1st. That the planets move in ellipses, of which the sun is in one of the foci.

2nd. The s.p.a.ces described by the ideal radius which joins each planet to the sun are proportional to the times of their description. In other words, the nearer a planet is to the sun, the faster it moves.

3rd. The squares of the times of revolution are as the cubes of the major axes of the orbits.

Such were the laws of Kepler, the basis of modern astronomy, which led in the hands of Newton to the simple explanation by universal gravitation, which itself is now asking to be explained.

We are not to suppose that the system of Copernicus was universally accepted even by astronomers of note. By some an attempt was made to invent a system which should have all the advantages of this, and yet if possible save the immobility of the earth. Such was that of Tycho Brahe, who was born three years after the death of Copernicus, and died in 1601. He was one of the most laborious and painstaking observers of his time, although by the peculiarity of fate he is known generally only by his false system.

[Ill.u.s.tration: FIG. 21.--TYCHO BRAHE'S SYSTEM.]

In 1577, Tycho Brahe wrote a little treatise, _Tychonis Brahe, Dani, De Mundi aetherei Recentioribus phenomenis, a propos_ of a comet that had lately appeared. He speaks at length of his system as follows:--"I have remarked that the ancient system of Ptolemy is not at all natural, and too complicated. But neither can I approve of the new one introduced by the great Copernicus after the example of Aristarchus of Samos. This heavy ma.s.s of earth, so little fit for motion, could not be displaced in this manner, and moved in three ways, like the celestial bodies, without a shock to the principles of physics. Besides, it is opposed to Scripture! I think then," he adds, "that we must decidedly and without doubt place the earth immovable in the centre of world, according to the belief of the ancients and the testimony of Scripture. In my opinion the celestial motions are arranged in such a way that the sun, the moon, and the sphere of the fixed stars, which incloses all, have the earth for their centre. The five planets turn about the sun as about their chief and king, the sun being constantly in the centre of their orbits, and accompany it in its annual motion round the earth." This system perfectly accounts for the apparent motions of the planets as seen from the earth, and is essentially a variation on the Copernican, rather than on the Ptolemaic system, but it lent itself less readily to future discoveries. It simply amounts, as far as the solar system is concerned, to impressing upon all the rest of it the motions of the earth, so as to leave the latter at rest; and were the sun only as large with respect to the earth as it seems, were the planets really smaller than the moon, and the stars only at a short distance, and smaller than the planets, it might seem more natural that they should move than the earth; but when all these suppositions were disproved, the very argument of Tycho Brahe for the stability of the earth turned the other way, and proved as incontestably that it moved. In the Copernican system, however, these questions are of no consequence; if the sun be at rest, this ma.s.s makes no difference; if the earth moves like the planets, their relative size does not alter anything; and if stars are immovable they may be at any distance and of any magnitude.

The objections of Tycho Brahe to the earth's motion were: First, that it was too heavy--we know now, however, that some other planets are heavier--and that the sun, which he would make move instead, is 340,000 times as heavy. Secondly, that if the earth moved, all loose things would be carried from east to west; but we have experience of many loose things being kept by friction on moving bodies, and can conceive how, all things may be kept by the attraction of the earth under the influence of its own motion. Thirdly, that he could not imagine that the earth was turned upside down every day, and that for twelve hours our heads are downwards.

But the existence of the antipodes overcomes this objection, and shows that there is no up and down in the universe, but each man calls that _down_ which is nearer to the centre of the earth than himself.

A variation on Tycho Brahe's system was attempted by one Longomonta.n.u.s, who had lived with him for ten years. It consisted in admitting the diurnal rotation, but not the annual revolution, of the earth; but it made no progress, and was soon forgotten.

More remarkable than this was the attempt by Descartes in the same direction, namely, to hold the principles of Copernicus, and yet to teach the immobility of the earth. His idea of immobility was however very different from that of Tycho Brahe, or of any one else, and would only be called so by those who were bound to believe it at all costs.

His Theory of Vortices, as it is called, will be best given in his own words as contained in his _Les Principes de la Philosophie_, third part, chap. xxvi., ent.i.tled, "That the earth is at rest in its heaven, which does not prevent its being carried along with it, and that it is the same with all the planets."

