Every one who notices the stars at all,—and who that thinks and can see does not?—must have observed during the autumn of 1877 two bright stars in the southern heavens. One of these shone with a lustre which but for its ruddy hue would have caused the star to be taken for the planet Jupiter; the other shone with a somewhat yellowish light, and was much fainter, though surpassing most of the fixed stars in brightness. The former was the planet Mars, the latter the ringed planet Saturn. The motions of these two stars with respect to each other and to the neighbouring stars were sufficiently conspicuous to attract attention. During October these stars attracted still more attention, because they drew nearer and nearer together, to all appearance, until on November 4th they were at their nearest, when the distance separating them was about one-third the apparent diameter of the moon, so that in a telescope showing at one view the whole disc of the moon, Mars and Saturn on the night of November 4th appeared like a splendid double star, the primary a fine red orb, the companion a smaller body, but attended by a splendid ring system and companion moons.

It was strange when we looked at these two stars, the yellow one apparently much smaller than the brighter, and the pair seemingly close together, to consider how thoroughly the reality differed from these appearances. The fainter and seemingly the smaller of the two stars was in reality some four thousand times larger than the brighter, and had, among eight orbs attending upon it, one nearly as large as the ruddy planet which as actually seen so completely outshone Saturn himself. Again, instead of being near each other, those two bodies were in reality separated by a distance exceeding some sixteen times that which separated us from the nearer of the two.

I propose now to consider some of the more interesting characteristics of these two planets, presenting specially those features which mark Saturn as the representative of one family of bodies, and Mars as the representative of another and an entirely different family.
Fig. 16.—The paths of Mars and Saturn during the autumn of 1877.

It will be well to consider Mars first; for although, as will presently be seen, Saturn came earlier of the two to the portion of his path where he was most favourably seen, there was nothing specially remarkable about the approach of Saturn on that occasion, whereas Mars in the year 1877 made a nearer approach to the earth than he has for thirty-two years past, or will for some forty-seven years to come.

In the first place, let us note the apparent paths on which the two planets have been and are now travelling.

Fig. 16 presents that part of the zodiac along which lay the apparent paths of Mars and Saturn in 1877. The stars marked with Greek letters belong to the constellation Aquarius, or the Water-Bearer (his jar is formed by the stars in the upper right-hand corner of the picture),—with a single exception, the star marked κ, which, with those close to it not lettered, belongs to the constellation Pisces, or the Fishes. Thus the loops traversed by the two planets in 1877 both fell in the constellation of the Water-Bearer; but, as will be seen from the symbols on the ecliptic, these loops lie in the zodiacal sign Pisces, which begins at κ and ends at γ. The signs have long since passed away, in fact, from the constellations to which they originally belonged.

It will be noticed that Mars described a wide loop ranging to a considerable distance from the ecliptic (or sun\'s track). Saturn, on the other hand, travelled on a narrow and shorter loop lying much nearer to the ecliptic, his whole track, except just where he was turning,—his stationary points,—lying nearly parallel to the ecliptic. It may be well to mention the reason of this well-marked difference. Mars does not in reality range even quite so widely from the plane of the ecliptic as Saturn does. Nay, his path is even less inclined to the ecliptic. (This may sound like repetition, but the inclination of a planet\'s path to the ecliptic is one thing, the range of the planet north and south of the ecliptic, in miles, is another. Mercury, for example, has of all planets the path most inclined to the ecliptic, but Mercury never attains anything like the same distance from the plane of the ecliptic which is attained by the remote planet Uranus, whose path is of all others the least inclined to the plane of the ecliptic. In fact, none of the planets, except Venus and Mars, have so small a range from the ecliptic in actual distance as Mercury has.) The reason why the range of Mars from the ecliptic appeared so much greater than that of Saturn, in 1877, is similar to the reason why Mars, though much smaller than Saturn, largely outshone him. Mars looked larger because he was nearer, his loop looked larger because his real path was nearer. For the same reason that a hut close by seems to stand higher above the horizon than a palace at a distance, or a mountain yet further away, so the displacement of Mars from the ecliptic plane appeared greater than that of Saturn, though in reality much less.

Let us consider how the paths of these planets are really situated. I know of no better way of showing this than by drawing the paths of the two families of planets separately. It is in fact utterly impossible to give an accurate yet clear view of the solar system in a single picture; and the student may take it for granted that every drawing or plate in which this has ever been attempted is from one cause or another misleading.

