The Story of the Heavens Part 14

Venus and Mars have, from one point of view, quite peculiar claims on our attention. They are our nearest planetary neighbours, on either side. We may naturally expect to learn more of them than of the other planets farther off. In the case of Venus, however, this antic.i.p.ation can hardly be realised, for, as we have already pointed out, its dense atmosphere prevents us from making a satisfactory telescopic examination. When we turn to our other planetary neighbour, Mars, we are enabled to learn a good deal with regard to his appearance. Indeed, with the exception of the moon, we are better acquainted with the details of the surface of Mars than with those of any other celestial body.

This beautiful planet offers many features for consideration besides those presented by its physical structure. The orbit of Mars is one of remarkable proportions, and it was by the observations of this...o...b..t that the celebrated laws of Kepler were discovered. During the occasional approaches of Mars to the earth it has been possible to measure its distance with accuracy, and thus another method of finding the sun's distance has arisen which, to say the least, may compete in precision with that afforded by the transit of Venus. It must also be observed that the greatest achievement in pure telescopic research which this century has witnessed was that of the discovery of the satellites of Mars.

To the unaided eye this planet generally appears like a star of the first magnitude. It is usually to be distinguished by its ruddy colour, but the beginner in astronomy cannot rely on its colour only for the identification of Mars. There are several stars nearly, if not quite, as ruddy as this globe. The bright star Aldebaran, the brightest star in the constellation of the Bull, has often been mistaken for the planet.

It often resembles Betelgeuze, a brilliant point in the constellation of Orion. Mistakes of this kind will be impossible if the learner has first studied the princ.i.p.al constellations and the more brilliant stars. He will then find great interest in tracing out the positions of the planets, and in watching their ceaseless movements.

[Ill.u.s.tration: Fig. 48.--The Orbits of the Earth and of Mars, showing the Favourable Opposition of 1877.]

The position of each orb can always be ascertained from the almanac.

Sometimes the planet will be too near the sun to be visible. It will rise with the sun and set with the sun, and consequently will not be above the horizon during the night. The best time for seeing one of the planets situated like Mars will be during what is called its opposition.

This state of things occurs when the earth intervenes directly between the planet and the sun. In this case, the distance from Mars to the earth is less than at any other time. There is also another advantage in viewing Mars during opposition. The planet is then at one side of the earth and the sun at the opposite side, so that when Mars is high in the heavens the sun is directly beneath the earth; in other words, the planet is then at its greatest elevation above the horizon at midnight.

Some oppositions of Mars are, however, much more favourable than others.

This is distinctly shown in Fig. 48, which represents the orbit of Mars and the orbit of the Earth accurately drawn to scale. It will be seen that while the orbit of the earth is very nearly circular, the orbit of Mars has a very decided degree of eccentricity; indeed, with the exception of the orbit of Mercury, that of Mars has the greatest eccentricity of any orbit of the larger planets in our system.

The value of an opposition of Mars for telescopic purposes will vary greatly according to circ.u.mstances. The favourable oppositions will be those which occur as near as possible to the 26th of August. The other extreme will be found in an opposition which occurs near the 22nd of February. In the latter case the distance between the planet and the earth is nearly twice as great as the former. The last opposition which was suitable for the highest cla.s.s of work took place in the year 1877.

Mars was then a magnificent object, and received much, and deserved, attention. The favourable oppositions follow each other at somewhat irregular intervals; the last occurred in the year 1892, and another will take place in the year 1909.

The apparent movements of Mars are by no means simple. We can imagine the embarra.s.sment of the early astronomer who first undertook the task of attempting to decipher these movements. The planet is seen to be a brilliant and conspicuous object. It attracts the astronomer's attention; he looks carefully, and he sees how it lies among the constellations with which he is familiar. A few nights later he observes the same body again; but is it exactly in the same place? He thinks not.

He notes more carefully than before the place of the planet. He sees how it is situated with regard to the stars. Again, in a few days, his observations are repeated. There is no longer a trace of doubt about the matter--Mars has decidedly changed his position. It is veritably a wanderer.

Night after night the primitive astronomer is at his post. He notes the changes of Mars. He sees that it is now moving even more rapidly than it was at first. Is it going to complete the circuit of the heavens? The astronomer determines to watch the orb and see whether this surmise is justified. He pursues his task night after night, and at length he begins to think that the body is not moving quite so rapidly as at first. A few nights more, and he is sure of the fact: the planet is moving more slowly. Again a few nights more, and he begins to surmise that the motion may cease; after a short time the motion does cease, and the object seems to rest; but is it going to remain at rest for ever?

