaying phases and being alternately a morning and an evening star.

It is rather interesting to consider the appearances which would be produced in our skies were the earth embellished with a system of rings similar to those of Saturn. In consequence of the curving of the terrestrial surface, they would not be seen at all from within the Arctic or Antarctic circles, as they would be always below the horizon.

From the equator they would be continually seen edgewise, and so would appear merely as line of light stretching right across the heaven and pa.s.sing through the zenith. But the dwellers in the remaining regions would find them very objectionable, for they would cut off the light of the sun during lengthy periods of time.

Saturn was a sore puzzle to the early telescopic observers. They did not for a long time grasp the fact that it was surrounded by a ring--so slow is the human mind to seek for explanations out of the ordinary course of things. The protrusions of the ring on either side of the planet, at first looked to Galileo like two minor globes placed on opposite sides of it, and slightly overlapping the disc. He therefore informed Kepler that "Saturn consists of three stars in contact with one another." Yet he was genuinely puzzled by the fact that the two attendant bodies (as he thought them) always retained the same position with regard to the planet's disc, and did not appear to revolve around it, nor to be in any wise shifted as a consequence of the movements of our earth.