"I adhere," he says, "to the hypothesis of Copernicus, because it seems to me the simplest and clearest. There is no vacuum anywhere in s.p.a.ce.... The heavens are full of a universal liquid substance. This is an opinion now commonly received among astronomers, because they cannot see how the phenomena can be explained without it. The substance of the heavens has the common property of all liquids, that its minutest particles are easily moved in any direction, and when it happens that they all move in one way, they necessarily carry with them all the bodies they surround, and which are not prevented from moving by any external cause. The matter of the heaven in which the planets are turns round continually like a vortex, which has the Sun for its centre. The parts that are nearest the Sun move faster than those that are at a greater distance; and all the planets, including the earth, remain always suspended in the same place in the matter of the heaven. And just as in the turns of rivers, when the water turns back on itself and twists round in circles, if any twig or light body floats on it, we see it carry them round, and make them move with it, and even among these twigs we may see some turning on their own centre, and those that are nearest to the middle of the vortex moving quicker than those on the outside; so we may easily imagine it to be with the planets, and this is all that is necessary to explain the phenomena. The matter that is round Saturn takes about thirty years to run its circle; that which surrounds Jupiter carries it and its satellites round in twelve years, and so on.... The satellites are carried round their primaries by smaller vortices.... The earth is not sustained by columns, nor suspended in the air by ropes, but it is environed on all sides by a very liquid heaven.

It is at rest, and has no propulsion or motion, since we do not perceive any in it. This does not prevent it being carried round by its heaven, and following its motion without moving itself, just as a vessel which is not moved by winds or oars, and is not retained by anchors, remains in repose in the middle of the sea, although the flood of the great ma.s.s of water carries it insensibly with it. Like the earth, the planets remain at rest in the region of heaven where each one is found.

Copernicus made no difficulty in allowing that the earth moves. Tycho, to whom this opinion seemed absurd and unworthy of common sense, wished to correct him, but the earth has far more motion in his hypothesis than in that of Copernicus."

[Ill.u.s.tration: FIG. 22.--DESCARTES' THEORY OF VORTICES.]

Such is the celebrated theory of vortices. The comparison of the rotation of the earth and planets and their revolution round the sun to the turning of small portions of a rapid stream, may contain an idea yet destined to be developed to account for these motions; but as used by Descartes it is a mere playing upon words admirably adapted to secure the concurrence of all parties; those who believed in the motion of the earth seeing that it did not interfere with their ideas in the least, and those who believed in its stability being gratified to find some way by which they might still cling to that belief and yet adopt the new ideas. This was its purpose, and that purpose it well served; but as a philosophical speculation it was worthless. When former astronomers declared that any planet moved, whether it were the earth or any other, they had no idea of attraction, but supposed the planet fixed to a sphere; this sphere moving and carrying the planet with it was what they meant by the planet moving: the theory of vortices merely subst.i.tuted a liquid for a solid sphere, with this disadvantage, that if the planet were fixed to a solid moving sphere, it _must_ move; if only placed in a liquid one, that liquid might pa.s.s it if it did not have motion of its own.

[Ill.u.s.tration: FIG. 23.--VORTICES OF THE STARS.]

A variation on Descartes' system of vortices was proposed in the eighteenth century, which supposed that the sun, instead of being fixed in the centre of the system, itself circulated round another centre, carrying Mercury with it. This motion of the sun was intented to explain the changes of magnitude of its disc as seen from the earth, and the diurnal and annual variations in its motion, without discarding its circular path.

[Ill.u.s.tration: FIG. 24.--VARIATION OF DESCARTES' THEORY.]

We have thus noticed all the chief astronomical systems that have at any time been entertained by astronomers. They one and all have given way before the universally acknowledged truth about which there is no longer any dispute. Systems are not now matters of opinion or theory. We speak of facts as certain as any that can be ascertained in any branch of knowledge. We have much to learn, but what we have settled as the basis of our knowledge will never more be altered as far as we can see.

Of course there have been always fantastic fancies put forth about the solar system, but they are more amusing than instructive. Some have said that there is no sun, moon, or stars, but that they are reflections from an immense light under the earth. Some savage races say that the moon when decreasing breaks up into stars, and is renewed each month by a creative act. The Indians used to say that it was full of nectar which the G.o.ds ate up when it waned, and which grew again when it waxed. The Brahmins placed the earth in the centre, and said that the stars moved like fishes in a sea of liquid. They counted nine planets, of which two are invisible dragons which cause eclipses; which, since they happen in various parts of the zodiac, show that these dragons revolve like the rest. They said the sun was nearer than the moon, perhaps because it is hotter and brighter. Berosus the Chaldean gave a very original explanation of the phases and eclipses of the moon. He said it had one side bright, and the other side just the colour of the sky, and in turning it represented the different colours to us.

Before concluding this chapter we may notice what information we possess as to the origin of the names by which the planets are known. These names have not always been given to them, and date only from the time when the poets began to a.s.sociate the Grecian mythology with astronomy.

