The Beauties of Nature, and the Wonders of the World We Live In Part 21

If we take the numbers 0, 3, 6, 12, 24, 48, 96, each one (after the second) the double of that preceding, and add four, we have the series.

4 7 10 16 28 52 100

Now the distances of the Planets from the Sun are as follow:--

Mercury. Venus. Earth. Mars. Jupiter. Saturn.

3.9 7.2 10 15.2 52.9 95.4

For this sequence, which was first noticed by Bode, and is known as Bode's law, no explanation can yet be given. It was of course at once observed that between Mars and Jupiter one place is vacant, and it has now been ascertained that this is occupied by a zone of Minor Planets, the first of which was discovered by Piazzi on January 1, 1801, a worthy prelude to the succession of scientific discoveries which form the glory of our century. At present over 300 are known, but certainly these are merely the larger among an immense number, some of them doubtless mere dust.

JUPITER

Beyond the Minor Planets we come to the stupendous Jupiter, containing 300 times the ma.s.s, and being 1200 times the size of our Earth--larger indeed than all the other planets put together. It is probably not solid, and from its great size still retains a large portion of the original heat, if we may use such an expression. Jupiter usually shows a number of belts, supposed to be due to clouds floating over the surface, which have a tendency to arrange themselves in belts or bands, owing to the rotation of the planet. Jupiter has four moons or satellites.

SATURN

[Ill.u.s.tration: Fig. 53.--Saturn.]

Next to Jupiter in size, as in position, comes Saturn, which, though far inferior in dimensions, is much superior in beauty. To the naked eye Saturn appears as a brilliant star, but when Galileo first saw it through a telescope it appeared to him to be composed of three bodies in a line, a central globe with a small one on each side. Huyghens in 1655 first showed that in reality Saturn was surrounded by a series of rings (see Fig. 53). Of these there are three, the inner one very faint, and the outer one divided into two by a dark line. These rings are really enormous shoals of minute bodies revolving round the planet, and rendering it perhaps the most marvellous and beautiful of all the heavenly bodies.

While we have one Moon, Mars two, and Jupiter four, Saturn has no less than eight satellites.

URa.n.u.s

Saturn was long supposed to be the outermost body belonging to the solar system. In 1781, however, on the 13th March, William Herschel was examining the stars in the constellation of the Twins. One struck him because it presented a distinct disc, while the true fixed stars, however brilliant, are, even with the most powerful telescope, mere points of light. At first he thought it might be a comet, but careful observations showed that it was really a new planet. Though thus discovered by Herschel it had often been seen before, but its true nature was unsuspected. It has a diameter of about 31,700 miles.

Four satellites of Ura.n.u.s have been discovered, and they present the remarkable peculiarity that while all the other planets and their satellites revolve nearly in one plane, the satellites of Ura.n.u.s are nearly at right angles, indicating the presence of some local and exceptional influence.

NEPTUNE

The study of Ura.n.u.s soon showed that it followed a path which could not be accounted for by the influence of the Sun and the other then known planets. It was suspected, therefore, that this was due to some other body not yet discovered. To calculate where such a body must be so as to account for these irregularities was a most complex and difficult, and might have seemed almost a hopeless, task. It was, however, solved almost simultaneously and independently by Adams in this country, and Le Verrier in France.

Neptune, so far as we yet know the out-most of our companions, is 35,000 miles in diameter, and its mean distance from the Sun is 2,780,000,000 miles.

ORIGIN OF THE PLANETARY SYSTEM

The theory of the origin of the Planetary System known as the "Nebular Hypothesis," which was first suggested by Kant, and developed by Herschel and Laplace, may be fairly said to have attained a high degree of probability. The s.p.a.ce now occupied by the solar system is supposed to have been filled by a rotating spheroid of extreme tenuity and enormous heat, due perhaps to the collision of two originally separate bodies. The heat, however, having by degrees radiated into s.p.a.ce, the gas cooled and contracted towards a centre, destined to become the Sun.

Through the action of centrifugal force the gaseous matter also flattened itself at the two poles, taking somewhat the form of a disc.

For a certain time the tendency to contract, and the centrifugal force, counterbalanced one another, but at length a time came when the latter prevailed and the outer zone detached itself from the rest of the sphere. One after another similar rings were thrown off, and then breaking up, formed the planets and their satellites.

That each planet and satellite did form originally a ring we still have evidence in the wonderful and beautiful rings of Saturn, which, however, in all probability will eventually form spherical satellites like the rest. Thus then our Earth was originally a part of the Sun, to which again it is destined one day to return. M. Plateau has shown experimentally that by rotating a globe of oil in a mixture of water and spirit having the same density this process may be actually repeated in miniature.

This brilliant, and yet simple, hypothesis is consistent with, and explains many other circ.u.mstances connected with the position, magnitude, and movements of the Planets and their satellites.