Has its long journey been finished? For many nights this seems to be the case, but at length the astronomer suspects that the planet must be commencing to move backwards. A few nights more, and the fact is confirmed beyond possibility of doubt, and the extraordinary discovery of the direct and the retrograde movement of Mars has been accomplished.

[Ill.u.s.tration: Fig. 49.--The Apparent Movements of Mars In 1877.]

In the greater part of its journey around the heavens Mars seems to move steadily from the west to the east. It moves backwards, in fact, as the moon moves and as the sun moves. It is only during a comparatively small part of its path that those elaborate movements are accomplished which presented such an enigma to the primitive observer. We show in the adjoining picture (Fig. 49) the track of the actual journey which Mars accomplished in the opposition of 1877. The figure only shows that part of its path which presents the anomalous features; the rest of the orbit is pursued, not indeed with uniform velocity, but with unaltered direction.

This complexity of the apparent movements of Mars seems at first sight fatal to the acceptance of any simple and elementary explanation of the planetary motion. If the motion of Mars were purely elliptic, how, it may well be said, could it perform this extraordinary evolution? The elucidation is to be found in the fact that the earth on which we stand is itself in motion. Even if Mars were at rest, the fact that the earth moves would make the planet appear to move. The apparent movements of Mars are thus combined with the real movements. This circ.u.mstance will not embarra.s.s the geometer. He is able to disentangle the true movement of the planet from its a.s.sociation with the apparent movement, and to account completely for the complicated evolutions exhibited by Mars.

Could we transfer our point of view from the ever-s.h.i.+fting earth to an immovable standpoint, we should then see that the shape of the orbit of Mars was an ellipse, described around the sun in conformity with the laws which Kepler discovered by observations of this planet.

Mars takes 687 days to travel round the sun, its average distance from that body being 141,500,000 miles. Under the most favourable circ.u.mstances the planet, at the time of opposition, may approach the earth to a distance not greater than about 35,500,000 miles. No doubt this seems an enormous distance, when estimated by any standard adapted for terrestrial measurements; it is, however, hardly greater than the distance of Venus when nearest, and it is much less than the distance from the earth to the sun.

We have explained how the _form_ of the solar system is known from Kepler's laws, and how the absolute size of the system and of its various parts can be known when the direct measurement of any one part has been accomplished. A close approach of Mars affords a favourable opportunity for measuring his distance, and thus, in a different way, solving the same problem as that investigated by the transit of Venus.

We are thus led a second time to a knowledge of the distance of the sun and the distances of the planets generally, and to many other numerical facts about the solar system.

On the occasion of the opposition of Mars in 1877 a successful attempt was made to apply this refined process to the solution of the problem of celestial measurement. It cannot be said to have been the first occasion on which this method was suggested, or even practically attempted. The observations of 1877 were, however, conducted with such skill and with such minute attention to the necessary precautions as to render them an important contribution to astronomy. Dr. David Gill, now her Majesty's Astronomer at the Cape of Good Hope, undertook a journey to the Island of Ascension for the purpose of observing the parallax of Mars in 1877.

On this occasion Mars approached to the earth so closely as to afford an admirable opportunity for the application of the method. Dr. Gill succeeded in obtaining a valuable series of measurements, and from them he concluded the distance of the sun with an accuracy somewhat superior to that attainable by the transit of Venus.

There is yet another method by which Mars can be made to give us information as to the distance of the sun. This method is one of some delicacy, and is interesting from its connection with the loftiest enquiries in mathematical astronomy. It was foreshadowed in the Dynamical theory of Newton, and was wrought to perfection by Le Verrier.

It is based upon the great law of gravitation, and is intimately a.s.sociated with the splendid discoveries in planetary perturbation which form so striking a chapter in modern astronomical discovery.

There is a certain relation between two quant.i.ties which at first sight seems quite independent. These quant.i.ties are the ma.s.s of the earth and the distance of the sun. The distance of the sun bears to a certain distance (which can be calculated when we know the intensity of gravitation at the earth's surface, the size of the earth and the length of the year) the same proportion that the cube root of the sun's ma.s.s bears to the cube root of that of the earth. There is no uncertainty about this result, and the consequence is obvious. If we have the means of weighing the earth in comparison with the sun, then the distance of the sun can be immediately deduced. How are we to place our great earth in the weighing scales? This is the problem which Le Verrier has shown us how to solve, and he does so by invoking the aid of the planet Mars.