The earlier names had reference rather to their several characters, although there appear to have been among every people two sets of names applied to them.

The earliest Greek names referred to their various degrees of brilliancy: thus Saturn, which is not easily distinguished, was called Phenon, or _that which appears_; Jupiter was named Phaeton, _the brilliant_; Mars was Pysos, or _flame-coloured_; Mercury, Stilbon, _the sparkling_; Venus, Phosphorus; and Lucifer, _the light-bearer_. They called the latter also Calliste, _the most beautiful_. It was also known then as now under the appellations of the morning star and evening star, indicating its special position.

With the ancient Accadians, the planets had similar names, among others.

Thus, "Mars was sometimes called _the vanishing star_, in allusion to its recession from the earth, and Jupiter the _planet of the ecliptic_, from its neighbourhood to the latter" (Sayce). The name of Mars raises the interesting question as to whether they had noticed its phases as well as its movements--especially when, with reference to Venus, it is recorded in the "Observations of Bel," that "it rises, and in its...o...b..t duly grows in size." They had also a rather confusing system of nomenclature by naming each planet after the star that it happened to be the nearest to at any point of its course round the ecliptic.

Among less cultivated nations also the same practice held, as with the natives of South America, whose name for the sun is a word meaning _it brings the day_; for the moon, _it brings the night_; and for Venus, _it announces the day_.

But even among the Eastern nations, from whom the Greeks and Romans borrowed their astronomical systems, it soon became a practice to a.s.sociate these planets with the names of the several divinities they worshipped. This was perhaps natural from the adoration they paid to the celestial luminaries themselves on account of their real or supposed influence on terrestrial affairs; and, moreover, as time went on, and heroes had appeared, and they had to find them dwelling-places in the heavens, they would naturally a.s.sociate them with one or other of the most brilliant and remarkable luminaries, to which they might suppose them translated. Beyond these general remarks, only conjectures can be made why any particular divinity should among the Greeks be connected with the several planets as we now know them. Such conjectures as the following we may make. Thus Jupiter, the largest, would take first rank, and be called after the name of the chief divinity. The soft and sympathising Venus--appearing at the twilight--would well denote the evening star. Mars would receive its name from its red appearance, naturally suggesting carnage and the G.o.d of war. Saturn, or Kronos, the G.o.d of time, is personified by the slow and almost imperceptible motion of that remote planet. While Mercury, the fiery and quick G.o.d of thieves and commerce, is well matched with the hide-and-seek planet which so seldom can be seen, and moves so rapidly.

These were the only planets known to the ancients, and were indeed all that could be discovered without a telescope. If the ancient Babylonians possessed telescopes, as has been conjectured from their speaking, as we have noticed above, of the increase of the size of Venus, and from the finding a crystal lens among the ruins of Nineveh, they did not use them for this purpose.

The other planets now known have a far shorter history. Ura.n.u.s was discovered by Sir William Herschel on the 13th of March, 1781, and was at first taken for a comet. Herschel proposed to call it Georgium Sidus, after King George III. Lalande suggested it should be named Herschel, after its discoverer, and it bore this name for some time. Afterwards the names, Neptune, Astroea, Cybele, and Ura.n.u.s were successively proposed, and the latter, the suggestion of Bode, was ultimately adopted. It is the name of the most ancient of the G.o.ds, connected with the then most modern of planets in point of discovery, though also most ancient in formation, if recent theories be correct. Neptune, as everybody knows, was calculated into existence, if one may so speak, by Adams and Leverrier independently, and was first seen, in the quarter indicated, by Dr. Galle at Berlin, in September, 1846, and by universal consent it received the name it now bears.

There are now also known a long series of what are called minor planets, all circulating between Mars and Jupiter, with their irregular orbits inextricably mingled together. Their discovery was led to in a remarkable manner. It was observed that the distances of the several planets might approximately be expressed by the terms of a certain mathematical series, if one term was supplied between Mars and Jupiter--a fact known by the name of Bode's law. When the new planet, Ura.n.u.s, was found to obey this law, the feeling was so strong that there must be something to represent this missing term, that strong efforts were made to discover it, which led to success, and several, whose names are derived from the minor G.o.ds and G.o.ddesses, are now well known.

