Chapter XIII: Transits of Venus 273 (4)

Now, it is clear that when the moon is at M_{1}, her illuminated face is turned from the earth, E. She therefore cannot be seen; and accordingly, in fig. 12, she is presented as a black disc at 1 to correspond with her invisibility when she is as at M_{1}. She passes on to M_{2}; and now from E a part of her illuminated half can be seen towards the sun, which would be towards the right, if we imagine an eye at E looking towards M_{2}. Her appearance then is as shown at 2, fig. 13. In any intermediate portion between M_{1} and M_{2}, the sickle of light is visible but narrower. We see also that all this time the moon's place on the sky cannot be far from the sun's place, for the line from E to M_{2} is not greatly inclined to the line from E to S. When the moon has got round to M_{3}, the observer on the earth sees as much of the dark half as of the bright half of the moon, the bright half being seen, of course, towards the sun. Thus the moon appears as at 3, fig. 13, Again as to position, the moon is now a quarter of a circuit of the heavens from the sun, for the line from E to M_{3} is square to the line from E to S. We see similarly that when at M_{4} the moon appears as shown at 4, fig. 13, for now the observer at E sees as small a part of the moon's dark side as he had seen of her bright side when she was at M_{2}. When she is at M_{5} the observer at E sees her bright face only, the dark face being turned directly from him. She, therefore, appears as at 5, fig. 13. Also being now exactly opposite the sun, as we see from fig. 12, she is at her highest when the sun is at his lowest, or at midnight; and at this time she rules the night as the sun rules the day.[10] As the moon passes on to M_{6}, a portion of her dark half comes into view, the bright side being now towards the left, as we look at M_{5} from E, fig. 12. Her appearance, therefore, is as shown at 5. When at M_{7} she is seen as at 7, half-bright and half-dark, as when she was at M_{3}, but the halves interchanged. At M_{8} she appears as at 8, and, lastly, at M_{1} she is again undiscernible.

The ancient Chaldean astronomers could have little doubt as to the validity of this explanation. In fact, while it is the explanation obviously suggested by observed facts, one cannot see how any other could have occurred to them.

But if they had had any doubts for a while, the occurrence of eclipses would soon have removed those doubts. They must early have noticed that at times the full moon became first partly obscured, then either wholly disappeared or changed in colour to a deep coppery red, and after a while reappeared. Sometimes the darkening was less complete, so that at the time of greatest darkness a portion of the moon seemed eaten out, though not by a well defined or black shadow. These phenomena, they would find, occurred only at the time of full moon. And if they were closely observant, they would find that these eclipses of the moon only occurred when the full moon was on or near the great circle round the stellar heavens, which they had learned to be the sun's track. They could hardly fail to infer that these darkenings of the moon were caused by the earth's shadow, near which the moon must always pass when she is full, and through which she must sometimes pass more or less fully; in fact, whenever, at the time of full, she is on or near the plane in which the earth travels round the sun. Solar eclipses would probably be observed later. For though a total eclipse of the sun is a much more striking phenomenon than a total eclipse of the moon, yet the latter are far more common. A partial eclipse of the sun may readily pass unnoticed, unless the sun's rays are so mitigated by haze or mist that it is possible to look at his disc without pain. Whenever solar eclipses came to be noted, and we know from the Chaldean discovery of the great eclipse period, called the _Saros_, that they were observed at least two thousand years before the Christian era, the fact that the moon is an opaque body circling round the earth, and much nearer to the earth than the sun is, must be regarded as demonstrated. Not only would eclipses of the sun be observed to occur only when the moon was passing between the earth and the sun, but in an eclipse of the sun, whether total or partial, the round black body cutting off the sun's light wholly or partially would be seen to have the familiar dimensions of the lunar orb.

Leaving solar and lunar eclipses for description on another occasion, I will now proceed to consider a peculiarity of moonlight which must very early have attracted attention,--I mean the phenomenon called the harvest-moon.

