A Text-Book of Astronomy Part 17

The irregularities in the shading of the illuminated parts of the disk are too conspicuous in Fig. 94, on account of difficulties of reproduction; these shadings are at the best hard to see in the telescope, and distinct permanent markings upon the planet are wholly lacking. This absence of markings makes almost impossible a determination of the planet's time of rotation about its axis, and astronomers are divided in this respect into two parties, one of which maintains that Venus, like the earth, turns upon its axis in some period not very different from 24 hours, while the other contends that, like the moon, it turns always the same face toward the center of its...o...b..t, making a rotation upon its axis in the same period in which it makes a revolution about the sun. The reason why no permanent markings are to be seen on this planet is easily found. Like Jupiter and Saturn, its atmosphere is at all times heavily cloud-laden, so that we seldom, if ever, see down to the level of its solid parts. There is, however, no reason here to suppose the interior parts hot and gaseous. It is much more probable that Venus, like the earth, possesses a solid crust whose temperature we should expect to be considerably higher than that of the earth, because Venus is nearer the sun. But the cloud layer in its atmosphere must modify the temperature in some degree, and we have practically no knowledge of the real temperature conditions at the surface of the planet.

It is the clouds of Venus which in great measure are responsible for its marked brilliancy, since they are an excellent medium for reflecting the sunlight, and give to its surface an albedo greater than that of any other planet, although Saturn is nearly equal to it.

Of course, the presence of such cloud formations indicates that Venus is surrounded by a dense atmosphere, and we have independent evidence of this in the shape of its disk when the planet is very nearly between the earth and sun. The illuminated part, from tip to tip of the horns, then stretches more than halfway around the planet's circ.u.mference, and shows that a certain amount of light must have been refracted through its atmosphere, thus making the horns of the crescent appear unduly prolonged. This atmosphere is shown by the spectroscope to be not unlike that of the earth, although, possibly, more dense.

MERCURY

149. CHIEF CHARACTERISTICS.--Mercury, on account of its nearness to the sun, is at all times a difficult object to observe, and Copernicus, who spent most of his life in Poland, is said, despite all his efforts, to have gone to his grave without ever seeing it. In our more southern lat.i.tude it can usually be seen for about a fortnight at the time of each elongation--i. e., when at its greatest angular distance from the sun--and the student should find from Fig. 16 the time at which the next elongation occurs and look for the planet, shining like a star of the first magnitude, low down in the sky just after sunset or before sunrise, according as the elongation is to the east or west of the sun.

When seen in the morning sky the planet grows brighter day after day until it disappears in the sun's rays, while in the evening sky its brilliancy as steadily diminishes until the planet is lost. It should therefore be looked for in the evening as soon as possible after it emerges from the sun's rays.

Mercury, as the smallest of the planets, is best compared with the moon, which it does not greatly surpa.s.s in size and which it strongly resembles in other respects. Careful comparisons of the amount of light reflected by the planet in different parts of its...o...b..t show not only that its albedo agrees very closely with that of the moon, but also that its light changes with the varying phase of the planet in almost exactly the same way as the amount of moonlight changes. We may therefore infer that its surface is like that of the moon, a rough and solid one, with few or no clouds hanging over it, and most probably covered with very little or no atmosphere. Like Venus, its rotation period is uncertain, with the balance of probability favoring the view that it rotates upon its axis once in 88 days, and therefore always turns the same face toward the sun.

If such is the case, its climate must be very peculiar: one side roasted in a perpetual day, where the direct heating power of the sun's rays, when the planet is at perihelion, is ten times as great as on the moon, and which six weeks later, when the planet is at its farthest from the sun, has fallen off to less than half of this. On the opposite side of the planet there must reign perpetual night and perpetual cold, mitigated by some slight access of warmth from the day side, and perhaps feebly imitating the rapid change of season which takes place on the day side of the planet. This view, however, takes no account of a possible deviation of the planet's axis from being perpendicular to the plane of its...o...b..t, or of the librations which must be produced by the great eccentricity of the orbit, either of which would complicate without entirely destroying the ideal conditions outlined above.