If Mars in his revolution around the sun were solely swayed by the attraction of the sun, he would, in accordance with the well-known laws of planetary motion, follow for ever the same elliptic path. At the end of one century, or even of many centuries, the shape, the size, and the position of that ellipse would remain unaltered. Fortunately for our present purpose, a disturbance in the orbit of Mars is produced by the earth. Although the ma.s.s of our globe is so much less than that of the sun, yet the earth is still large enough to exercise an appreciable attraction on Mars. The ellipse described by the planet is consequently not invariable. The shape of that ellipse and its position gradually change, so that the position of the planet depends to some extent upon the ma.s.s of the earth. The place in which the planet is found can be determined by observation; the place which the planet would have had if the earth were absent can be found by calculation. The difference between the two is due to the attraction of the earth, and, when it has been measured, the ma.s.s of the earth can be ascertained. The amount of displacement increases from one century to another, but as the rate of growth is small, ancient observations are necessary to enable the measures to be made with accuracy.

A remarkable occurrence which took place more than two centuries ago fortunately enables the place of Mars to be determined with great precision at that date. On the 1st of October, 1672, three independent observers witnessed the occultation of a star in Aquarius by the ruddy planet. The place of the star is known with accuracy, and hence we are provided with the means of indicating the exact point in the heavens occupied by Mars on the day in question. From this result, combined with the modern meridian observations, we learn that the displacement of Mars by the attraction of the earth has, in the lapse of two centuries, grown to about five minutes of arc (294 seconds). It has been maintained that this cannot be erroneous to the extent of more than a second, and hence it would follow that the earth's ma.s.s is determined to about one three-hundredth part of its amount. If no other error were present, this would give the sun's distance to about one nine-hundredth part.

[Ill.u.s.tration: Fig. 50.--Relative Sizes of Mars and the Earth.]

Notwithstanding the intrinsic beauty of this method, and the very high auspices under which it has been introduced, it is, we think, at present hardly worthy of reliance in comparison with some of the other methods.

As the displacement of Mars, due to the perturbing influence of the earth, goes on increasing continually, it will ultimately attain sufficient magnitude to give a very exact value of the earth's ma.s.s, and then this method will give us the distance of the sun with great precision. But interesting and beautiful though this method may be, we must as yet rather regard it as a striking confirmation of the law of gravitation than as affording an accurate means of measuring the sun's distance.

[Ill.u.s.tration: Fig. 51.--Drawing of Mars (July 30th, 1894).]

[Ill.u.s.tration: Fig. 52.--Drawing of Mars (August 16th, 1894).]

[Ill.u.s.tration: Fig. 53.--Elevations and Depressions on the "Terminator"

of Mars (August 24th, 1894).]

[Ill.u.s.tration: Fig. 54.--The Southern Polar Cap on Mars (July 1, 1894).]

The close approaches of Mars to the earth afford us opportunities for making a careful telescopic scrutiny of his surface. It must not be expected that the details on Mars could be inspected with the same minuteness as those on the moon. Even under the most favourable circ.u.mstances, Mars is still more than a hundred times as far as the moon, and, therefore, the features of the planet have to be at least one hundred times as large if they are to be seen as distinctly as the features on the moon. Mars is much smaller than the earth. The diameter of the planet is 4,200 miles, but little more than half that of the earth. Fig. 50 shows the comparative sizes of the two bodies. We here reproduce two of the remarkable drawings[16] of Mars made by Professor William H. Pickering at the Lowell Observatory, Flagstaff A.T. Fig. 51 was taken on the 30th of July, 1894, and Fig. 52 on the 16th of August, 1894.

The southern polar cap on Mars, as seen by Professor William H.

Pickering at Lowell Observatory on the 1st of July, 1894, is represented in Fig. 54.[17] The remarkable black mark intruding into the polar area will be noticed. In Fig. 53 are shown a series of unusually marked elevations and depressions upon the "terminator" of the planet, drawn as accurately as possible to scale by the same skilful hand on the 24th of August, 1894.

In making an examination of the planet it is to be observed that it does not, like the moon, always present the same face towards the observer.

Mars rotates upon an axis in exactly the same manner as the earth. It is not a little remarkable that the period required by Mars for the completion of one rotation should be only about half an hour greater than the period of rotation of the earth. The exact period is 24 hours, 37 minutes, 22-3/4 seconds. It therefore follows that the aspect of the planet changes from hour to hour. The western side gradually sinks from view, the eastern side gradually a.s.sumes prominence. In twelve hours the aspect of the planet is completely changed. These changes, together with the inevitable effects of foreshortening, render it often difficult to correlate the objects on the planet with those on the maps. The latter, it must be confessed, fall short of the maps of the moon in definiteness and in certainty; yet there is no doubt that the main features of the planet are to be regarded as thoroughly established, and some astronomers have given names to all the prominent objects.