The moon circuits the heavens in a path but slightly inclined to that of the sun, called the ecliptic, and for our present purpose we may speak of the moon as travelling in the ecliptic. Now we know that during the winter half of the year the sun is south of the equator, the circle of the heavenly sphere which passes through the east and west points of the horizon, and has its plane square to the polar axis of the heavens. During the other or summer half of the year he is north of the equator. In the former case the sun is above the horizon less than half the twenty-four hours, day being so much shorter as the sun is farther south of the equator; whereas in the latter case the sun is above the horizon more than twelve hours, day being so much the longer as the sun is farther north of the equator. Precisely similar changes affect the moon, only, instead of taking place in a year (the time in which the sun circuits the stellar heavens), they occur in what is called a sidereal month, the time in which the moon completes her circuit of the stellar heavens. For about a fortnight the moon is above the horizon longer than she is below the horizon, while during the next fortnight she is below the horizon longer than she is above the horizon. Now clearly when the length of what we may call the moon's diurnal path (meaning her path above the horizon) is lengthening most, the time of her rising on successive nights must change least. She comes to the south later and later each successive night by about 50½ minutes, because she is always travelling towards the east at such a rate as to complete one circuit in about four weeks; and losing thus one day in 28, she losses about 50½ minutes per day. If the interval between her rising and her arriving to the south were always the same, she would rise 50½ minutes later night after night. But if the interval is lengthening, say by 10 minutes per night, she would of course rise only 40½ minutes later: if the interval _is_ lengthening 20 minutes per night, she would rise only 30½ minutes later, and so forth. But the lunar diurnal arc is lengthening all the time she is passing from her position farthest south of the equator to her position farthest north, just in the same way as the solar day is lengthening from mid-winter to midsummer, only to a much greater degree. And as the solar day lengthens fastest at spring when the sun crosses the equator from south to north, so the time the moon is above the horizon lengthens most, day by day, when the moon is crossing the equator from south to north. It lengthens, _then_, from an hour to an hour and 20 minutes in one day, that is, the interval between moon-rise and moon-setting increases from 30 to 40 minutes. At this time, then, whenever it happens in each lunar month, the moon's time of rising changes least: instead of the moon rising night after night 50½ minutes later, the actual difference varies only from 10 to 20 minutes.

Now if this happens at a time when the moon is not nearly full, it is not specially noticed, because the moon's light is not then specially useful. But if it happens when the moon is nearly full, it is noticed, because her light is then so useful. A moon nearly full, afterwards quite full, and then for a day or two still nearly full, rising night after night at nearly the same time, remaining also night after night longer above the horizon, manifestly serves man for the time being in the most convenient way possible. But it is clear that as the full moon is opposite the sun, and as to fulfil the condition described we have seen that she must be crossing the equator from south to north, the sun, opposite to her, must be at the part of his path where he crosses the equator from north to south. In other words, the time of year must be the autumnal equinox. Thus the moon which comes to "full" nearest to September 22 or 23 will behave in the convenient way described. At this time, moreover, when she rises night after night nearly at the same time, the nights are lengthening fastest while the time the moon is above the horizon is lengthening still more, and therefore, in all respects, the moon is then doing her best, so to speak, to illuminate the nights. At this season the moon is called the harvest-moon, from the assistance she sometimes renders to harvesters.

The moon which is full nearest to September 22-23 may precede or follow that date. In the former case only can it properly be called a harvest-moon. In the latter it is sometimes called the hunter's moon. The full moon occurring nearest to harvest time will always partake more or less of the qualities of a full moon occurring at the autumnal equinox: and similarly of a full moon following the autumnal equinox. So that, in almost every year, there may be said to be a harvest-moon and a hunter's moon. But, of course, it will very often happen that in any particular agricultural district the harvest has to be gathered in during the wrong half of the lunar month, that is, during the last and first, instead of the second and third quarters.

The reader must not fall into the mistake of supposing, as I have seen sometimes stated in text-books of astronomy, that we are more favoured in this respect than the inhabitants of the southern hemisphere. It is quite true that the same full moon shines on us as on our friends in New Zealand, Australia, and Cape Colony, and also that our autumn is their spring, and their spring our autumn. But the full moon we have in autumn behaves in the southern hemisphere not as with us, but as our spring full moon behaves; and the full moon of our spring, which is their autumn, behaves with them as our autumn moon behaves with us. It is, therefore, for them a harvest-moon if it occur before the equinox, and a hunter's moon if it occur after the equinox. A very little consideration will show why this is. In fact if, in the explanation given above, the words north and south be interchanged, and March 21-22 written for September 22-23, the explanation will be precisely that which I should have given respecting the harvest (or March) moon of the southern hemisphere, if I had been writing for southern readers.