MARS

150. APPEARANCE.--The one remaining member of the inner group, Mars, has in recent years received more attention than any other planet, and the newspapers and magazines have announced marvelous things concerning it: that it is inhabited by a race of beings superior in intelligence to men; that the work of their hands may be seen upon the face of the planet; that we should endeavor to communicate with them, if indeed they are not already sending messages to us, etc.--all of which is certainly important, if true, but it rests upon a very slender foundation of evidence, a part of which we shall have to consider.

Beginning with facts of which there is no doubt, this ruddy-colored planet, which usually shines about as brightly as a star of the first magnitude, sometimes displays more than tenfold this brilliancy, surpa.s.sing every other planet save Venus and presenting at these times especially favorable opportunities for the study of its surface. The explanation of this increase of brilliancy is, of course, that the planet approaches unusually near to the earth, and we have already seen from a consideration of Fig. 17 that this can only happen in the months of August and September. The last favorable epoch of this kind was in 1894. From Fig. 17 the student should determine when the next one will come.

[Ill.u.s.tration: FIG. 95.--Mars.--SCHAEBERLE.]

Fig. 95 presents nine drawings of the planet made at one of the epochs of close approach to the earth, and shows that its face bears certain faint markings which, though inconspicuous, are fixed and permanent features of the planet. The dark triangular projection in the lower half of the second drawing was seen and sketched by Huyghens, 1659 A. D. In Fig. 96 some of these markings are shown much more plainly, but Fig. 95 gives a better idea of their usual appearance in the telescope.

[Ill.u.s.tration: FIG. 96.--Four views of Mars differing 90 in longitude.--BARNARD.]

151. ROTATION.--It may be seen readily enough, from a comparison of the first two sketches of Fig. 95, that the planet rotates about an axis, and from a more extensive study it is found to be very like the earth in this respect, turning once in 24h. 37m. around an axis tipped from being perpendicular to the plane of its...o...b..t about a degree and a half more than is the earth's axis. Since it is this inclination of the axis which is the cause of changing seasons upon the earth, there must be similar changes, winter and summer, as well as day and night, upon Mars, only each season is longer there than here in the same proportion that its year is longer than ours--i. e., nearly two to one. It is summer in the northern hemisphere of Mars whenever the sun, as seen from Mars, stands in that constellation which is nearest the point of the sky toward which the planet's axis points. But this axis points toward the constellation Cygnus, and Alpha Cygni is the bright star nearest the north pole of Mars. As Pisces is the zodiacal constellation nearest to Cygnus, it must be summer in the northern hemisphere of Mars when the sun is in Pisces, or, turning the proposition about, it must be summer in the _southern_ hemisphere of Mars when the planet, as seen from the sun, lies in the direction of Pisces.

152. THE POLAR CAPS.--One effect of the changing seasons upon Mars is shown in Fig. 97, where we have a series of drawings of the region about its south pole made in 1894, on dates between May 21st and December 10th. Show from Fig. 17 that during this time it was summer in the region here shown. Mars crossed the prime radius in 1894 on September 5th. The striking thing in these pictures is the white spot surrounding the pole, which shrinks in size from the beginning to near the end of the series, and then disappears altogether. The spot came back again a year later, and like a similar spot at the north pole of the planet it waxes in the winter and wanes during the summer of Mars in endless succession.

[Ill.u.s.tration: FIG. 97.--The south polar cap of Mars in 1894.--BARNARD.]

Sir W. Herschel, who studied these appearances a century ago, compared them with the snow fields which every winter spread out from the region around the terrestrial pole, and in the summer melt and shrink, although with us they do not entirely disappear. This explanation of the polar caps of Mars has been generally accepted among astronomers, and from it we may draw one interesting conclusion: the temperature upon Mars between summer and winter oscillates above and below the freezing point of water, as it does in the temperate zones of the earth. But this conclusion plunges us into a serious difficulty. The temperature of the earth is made by the sun, and at the distance of Mars from the sun the heating effect of the latter is reduced to less than half what it is at the earth, so that, if Mars is to be kept at the same temperature as the earth, there must be some peculiar means for storing the solar heat and using it more economically than is done here. Possibly there is some such mechanism, although no one has yet found it, and some astronomers are very confident that it does not exist, and a.s.sert that the comparison of the polar caps with snow fields is misleading, and that the temperature upon Mars must be at least 100, and perhaps 200 or more, below zero.