In figs. 17 and 18 the shape and position of the planetary paths are correctly shown. Very little description is necessary, but it may be mentioned that on each orbit the point nearest to the sun is indicated by the initial letter of the planet, while the point farthest from the sun is indicated by the same letter accented. The places where each path crosses the plane of the earth\'s—which is supposed to be the plane of the paper—are marked ? and ?, the former sign marking where the planet in travelling round in the direction shown by the arrows crosses the plane of the earth\'s path from below upwards, while the latter marks the place where the planet in travelling round crosses the plane of the earth\'s path from above downwards.
Fig. 17.—The paths of Mercury, Venus, the Earth, and Mars, around the Sun.

Fig. 17 shows the paths of the inner family of planets of which our earth is a member. Fig. 18 shows the outer family of planets, and inside of it the ring of small planets called asteroids. Inside that ring, again, we see the paths of the inner family of planets; but they appear on a very small scale indeed. In fact, the scales appended to the two figures show that a length which represents 50,000,000 miles in fig. 17, represents 1,000,000,000 miles in fig. 18; or, in other words, the scale of fig. 18 is only one-twentieth of the scale of fig. 17. On the scale of fig. 17 the sun would be fairly represented by an ordinary pin-hole; on the scale of fig. 18 the sun would be scarcely visible. The dots round the orbits show the planets\' places at intervals of 10 days in fig. 17, and of 1000 days in fig. 18, starting always from the left side of orbit (on horizontal line through sun).
Fig. 18.—The paths of Jupiter, Saturn, Uranus, and Neptune, around the ring of small planets.

Now looking at fig. 18 and noting how small is the distance of the path of Mars from the earth\'s path, compared with the distance of Saturn\'s path, we understand why Saturn, despite his far superior size, shines far less brightly in our skies than Mars does. In fact, in October, 1877, the Earth and Mars were on the parts of their tracks which lay nearest together, that is, the parts occupying the lower right-hand corner of fig. 17; and turning to fig. 18, we perceive that the distance separating the two paths here is very small indeed compared with Saturn\'s distance.

So that, when we looked at Mars and Saturn as they shone in conjoined splendour in our skies, in 1877, we saw in the bright orb of Mars the planet whose track lies nearest to us in that direction, whereas in looking at Saturn the range of view passed athwart the track of Mars, through the ring of asteroids, and past the orbit of Jupiter, before entering the wide and barren region which separates the orbits of the two giant members of the solar system.

We study Mars under much more favourable conditions than either Jupiter or Saturn. And yet, at a first view, the telescopic aspect of this interesting planet is exceedingly disappointing. Galileo, who quite easily discovered the moons of Jupiter with his largest telescope, could barely detect with it the fact that Mars is not quite round at all times, but is seen sometimes in the shape of the moon two or three days before or after full. "I dare not affirm," he wrote on December 30, 1610, to his friend Castelli, "that I can observe the phases of Mars; yet, unless I mistake, I think I already perceive that he is not perfectly round." But even in a large telescope one can see very little except under very favourable conditions. It has only been by long and careful study, and piecing together the information obtained at various times, that astronomers have obtained a knowledge of the facts which appear in our text-books of astronomy. The possessor of a telescope who should expect, on turning the instrument towards Mars, to perceive what he has read in descriptions of the planet, would be considerably disappointed.

First noticed among the features of the planet were two white spots of light occupying the northern and southern parts of his disc. These are now known to be regions of snow and ice, like those which surround the poles of our own earth. But how different the reality must be from what we seem to see in the telescope! These two tiny white specks represent hundreds of thousands of square miles covered over with great masses of snow and ice, which doubtless are moved by disturbing forces similar to those which make our arctic regions for the most part impassable even for the most daring of our seamen.

The snow-caps of Mars change in size as the planet circuits round the sun, completing his year of seasons (which lasts 687 of our days). They are largest in the winter of Mars, smallest in the Martian summer; so that, as it is winter for one hemisphere when it is summer for the other, one of the snow-caps is larger than the other at the winter and summer seasons. In the same way, our arctic snows extend more widely during our winter, while the antarctic snows then retreat; whereas, during our summer, when it is winter in the southern hemisphere, the antarctic snows advance and our arctic snows retreat.

But we have still to learn why these white spots are known to be masses of snow. They might well from analogy be considered to be snows, since they behave like the snows of our polar regions. Yet that would be very different from proving them to be snow masses. I shall now show how this has been done, and afterwards describe the lands and seas of the planet, and give a short account of the recent interesting discovery of two moons attending on the planet which Tennyson had called the "moonless Mars."