* * * * *

Having thus considered the moon as a light-giver, both in respect of her monthly changes and of that yearly change which causes her services to be most useful in harvest time, let us consider what science tells us of the orb which thus usefully reflects to us the solar rays.

The moon is a globe about 2159½ miles in diameter, travelling round the earth at a mean distance of 238,818 miles. Her path round the earth is not, however, a circle, but an ellipse, which itself is constantly varying in shape. The average eccentricity of the moon's path is such that her greatest and least distances, as she circuits round it, are 251,953 miles and 225,683 miles respectively; but when it is most eccentric, her greatest and least distances are 252,948 miles and 221,593 miles respectively; while, when it is least eccentric, they are respectively 250,324 miles and 227,312 miles. The earth's surface exceeds the moon's nearly 13½ times, the actual number of square miles in the moon's surface amounting to 14,600,000. This is nearly equal to Europe and Africa together, or, more nearly still, to North and South America together, without their islands. In volume our earth exceeds the moon rather more than 49¼ times: or, more nearly, if the earth's volume be represented by 10,000, the moon's will be represented by 209. The materials of the moon's globe are either lighter or (more probably) they are less closely compacted than those forming our earth,--for, according to the best modern estimates, the earth exceeds the moon in mass nearly 81½ times. Assuming as the most probable value of the earth's mean density about 5-7/10 times the density of water, the moon's mean density is equal to 3-46/100 times that of water. Gravity at her surface is accordingly much less than at the surface of the earth; a quantity of matter weighing six pounds at the surface of the earth would weigh almost exactly one pound at the surface of the moon.

The moon circuits once round the earth in 27d. 7h. 43m. 11.5s. This is the time in which, viewed from the earth, she seems to complete one circuit round the stellar heavens, and is therefore called a sidereal month. But as the earth is all the time travelling the same way round the sun, the lunar month is longer. Thus, suppose S (fig. 14) to be the sun, E the earth at the beginning of a lunar month, M_{1} M_{2} M_{3} M_{4} the moon's path, and M_{1} the moon's place on the line joining E and S. If the earth remained at rest while the moon went round the path M_{1} M_{3}, then after completing one circuit the moon would again be at M_{1} on the line joining E and S, or it would be new moon again. But the earth is moving onwards along the arc EE´ of her circuit round the sun. So that when the moon has completed one circuit she is at M_{4} (E´m_{1} drawn parallel to EM_{1}) and has still to travel some distance before she gets round to M´ on the line joining S and E´. The lunation, or interval between successive new moons, has an average duration of 29d. 12h. 44m. 38s., exceeding a sidereal month by 2d. 5h.

It would not, however, be correct to regard the earth as the true centre of the moon's motion. The moon is in reality a planet circling round the sun, but largely perturbed by the attraction of its companion planet the earth. If the moon's path in the course of a year were carefully drawn to scale, or, better, were modelled by means of a fine wire, it would scarcely be distinguishable from a similar picture or model of the earth's path round the sun. Or thus, the entire width of the moon's track is about 477,636 miles, while the diameter of the orbit along which she and the earth both travel is nearly 104,000,000 miles, or 385 times as great. If we draw then a circle 3-85/100 inches in diameter to represent the earth's path round the sun, somewhat eccentrically placed, and the circular line is 1-100th of an inch wide, the moon's track would be fairly represented by a curve touching alternately the inside and the outside edge of this circular line, at equidistant points dividing the circle into about 24¾ parts.

Regarding the moon as a planet, she may be said to have a year, and seasons, and day and night, as the earth has, but very unlike our seasons and days. Her axis is inclined only 1½ degrees from uprightness to her path, whereas our earth's axis is inclined 23½ degrees. The sun's range of mid-day altitude is in fact not quite equal to the range of our sun in mid-day height, from four days before to four days after either spring or autumn. The lunar day lasts a lunar month, daytime and night-time each lasting rather more than a fortnight. The lunar year of seasons is not, as is commonly stated, the same in length as ours. She goes round the sun in the same time, so that her sidereal year is the same as ours; but owing to the swaying round of her axis her year of seasons or tropical year is shorter. Our tropical year is also shorter than the sidereal year, but very little shorter, because the earth's axis sways round once only in 25,868 years. The moon's axis sways round once in 18⅗ years, and accordingly the year of seasons is much more effectively shortened. It lasts, in fact, only 346d. 14h. 34m. of our time; and contains only 11¾ lunar days. So that I cannot altogether agree with Sir W. Herschel's statement, that "the moon's situation with respect to the sun is much like that of our earth, and by a rotation on its axis it enjoys an agreeable variety of seasons, and of day and night."