153. ATMOSPHERE AND CLIMATE.--In this connection one feature of Mars is of importance. The markings upon its surface are always visible when turned toward the earth, thus showing that the atmosphere contains no such amount of cloud as does our own, but on the whole is decidedly clear and sunny, and presumably much less dense than ours. We have seen in comparing the earth and the moon how important is the service which the earth's atmosphere renders in storing the sun's heat and checking those great vicissitudes of temperature to which the moon is subject; and with this in mind we must regard the smaller density and cloudless character of the atmosphere of Mars as unfavorable to the maintenance there of a temperature like that of the earth. Indeed, this cloudlessness must mean one of two things: either the temperature is so low that vapors can not exist in any considerable quant.i.ty, or the surface of Mars is so dry that there is little water or other liquid to be evaporated. The latter alternative is adopted by those astronomers who look upon the polar caps as true snow fields, which serve as the chief reservoir of the planet's water supply, and who find in Fig. 98 evidence that as the snow melts and the water flows away over the flat, dry surface of the planet, vegetation springs up, as shown by the dark markings on the disk, and gradually dies out with the advancing season.

Note that in the first of these pictures the season upon Mars corresponds to the end of May with us, and in the last picture to the beginning of August, a period during which in much of our western country the luxuriant vegetation of spring is burned out by the scorching sun. From this point of view the permanent dark spots are the low-lying parts of the planet's surface, in which at all times there is a sufficient acc.u.mulation of water to support vegetable life.

[Ill.u.s.tration: FIG. 98.--The same face of Mars at three different seasons.--LOWELL.]

154. THE Ca.n.a.lS.--In Fig. 98 the lower part of the disk of Mars shows certain faint dark lines which are generally called ca.n.a.ls, and in Plate III there is given a map of Mars showing many of these ca.n.a.ls running in narrow, dusky streaks across the face of the planet according to a pattern almost as geometrical as that of a spider's web. This must not be taken for a picture of the planet's appearance in a telescope. No man ever saw Mars look like this, but the map is useful as a plain representation of things dimly seen. Some of the regions of this map are marked Mare (sea), in accordance with the older view which regarded the darker parts of the planet--and of the moon--as bodies of water, but this is now known to be an error in both cases. The curved surface of a planet can not be accurately reproduced upon the flat surface of paper, but is always more or less distorted by the various methods of "projecting" it which are in use. Compare the map of Mars in Plate III with Fig. 99, in which the projection represents very well the equatorial parts of the planet, but enormously exaggerates the region around the poles.

It is a remarkable feature of the ca.n.a.ls that they all begin and end in one of these dark parts of the planet's surface; they show no loose ends lying on the bright parts of the planet. Another even more remarkable feature is that while the larger ca.n.a.ls are permanent features of the planet's surface, they at times appear "doubled"--i. e., in place of one ca.n.a.l two parallel ones side by side, lasting for a time and then giving place again to a single ca.n.a.l.

It is exceedingly difficult to frame any reasonable explanation of these ca.n.a.ls and the varied appearances which they present. The source of the wild speculations about Mars, to which reference is made above, is to be found in the suggestion frequently made, half in jest and half in earnest, that the ca.n.a.ls are artificial water courses constructed upon a scale vastly exceeding any public works upon the earth, and testifying to the presence in Mars of an advanced civilization. The distinguished Italian astronomer, Schiaparelli, who has studied these formations longer than any one else, seems inclined to regard them as water courses lined on either side by vegetation, which flourishes as far back from the central channel as water can be supplied from it--a plausible enough explanation if the fundamental difficulty about temperature can be overcome.

[Ill.u.s.tration: FIG. 99.--A chart of Mars, 1898-'99.--CERULLI.]