The Planets, for instance, lie more or less in the same plane, they revolve round the Sun and rotate on their own axis in the same direction--a series of coincidences which cannot be accidental, and for which the theory would account. Again the rate of cooling would of course follow the size; a small body cools more rapidly than a large one. The Moon is cold and rigid; the Earth is solid at the surface, but intensely hot within; Jupiter and Saturn, which are immensely larger, still retain much of their original heat, and have a much lower density than the Earth; and astronomers tell us on other grounds that the Sun itself is still contracting, and that to this the maintenance of its temperature is due.

Although, therefore, the Nebular Theory cannot be said to have been absolutely proved, it has certainly been brought to a high state of probability, and is, in its main features, generally accepted by astronomers.

The question has often been asked whether any of the heavenly bodies are inhabited, and as yet it is impossible to give any certain answer. It seems _a priori_ probable that the millions of suns which we see as stars must have satellites, and that some at least of them may be inhabited. So far as our own system is concerned the Sun is of course too hot to serve as a dwelling-place for any beings with bodies such as ours. The same may be said of Mercury, which is at times probably ten times as hot as our tropics. The outer planets appear to be still in a state of vapour. The Moon has no air or water.

Mars is in a condition which most nearly resembles ours. All, however, that can be said is that, so far as we can see, the existence of living beings on Mars is not impossible.

COMETS

The Sun, Moon, and Stars, glorious and wonderful as they are, though regarded with great interest, and in some cases wors.h.i.+pped as deities, excited the imagination of our ancestors less than might have been expected, and even now attract comparatively little attention, from the fact that they are always with us. Comets, on the other hand, both as rare and occasional visitors, from their large size and rapid changes, were regarded in ancient times with dread and with amazement.

Some Comets revolve round the Sun in ellipses, but many, if not the majority, are visitors indeed, for having once pa.s.sed round the Sun they pa.s.s away again into s.p.a.ce, never to return.

The appearance which is generally regarded as characteristic of a Comet is that of a head with a central nucleus and a long tail. Many, however, of the smaller ones possess no tail, and in fact Comets present almost innumerable differences. Moreover the same Comet changes rapidly, so that when they return, they are identified not in any way by their appearance, but by the path they pursue.

Comets may almost be regarded as the ghosts of heavenly bodies. The heads, in some cases, may consist of separate solid fragments, though on this astronomers are by no means agreed, but the tails at any rate are in fact of almost inconceivable tenuity. We know that a cloud a few hundred feet thick is sufficient to hide, not only the stars, but even the Sun himself. A Comet is thousands of miles in thickness, and yet even extremely minute stars can be seen through it with no appreciable diminution of brightness. This extreme tenuity of comets is moreover shown by their small weight. Enormous as they are I remember Sir G. Airy saying that there was probably more matter in a cricket ball than there is in a comet. No one, however, now doubts that the weight must be measured in tons; but it is so small, in relation to the size, as to be practically inappreciable. If indeed they were comparable in ma.s.s even to the planets, we should long ago have perished. The security of our system is due to the fact that the planets revolve round the Sun in one direction, almost in circles, and very nearly in the same plane. Comets, however, enter our system in all directions, and at all angles; they are so numerous that, as Kepler said, there are probably more Comets in the sky than there are fishes in the sea, and but for their extreme tenuity they would long ago have driven us into the Sun.

When they first come in sight Comets have generally no tail; it grows as they approach the Sun, from which it always points away. It is no mere optical illusion; but while the Comet as a whole is attracted by the Sun, the tail, how or why we know not, is repelled. When once driven off, moreover, the attraction of the Comet is not sufficient to recall it, and hence perhaps so many Comets have now no tails.

Donati's Comet, the great Comet of 1858, was first noticed on the 2d June as a faint nebulous spot. For three months it remained quite inconspicuous, and even at the end of August was scarcely visible to the naked eye. In September it grew rapidly, and by the middle of October the tail extended no less than 40 degrees, after which it gradually disappeared.

Faint as is the light emitted by Comets, it is yet their own, and spectrum a.n.a.lysis has detected the presence in them of carbon, hydrogen, nitrogen, sodium, and probably of iron.

Comets then remain as wonderful, and almost as mysterious, as ever, but we need no longer regard "a comet as a sign of impending calamity; we may rather look upon it as an interesting and a beautiful visitor, which comes to please us and to instruct us, but never to threaten or to destroy."[69] We are free, therefore, to admire them in peace, and beautiful, indeed, they are.

"The most wonderful sight I remember," says Hamerton, "as an effect of calm, was the inversion of Donati's Comet, in the year 1858, during the nights when it was sufficiently near the horizon to approach the rugged outline of Graiganunie, and be reflected beneath it in Loch Awe. In the sky was an enormous aigrette of diamond fire, in the water a second aigrette, scarcely less splendid, with its brilliant point directed upwards, and its broad, shadowy extremity ending indefinitely in the deep. To be out on the lake alone, in a tiny boat, and let it rest motionless on the gla.s.sy water, with that incomparable spectacle before one, was an experience to be remembered through a lifetime. I have seen many a glorious sight since that now distant year, but nothing to equal it in the a.s.sociation of solemnity with splendour."[70]