When the moon is examined with a telescope her surface is seen to be marked by many irregularities. There are large dark regions which were formerly thought to be seas, but are now known to be land-surfaces. Some of these regions are singularly level, and have been thought to be old sea-bottoms. Mountains and mountain ranges are another important feature of the moon's surface. Some, like our Rocky Mountains and Andes, form long continuous chains; others form elevated plateaus whence ridges extend in various directions. A very striking form is that of narrow ridges little raised above the general level, but reaching over enormous areas of the moon's globe. It is a system of this kind, radiating from a great lunar crater called Tycho, which gives to small photographs of the moon the appearance of a peeled orange. They are supposed to indicate the action of tremendous forces of upheaval, in past ages, bursting open portions of the moon's crust.

But the most characteristic of all the lunar features are the crater mountains, which exist on a scale not only much larger relatively to the moon's globe than the scale on which terrestrial craters are formed, but much larger absolutely. They are also far more numerous. Some parts of the moon's surface, especially in the bright south-western quarter of her face, are literally crowded with craters of various dimensions.

There are few signs of the former emission of lava from the lunar craters. Within some of them recent changes have been suspected. A remarkable instance is that of the crater Linné, marked in Mädler's map as a deep, well-walled crater, some four miles in diameter. At present only a small crater can be seen in its place. The surrounding region is rather conspicuously bright. It is not necessary to infer that there has been any volcanic disturbance, however. Far more probably the walls have been thrown down through the long-continued action of that alternate expansion and contraction, which must affect the moon's crust as the long fortnightly day proceeds, and then the equally long lunar night.

There are many well-marked valleys on the moon, besides clefts and ravines. The features called _rilles_ are among the most perplexing objects on the moon's surface. Webb, in his charming and most useful little book, "Celestial Objects for Common Telescopes," thus describes them: "These most singular furrows pass chiefly through levels, intersect craters (proving a more recent date), reappear beyond obstructing mountains, as though carried through by a tunnel, and commence and terminate with little reference to any conspicuous feature of the neighbourhood. The idea of artificial formation is negatived by their magnitude; they have been more probably referred to cracks in a shrinking surface." Some observations would seem to show that they have been formed from rows of closely-adjacent small craters. _Faults_, also, or closed cracks where the surface is higher on one side than on the other, have been recognised from the careful study of the shadows on the moon's disc.

From measurements of the shadows of lunar mountains, it appears that their average height is about five miles. In comparing this elevation with that assigned to terrestrial mountains, it must be remembered that these are measured from the sea-level; if the average height of terrestrial mountains were determined with reference to the sea-bottom it would be far greater. Still, even taking this circumstance into account, the average height of the lunar mountains bears a far greater ratio to the diameter of the globe on which they stand than the average height of our mountains to the earth's diameter.

Several circumstances agree in showing that the moon's atmosphere must be exceedingly rare. The shadows of lunar mountains are either actually black or nearly so. When the moon hides the sun in total eclipse, no sign can be seen of any refractive effort exerted on the sun's rays. When a star is hidden (or _occulted_) by the moon, the star vanishes in an instant and reappears with equal suddenness. It is certain from these phenomena that the moon has either no air, or air exceedingly tenuous. It is equally clear that she has no water, for if she had we should undoubtedly be able to recognise the occasional formation or dissipation of mist and vapour over parts of the moon's surface. No signs of such phenomena have ever been observed. The moon is certainly at present a waterless globe, so far at least as her surface is concerned.