Even before the poles of Mars had been discovered, observers had perceived that the planet has marks upon its surface. Cassini, in 1666, at Paris, found by observing these spots that the planet turns on its axis once in about twenty-four hours forty minutes. In the same year Dr. Hooke observed Mars. He was in doubt whether the planet turned once round or twice round in about twenty-four hours; for with his imperfect telescope two opposite faces of the planet seemed so much alike that he was doubtful whether they really were two different faces or the same. Fortunately he published two pictures of the planet, taken on the same night in March, 1666, and we have been able to keep such good count of Mars\'s turning on his axis, that we know exactly how many times he has turned since that distant time. However, at present, we need not further consider the turning motion of Mars, but rather what the telescope has shown us about him. Only, let it be remembered that he has a day of about twenty-four hours thirty-seven minutes, and is in this respect much like our earth.

Maraldi, Cassini\'s nephew, early in the last century observed several spots on Mars, and, in particular, one somewhat triangular dark spot, which was one of Hooke\'s markings, but more clearly seen by Maraldi. About this time it was seen that the darker markings have a somewhat greenish colour; and towards the end of last century, or, more exactly, about a hundred years ago, the idea was maintained by Sir W. Herschel that the dark-greenish markings are seas, while the lighter parts of Mars, to which the planet owes its somewhat ruddy colour, are lands. Sir W. Herschel also was the first to show that Mars, like our earth, has seasons. It had been supposed by Cassini, Maraldi, and others, that the axis of Mars is upright to the level of the path in which he travels. Of course, if this were so, the light of the sun would always fall on the planet in the same way; for the sun is in that level. But the axis, like that of our own earth, is bowed considerably from uprightness; so that at one part of his year the sun\'s rays fall more fully on his northern regions, and his southern regions are correspondingly turned away from the sun; then it is summer in his northern regions, winter in his southern. At the opposite season the reverse holds, and then winter prevails over his northern and summer over his southern regions. Midway between these two seasons, the sun\'s rays are equably distributed over both hemispheres of Mars, and then the days and nights are equal, and it is spring in that hemisphere which is passing from winter to summer, and autumn in the other hemisphere which is passing from summer to winter. All these changes are precisely like those which take place in the case of our own earth. Only, the year of Mars, and therefore his seasons, are longer. He takes 687 days in travelling round the sun, giving nearly 172 days, or more than five and a half of our months, for each season.
Fig. 19. Fig. 20. Fig. 21.

Figs. 19-21.—Three Views of Mars.

Figs. 19, 20, and 21 are three views of Mars, drawn by Mr. Nathaniel Green, an excellent observer, who has paid special attention to this planet. Fig. 19 shows a faintly-marked sea running north and south (the upper part of the picture being the south, because that is the way in which the telescope used by astronomers inverts objects.) This is one of the markings which deceived Hooke. This picture was drawn on May 30, 1873, at half-past seven in the evening. The second picture was drawn two days earlier, at eight in the evening; but it shows the planet as it would have looked on May 30 at about a quarter past nine in the evening, by which time the sea running north and south had been carried over to the right and lost to view. But another north and south sea had come into view on the right. The third picture shows a view taken three hours later, or at eleven on May 28, when the planet appeared precisely as he would have appeared at a quarter past eleven in the early morning of May 31, had weather then permitted Mr. Green to continue his observations. You see in it the great north and south sea which Maraldi had noticed, the other of those two which had deceived Hooke.

It will be seen from these drawings, which, be it remembered, were taken at the telescope, that it is possible from a great number of such drawings to make a chart of Mars, showing its lands and seas not as they are seen in the telescope, but as they might be laid down by inhabitants of Mars in a map or planisphere. This has been done, with gradually increasing accuracy,—first by Sir W. Herschel, next by Beer and M?dler, then by Phillips, and lastly by myself. (In claiming for my own chart greater accuracy, I am simply asserting the superior completeness of the list of telescopic drawings which I was able to consult.) The result is shown in the accompanying chart (fig. 22), which presents the whole surface of Mars divided into lands and seas and polar snows, with the names attached of various observers who have at sundry times contributed to our knowledge of the planet\'s features.