All these planets, like the signs of the zodiac, are indicated by astronomers by certain symbols, which, as they derive their form from the names or nature of the planets, may properly here be explained. The sign of Neptune is [symbol: neptune], representing the trident of the sea; for Ura.n.u.s [symbol: ura.n.u.s], which is the first letter of Herschel with a little globe below; [symbol: saturn] is the sickle of time, or Saturn; [symbol: jupiter] is the representation of the first letter of Zeus or Jupiter; [symbol: mars] is the lance and buckler of Mars; [symbol: venus] the mirror of Venus; [symbol: mercury] the wand of Mercury; [symbol: sun] the sun's disc; and [symbol: moon] the crescent of the moon.

[Ill.u.s.tration: PLATE IX.--THE SOLAR SYSTEM.]

The more modern discoveries have, of course, been all made by means of the telescope, and a few words on the history of its discovery may fitly close this chapter.

According to Olbers, a concave and convex lens were first used in combination, to render objects less distant in appearance, in the year 1606. In that year the children of one Jean Lippershey, an optician of Middelburg, in Zealand, were playing with his lenses, and happened to hold one before the other to look at a distant clock. Their great surprise in seeing how near it seemed attracted their father's attention, and he made several experiments with them, at last fixing them as in the modern telescope--in draw tubes. On the 2nd of October, 1606, he made a pet.i.tion to the States-General of Holland for a patent.

The aldermen, however, saw no advantage in it, as you could only look with one eye instead of two. They refused the patent, and though the discovery was soon found of value, Lippershey reaped no benefit.

Galileo was the first to apply the telescope to astronomical observations. He did not have it made in Holland, but constructed it himself on Lippershey's principle. This was in 1609. Its magnifying power was at first 4, and he afterwards increased it to 7, and then to 30. With this he discovered the phases of Venus, the spots on the sun, the four satellites of Jupiter, and the mountains of the moon.

[Ill.u.s.tration: PLATE X.--THE DISCOVERY OF THE TELESCOPE.]

Kepler, in 1611, made the first astronomical telescope with two concave gla.s.ses.

Huyghens increased the magnifying power successively to 48, 50, and 92, and discovered Saturn's ring and his satellite No. 4.

Ca.s.sini, the first director of the Paris Observatory, brought it to 150, aided by Auzout Campani of Rome, and Rives of London. He observed the rotation of Jupiter (1665), that of Venus and Mars (1666), the fifth and third satellites of Saturn (1671), and afterwards the two nearer ones (1684); the other satellites of this planet were discovered, the sixth and seventh, by Sir William Herschel (1789), and the eighth by Bond and Lasel (1848).

We may add here that the satellites of Ura.n.u.s were discovered, six by Herschel from 1790 to 1794, and two by La.s.sel in 1851, the latter also discovering Neptune's satellite in 1847.

The rotation of Saturn was discovered by Herschel in 1789, and that of Mercury by Schroeter in 1800.

The earliest telescopes which were reflectors were made by Gregory in 1663 and Newton in 1672. The greatest instruments of our century are that of Herschel, which magnifies 3,000 times, and Lord Rosse's, magnifying 6,000 times, the Foucault telescope at Ma.r.s.eilles, of 4,000, the reflector at Melbourne, of 7,000, and the Newall refractor.

[Ill.u.s.tration: PLATE XI.--THE FOUNDATION OF PARIS OBSERVATORY.]

The exact knowledge of the heavens, which makes so grand a feature in modern science, is due, however, not only to the existence of instruments, but also to the establishment of observatories especially devoted to their use. The first astronomical observatory that was constructed was that at Paris. In 1667 Colbert submitted the designs of it to Louis XIV., and four years afterwards it was completed. The Greenwich Observatory was established in 1676, that of Berlin in 1710, and that of St. Petersburg in 1725. Since then numerous others have been erected, private as well as public, in all parts of the world, and no night pa.s.ses without numerous observations being taken as part of the ordinary duty of the astronomers attached to them.

CHAPTER IX.

THE TERRESTRIAL WORLD OF THE ANCIENTS.--COSMOGRAPHY AND GEOGRAPHY.

With respect to the shape and position of the earth itself in the material universe, and the question of its motion or immobility, we cannot go so far back as in the case of the heavens, since it obviously requires more observation, and is not so pressing for an answer.

Amongst the Greeks several authors appear to have undertaken the subject, but only one complete work has come down to us which undertakes it directly. This is a work attributed to Aristotle, _De Mundo_. It is addressed to Alexander, and by some is considered to be spurious, because it lacks the majestic obscurity that in his acknowledged works repels the reader. Although, however, it is not as obscure as it might be, for the writer, it is quite bad enough, and its dryness and vagueness, its mixture of metaphysical and physical reasoning, logic and observation, and the change that has naturally pa.s.sed over the meanings of many common words since they were written, render it very tedious and unpleasant reading.