The markings on the surface of Mars are of two cla.s.ses. Some of them are of an iron-grey hue verging on green, while the others are generally dark yellow or orange, occasionally verging on white. The former have usually been supposed to represent the tracts of ocean, the latter the continental ma.s.ses on the ruddy planet. We possess a great number of drawings of Mars, the earliest being taken in the middle of the seventeenth century. Though these early sketches are very rough, and are not of much value for the solution of questions of topography, they have been found very useful in aiding us to fix the period of rotation of the planet on its axis by comparison with our modern drawings.

Early observers had already noticed that each of the poles of Mars is distinguished by a white spot. It is, however, to William Herschel that we owe the first systematic study of these remarkable polar caps. This ill.u.s.trious astronomer was rewarded by a very interesting discovery. He found that these arctic tracts on Mars vary both in extent and distinctness with the seasons of the hemisphere on which they are situated. They attain a maximum development from three to six months after the winter solstice on that planet, and then diminish until they are smallest about three to six months after the summer solstice. The a.n.a.logy with the behaviour of the ma.s.ses of snow and ice which surround our own poles is complete, and there has until lately been hardly any doubt that the white polar spots of Mars are somewhat similarly const.i.tuted.

As the period of revolution of Mars around the sun is so much longer than our year, 687 days instead of 365, the seasons of the planet are, of course, also much longer than the terrestrial seasons. In the northern hemisphere of Mars the summer lasts for no fewer than 381 days, and the winter must be 306 days. In both hemispheres the white polar cap in the course of the long winter season increases until it reaches a diameter of 45 to 50, while the long summer reduces it to a small area only 4 or 5 in diameter. It is remarkable that one of these white regions--that at the south pole--seems not to be concentric with the pole, but is placed so much to one side that the south pole of Mars appears to be quite free from ice or snow once a year.

Although many valuable observations of Mars were made in the course of the nineteenth century, it is only since the very favourable opposition of 1877 that the study of the surface of Mars has made that immense progress which is one of the most remarkable features of modern astronomy. Among the observers who produced valuable drawings of the planet in 1877 we may mention Mr. Green, whose exquisite pictures were published by the Royal Astronomical Society, and Professor Schiaparelli, of Milan, who almost revolutionised our knowledge of this planet.

Schiaparelli had a refractor of only eight inches aperture at his disposal, but he was doubtless much favoured by the purity of the Italian sky, which enabled him to detect in the bright portions of the surface of Mars a considerable number of long, narrow lines. To these he gave the name of ca.n.a.ls, inasmuch as they issued from the so-called oceans, and could be traced across the reputed continents for considerable distances, which sometimes reached thousands of miles.

The ca.n.a.ls seemed to form a kind of network, which connected the various seas with each other. A few of the more conspicuous of these so-called ca.n.a.ls appeared indeed on some of the drawings made by Dawes and others before Schiaparelli's time. It was, however, the ill.u.s.trious Italian astronomer who detected that these narrow lines are present in such great numbers as to form a notable feature of the planet. Some of these remarkable features are shown in Figs. 51 and 52, which are copied from drawings made by Professor William H. Pickering at the Lowell Observatory in 1894.

Great as had been the surprise of astronomers when Schiaparelli first proclaimed the discovery of these numerous ca.n.a.ls, it was, perhaps, surpa.s.sed by the astonishment with which his announcement was received in 1882 that most of the ca.n.a.ls had become double. Between December, 1881, and February, 1882, thirty of these duplications appear to have taken place. Nineteen of these were cases of a well-traced parallel line being formed near a previously existing ca.n.a.l. The remaining ca.n.a.ls were less certainly established, or were cases where the two lines did not seem to be quite parallel. A copy of the map of Mars which Schiaparelli formed from his observations of 1881-82 is given in Plate XVIII. It brings out clearly these strange double ca.n.a.ls, so unlike any features that we know on any other globe.

[Ill.u.s.tration: PLATE XVIII.

SCHIAPARELLI'S MAP OF MARS IN 1881-82.]

Subsequent observations by Schiaparelli and several other observers seem to indicate that this phenomenon of the duplication of the ca.n.a.ls is of a periodic character. It is produced about the times when Mars pa.s.ses through its equinoxes. One of the two parallel lines is often superposed as exactly as possible upon the track of the old ca.n.a.l. It does, however, sometimes happen that both the lines occupy opposite sides of the former ca.n.a.l and are situated on entirely new ground. The distance between the two lines varies from about 360 miles as a maximum down to the smallest limit distinguishable in our large telescopes, which is something less than thirty miles. The breadth of each of these remarkable channels may range from the limits of visibility, say, up to more than sixty miles.