SHOOTING STARS

On almost any bright night, if we watch a short time some star will suddenly seem to drop from its place, and, after a short plunge, to disappear. This appearance is, however, partly illusory. While true stars are immense bodies at an enormous distance, Shooting Stars are very small, perhaps not larger than a paving stone, and are not visible until they come within the limits of our atmosphere, by the friction with which they are set on fire and dissipated. They are much more numerous on some nights than others. From the 9th to the 11th August we pa.s.s through one cl.u.s.ter which is known as the Perseids; and on the 13th and 14th November a still greater group called by astronomers the Leonids. The Leonids revolve round the Sun in a period of 33 years, and in an elliptic orbit, one focus of which is about at the same distance from the Sun as we are, the other at about that of Ura.n.u.s. The shoal of stars is enormous; its diameter cannot be less than 100,000 miles, and its length many hundreds of thousands. There are, indeed, stragglers scattered over the whole orbit, with some of which we come in contact every year, but we pa.s.s through the main body three times in a century--last in 1866--capturing millions on each occasion. One of these has been graphically described by Humboldt:

"From half after two in the morning the most extraordinary luminary meteors were seen in the direction of the east. M. Bonpland, who had risen to enjoy the freshness of the air, perceived them first. Thousands of bodies and falling stars succeeded each other during the s.p.a.ce of four hours. Their direction was very regular from north to south. They filled a s.p.a.ce in the sky extending from due east 30 to north and south. In an amplitude of 60 the meteors were seen to rise above the horizon at east-north-east, and at east, to describe arcs more or less extended, and to fall towards the south, after having followed the direction of the meridian. Some of them attained a height of 40, and all exceeded 25 or 30. No trace of clouds was to be seen. M. Bonpland states that, from the first appearance of the phenomenon, there was not in the firmament a s.p.a.ce equal in extent to three diameters of the moon which was not filled every instant with bolides and falling stars. The first were fewer in number, but as they were of different sizes it was impossible to fix the limit between these two cla.s.ses of phenomena. All these meteors left luminous traces from five to ten degrees in length, as often happens in the equinoctial regions. The phosph.o.r.escence of these traces, or luminous bands, lasted seven or eight seconds. Many of the falling stars had a very distinct nucleus, as large as the disc of Jupiter, from which darted sparks of vivid light. The bodies seemed to burst as by explosion; but the largest, those from 1 to 1 15' in diameter, disappeared without scintillation, leaving behind them phosph.o.r.escent bands (trabes), exceeding in breadth fifteen or twenty minutes. The light of these meteors was white, and not reddish, which must doubtless be attributed to the absence of vapour and the extreme transparency of the air."[71]

The past history of the Leonids, which Le Verrier has traced out with great probability, if not proved, is very interesting. They did not, he considers, approach the Sun until 126 A.D., when, in their career through the heavens, they chanced to come near to Ura.n.u.s. But for the influence of that planet they would have pa.s.sed round the Sun, and then departed again for ever. By his attraction, however, their course was altered, and they will now continue to revolve round the Sun.

There is a remarkable connection between star showers and comets, which, however, is not yet thoroughly understood. Several star showers follow paths which are also those of comets, and the conclusion appears almost irresistible that these comets are made up of Shooting Stars.

We are told, indeed, that 150,000,000 of meteors, including only those visible with a moderate telescope, fall on the earth annually. At any rate, there can be no doubt that every year millions of them are captured by the earth, thus const.i.tuting an appreciable, and in the course of ages a constantly increasing, part of the solid substance of the globe.

THE STARS

We have been dealing in the earlier part of this chapter with figures and distances so enormous that it is quite impossible for us to realise them; and yet we have still others to consider compared with which even the solar system is insignificant.

In the first place, the number of the Stars is enormous. When we look at the sky at night they seem, indeed, almost innumerable; so that, like the sands of the sea, the Stars of heaven have ever been used as effective symbols of number. The total number visible to the naked eye is, however, in reality only about 3000, while that shown by the telescope is about 100,000,000. Photography, however, has revealed to us the existence of others which no telescope can show. We cannot by looking long at the heavens see more than at first; in fact, the first glance is the keenest. In photography, on the contrary, no light which falls on the plate, however faint, is lost; it is taken in and stored up. In an hour the effect is 3600 times as great as in a second. By exposing the photographic plate, therefore, for some hours, and even on successive nights, the effect of the light is as it were acc.u.mulated, and stars are rendered visible, the light of which is too feeble to be shown by any telescope.

The distances and magnitudes of the Stars are as astonis.h.i.+ng as their numbers, Sirius, for instance, being about twenty times as heavy as the Sun itself, 50 times as bright, and no less than 1,000,000 times as far away; while, though like other stars it seems to us stationary, it is in reality sweeping through the heavens at the rate of 1000 miles a minute; Maia, Electra, and Alcyone, three of the Pleiades, are considered to be respectively 400, 480, and 1000 times as brilliant as the Sun, Canopus 2500 times, and Arcturus, incredible as it may seem, even 8000 times, so that, in fact, the Sun is by no means one of the largest Stars. Even the minute Stars not separately visible to the naked eye, and the millions which make up the Milky Way, are considered to be on an average fully equal to the Sun in l.u.s.tre.