It has been thought that though there is no water and very little air on the side of the moon turned towards the earth, there may be both water and air on the farther unseen side. The theory has been long since given up, but the reasoning on which it depends is worth noting. Owing to the strange circumstance that the moon rotates on her axis in the same time in which she revolves round the earth, she always presents the same face towards the earth, or very nearly so. If her axis were exactly square to the path in which she circuits the earth, and if she revolved at a uniform rate, we should have exactly the same side constantly turned towards us. But as the axis is inclined about 6⅔° from uprightness to the path round the _earth_ (which, be it remembered, is not in the same plane as the path round the sun, but inclined 5° 8´ to it), the northern and southern parts of the moon are alternately swayed over by about 6⅔° into view. This apparent swaying is called a libration, and the libration just described is called the libration in latitude. Again, as the moon does not travel at a uniform rate round the earth, but faster than her mean rate when nearer to us, and slower when farther from us, she alternately gains and loses in her motion of revolution as compared with her motion of rotation, by a quantity varying between 5° and 7¾°, to which varying extent the parts east and west of her mean disc are alternately swayed into view. This is called the libration in longitude. Thus we see, beyond the edge of the _mean_ half turned towards us, a considerable fringe of the other half. If a globe, as PAP´B, fig. 15, were divided into two halves to represent the farther and nearer halves of the moon, and held so that that dividing circle were seen as PEP´ in the figure, then Ppep´P´ would represent the part brought into view at different times by the apparent swaying described above; while P_pep´_P´ would represent the parts swayed out of view. The regions thus alternately in view and out of view have their greatest breadth, not at the poles or east and west, but at mMm and m´M´m´, where the two librations act together. The narrow fringe bordering these regions is that brought into or out of view by changes in the place of the observer on earth, due to the earth's rotation. It is called the parallactic fringe, any change in the apparent position of a heavenly body, or part of one, on account of the earth's rotation, being termed _parallax_.

Lastly, let us return to the consideration of moonlight, as depending on the condition of the moon's surface, To one who observes the moon as seen on the sky, her light appears white; but it must not be supposed that she is a white body. Careful estimates of the quantity of light she reflects show that she is more nearly black than white, though in reality she is neither one nor the other. It has been said, and truly, that if the surface of the moon were covered with black velvet she would still appear white; for even black velvet reflects some light, and whatever light the moon reflected would show her by contrast with the blackness of the sky, as a luminous body or white. It follows from the observations made by Zöllner that if the moon's surface were covered with white snow she could give us about 4½ times as much light as she actually does. If she were covered with white paper she would give more than 4 times as much light as she does. If she had a surface of white sandstone her light would be nearly half as great again as it is. She gives rather more light than she would if her surface consisted entirely of weathered grey sandstone, or of clay marl, and more than twice as much light as she would give if her surface were of moist earth, or dark grey syenite. As some parts of her surface are obviously much brighter than others, we must infer that some parts shine with much more, and others with much less, brightness than weathered grey sandstone. Probably some parts are much brighter than white sandstone, and some much darker than dark grey syenite. From the degree in which her lustre changes with her changing aspect, Zöllner infers that her mountains have an average slope of about fifty-two degrees.

FOOTNOTES:

[10] It has been thought by some that, in the beginning, the moon was always opposite the sun, thus always ruling the night. Milton thus understood the account given in the first book of Genesis. For he says,--

Less bright the morn,
But opposite in levell'd west was set
His mirror, with full face, borrowing her light
From him; for other light she needed none
In that aspect; and still that distance keeps
Till night, then in the east her turn she shines,
Revolv'd on Heav'n's great axle.

It was only as a consequence of Adam's transgression that he conceives the angels sought to punish the human race by altering the movements of the celestial bodies--

To the blank moon
Her office they prescribe--

It is hardly necessary to say, perhaps, that this interpretation is not scientifically admissible.

IX.

_THE PLANET MARS._

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.

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 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).

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."

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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 Mädler, 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.

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 particular point; but I only mention that, in reality, no tints are actually wanting, though some are very much enfeebled.

The sun's light falling on any opaque object is reflected. If the object is white, the light gives exactly the same spectrum, only fainter. Thus, I take a piece of white paper on which the sun's rays are falling, and examine its light with one of Browning's spectroscopes. I get the ordinary solar spectrum. The cold white paper gives me in fact a spectrum which speaks of a heat so intense that the most stubborn metals are not merely melted but vaporized in it. But this heat resides in the sun, not in the paper.

Now, speaking generally, Mars also sends us sunlight, so that when we spread out with the spectroscope the rays coming from this planet, we get the solar spectrum, only of course very much enfeebled. _But_ close examination shows that other tints besides those missing from the solar spectrum are missing from the spectrum of Mars. He reflects to us the sunlight, almost as it reaches him, but he abstracts from it a few tints on his own account.