The duplication of the ca.n.a.ls is perhaps the most difficult problem which Mars offers to us for solution. Even if we admit that the ca.n.a.ls themselves represent inlets or channels through which the melted polar snow makes its way across the equatorial continents, it is not easy to see how the duplicate ca.n.a.ls can arise. This is especially true in those cases where the original channel seems to vanish and to be replaced by two quite new ca.n.a.ls, each about the breadth of the English Channel, and lying one on each side of the course of the old one. The very obvious explanation that the whole duplication is an optical illusion has been brought forward more than once, but never in a conclusive manner. We must, perhaps, be content to let the solution of this matter rest for the present, in the hope that the extraordinary attention which this planet is now receiving will in due time explain the present enigma.

The markings on the surface of this planet are, generally speaking, of a permanent character, so that when we compare drawings made one or two hundred years ago with drawings made more recently we can recognise in each the same features. This permanence is, however, not nearly so absolute as it is in the case of the moon. In addition to the ca.n.a.ls which we have already considered, many other parts of the surface of Mars alter their outlines from time to time. This is particularly the case with those dark spots which we call oceans, the contours of which sometimes undergo modifications in matters of detail which are quite unmistakable. Changes of colour are often observed on parts of the planet, and though some of these observations may perhaps be attributed to the influence of our own atmosphere on the planet's appearance, they cannot be all thus accounted for. Some of the phenomena must certainly be due to actual changes which have taken place on the surface of Mars.

As an example of such changes, we may refer to the north-western part of the notable feature, to which Schiaparelli has given the name of _Syrtis major_.[18] This has at various times been recorded as grey, green, blue, brown, and even violet. When this region (about the time of the autumnal equinox of the northern hemisphere) is situated in the middle of the visible disc, the eastern part is distinctly greener than the western. As the season progresses this characteristic colour gets feebler, until the green tint is to be perceived only on the sh.o.r.es of the Syrtis. The atmosphere of Mars is usually very transparent, and fortunately allows us to scrutinise the surface of the planet without putting obstacles in the way m the shape of Martian clouds. Such clouds, however, are not invariably absent. Our view of the surface is occasionally obstructed in such a manner as to make it certain that clouds or mist in the atmosphere of Mars must be the cause of the trouble.

Would we form an idea of the physical const.i.tution of the surface of Mars, then the question as to the character of the atmosphere of the planet is among the first to be considered. Spectroscopic observations do not in this case render us much a.s.sistance. Of course, we know that the planet has no intrinsic light. It merely s.h.i.+nes by reflected sunlight. The hemisphere which is turned towards the sun is bright, and the hemisphere which is turned away from the sun is dark. The spectrum ought, therefore, like that of the moon, to be an exact though faint copy of the solar spectrum, unless the sun's rays, by pa.s.sing twice through the atmosphere of Mars, suffered some absorption which could give rise to additional dark lines. Some of the earlier observers thought that they could distinctly make out some such lines due, as was supposed, to water vapour. The presence of such lines is, however, denied by Mr. Campbell, of the Lick Observatory, and Professor Keeler, at the Allegheny Observatory,[19] who, with their unrivalled opportunities, both instrumental and climatic, could find no difference between the spectra of Mars and the moon. If Mars had an atmosphere of appreciable extent, its absorptive effect should be noticeable, especially at the limb of the planet; but Mr. Campbell's observations do not show any increased absorption at the limb. It would therefore seem that Mars cannot have an extensive atmosphere, and this conclusion is confirmed in several other ways.

The distinctness with which we see the surface of this planet tends to show that the atmosphere must be very thin as compared with our own.

There can hardly be any doubt that an observer on Mars with a good telescope would be unable to distinguish much of the features of the earth's surface. This would be the case not only by reason of the strong absorption of the light during the double pa.s.sage through our atmosphere, but also on account of the great diffusion of the light caused by this same atmosphere. Also, it is needless to say, the great amount of cloud generally floating over the earth would totally obscure many parts of our planet from a Martian observer. But though, as already mentioned, we occasionally find parts of Mars rendered indistinct, it must be acknowledged that the clouds on Mars are very slight. We should expect that the polar caps, if composed of snow, would, when melting, produce clouds which would more or less hide the polar regions from our inspection; yet nothing of the kind has ever been seen.