But now it will be asked by the thoughtful reader, how can any one possibly be sure that the regions called continents and seas do really consist of land and water? At any rate, the doubt might well be entertained respecting the water. For land is a wide term, including all kinds of rock surface, sand, earthy soil, and so forth; but it may seem to require proof that the substance we call water really exists out yonder in space, either in the form of snow and ice at the Martian poles, or as flowing water in the Martian seas, or in the vaporous form in the planet\'s air.

Fig. 22.—Chart of Mars, from 27 drawings by Mr. Dawes.


Very strange, then, at first must the statement seem, that we are as sure of the existence of water in all these forms on Mars as if we had sent some messenger to the planet who had brought back for study by our chemists a block of Martian ice, a vessel full of Martian water, and a flask of Martian air saturated with aqueous vapour. Indeed, I do not know of any discovery effected by man which more strikingly displays the power of human ingenuity in mastering difficulties which, at a first view, seem altogether insuperable. When we know that a mass of ice as large as Great Britain would appear at the distance of Mars a mere bright point; that a sea as large as the Mediterranean would appear like a faint, greenish-blue, streak; and that cloud masses such as would cover the whole of Europe would only present the appearance of a whitish glare, how hopeless seems the task of attempting to determine what is the real chemical constitution of objects thus seen! It might well be thought that no possible explanation of the method used by astronomers could serve to establish its validity. Yet nothing can be simpler than the principle of the method, or more satisfactory than its application in this special case.

First, let the reader rid his mind of the difficulty arising from the enormous distance of the celestial bodies. To do this let him note that there are some things which a body close by can tell us no more certainly than a remote body. For instance, we are just as certain that Mars is a body capable of reflecting sunlight as we are that a cricket-ball is. We know as certainly, too, that the quality of Mars is such that more of the red of the sun\'s light is sent to us than of the other colours. For we perceive that Mars is a ruddy planet. Since distance in no way interferes with our perception of these general facts, and others like them, we need not necessarily find in mere distance any difficulty in the way of recognising some other facts. All that we require to be shown before admitting the validity of the evidence is, that it is of such a kind that distance does not affect its quality, however much distance may and must affect the quantity of evidence.

Now there is a means of taking the light which comes from a body shining either with its own or with reflected light, and analyzing it into its component colours. The spectroscope is the instrument by which this is accomplished. I do not propose to describe here the nature of this instrument, or the details of the various methods in which it is employed. I note only that it separates the rays of different colour coming from an object, and lays them side by side for us,—the red rays by themselves, the orange rays by themselves, and so with the yellow, green, blue, indigo, and violet. And not only are the rays of these colours set by themselves, but the red rays are sorted in order, from the deepest brown-red[11] to a tint of red (the lightest) which must almost be called orange; the orange in order, from orange which must almost be called red to a tint (the lightest orange) which must almost be called yellow; the yellow, from an almost orange yellow to a yellow just beginning to be tinged with green; the green, from an almost yellow green (the lightest) to a green which may almost be called blue (the darkest); the blue, from this tint to the beginning of the indigo; the indigo, from this tint to the first rays of the violet; and lastly the violet, through all the tints of this beautiful colour to a blackish-brown violet, where the visible spectrum ends. All these tints are sorted in order by the spectroscope, just as a skilful colourist might range in due sequence a myriad tints of colour. But this is only true of really white light, such light as comes from a glowing mass of metal burning at a white heat. In other cases (even when the light may seem white to the eye) some of the tints are found, when the spectroscope spreads out the colours for us, to be missing. And we know that this may be caused in two ways. Either the source of light never gave out those missing tints; or, the source of light gave them out, but some absorbing medium stopped them on their way before they reached the spectroscope with which we examine them. There may be cases where we cannot tell very easily which of these is the true cause. But sometimes we can, as the instances I have now to deal with will show you.

The sun\'s own light shows under this method of spectroscopic analysis millions of tints, in fact I might say millions of red tints, and so forth, right through the spectral list of colours. But also many thousands of tints are wanting. Imagine a rainbow-coloured ribbon, the colours ranged along its length, so that the ribbon is black at both ends, and that from the black of one end the colour merges into very deep red, and thence by insensible gradations through orange, yellow, green, blue, indigo, and violet, into the black of the other end. Then suppose that tens of thousands of the fine threads which run athwart the ribbon—i.e., the short cross threads—are drawn out. Then the ribbon, laid on a dark background showing through the spaces where the threads were drawn out, would represent the solar spectrum. We know then that the light of the sun\'s glowing mass either wants particular tints originally, or shines through vapours which prevent the free passage of rays of those colours. Both causes might be at work, not one only. At present we are not concerned with this particula............