[Ill.u.s.tration: PLATE III. MAP OF MARS (AFTER SCHIAPARELLI)]

155. SATELLITES.--In 1877, one of the times of near approach, Professor Hall, of Washington, discovered two tiny satellites revolving about Mars in orbits so small that the nearer one, Phobos, presents the remarkable anomaly of completing the circuit of its...o...b..t in less time than the planet takes for a rotation about its axis. This satellite, in fact, makes three revolutions in its...o...b..t while the planet turns once upon its axis, and it therefore rises in the west and sets in the east, as seen from Mars, going from one horizon to the other in a little less than 6 hours. The other satellite, Deimos, takes a few hours more than a day to make the circuit of its...o...b..t, but the difference is so small that it remains continuously above the horizon of any given place upon Mars for more than 60 hours at a time, and during this period runs twice through its complete set of phases--new, first quarter, full, etc. In ordinary telescopes these satellites can be seen only under especially favorable circ.u.mstances, and are far too small to permit of any direct measurement of their size. The amount of light which they reflect has been compared with that of Mars and found to be as much inferior to it as is Polaris to two full moons, and, judging from this comparison, their diameters can not much exceed a half dozen miles, unless their albedo is far less than that of Mars, which does not seem probable.

THE ASTEROIDS

156. MINOR PLANETS.--These may be dismissed with few words. There are about 500 of them known, all discovered since the beginning of the nineteenth century, and new ones are still found every year. No one pretends to remember the names which have been a.s.signed them, and they are commonly represented by a number inclosed in a circle, showing the order in which they were discovered--e. g., [circle 1] = Ceres, [circle 433] = Eros, etc. For the most part they are little more than chips, world fragments, adrift in s.p.a.ce, and naturally it was the larger and brighter of them that were first discovered. The size of the first four of them--Ceres, Pallas, Juno, and Vesta--compared with the size of the moon, according to Professor Barnard, is shown in Fig. 100. The great majority of them must be much smaller than the smallest of these, perhaps not more than a score of miles in diameter.

A few of the asteroids present problems of special interest, such as Eros, on account of its close approach to the earth; Polyhymnia, whose very eccentric orbit makes it a valuable means for determining the ma.s.s of Jupiter, etc.; but these are special cases and the average asteroid now receives scant attention, although half a century ago, when only a few of them were known, they were regarded with much interest, and the discovery of a new one was an event of some consequence.

It was then a favorite speculation that they were in fact fragments of an ill-fated planet which once filled the gap between the orbits of Mars and Jupiter, but which, by some mischance, had been blown into pieces.

This is now known to be well-nigh impossible, for every fragment which after the explosion moved in an elliptical orbit, as all the asteroids do move, would be brought back once in every revolution to the place of the explosion, and all the asteroid orbits must therefore intersect at this place. But there is no such common point of intersection.

[Ill.u.s.tration: FIG. 100.--The size of the first four asteroids.--BARNARD.]

157. LIFE ON THE PLANETS.--There is a belief firmly grounded in the popular mind, and not without its advocates among professional astronomers, that the planets are inhabited by living and intelligent beings, and it seems proper at the close of this chapter to inquire briefly how far the facts and principles here developed are consistent with this belief, and what support, if any, they lend to it.

At the outset we must observe that the word life is an elastic term, hard to define in any satisfactory way, and yet standing for something which we know here upon the earth. It is this idea, our familiar though crude knowledge of life, which lies at the root of the matter. Life, if it exists in another planet, must be in its essential character like life upon the earth, and must at least possess those features which are common to all forms of terrestrial life. It is an abuse of language to say that life in Mars may be utterly unlike life in the earth; if it is absolutely unlike, it is not life, whatever else it may be. Now, every form of life found upon the earth has for its physical basis a certain chemical compound, called protoplasm, which can exist and perpetuate itself only within a narrow range of temperature, roughly speaking, between 0 and 100 centigrade, although these limits can be considerably overstepped for short periods of time. Moreover, this protoplasm can be active only in the presence of water, or water vapor, and we may therefore establish as the necessary conditions for the continued existence and reproduction of life in any place that its temperature must not be permanently above 100 or below 0, C., and water must be present in that place in some form.

With these conditions before us it is plain that life can not exist in the sun on account of its high temperature. It is conceivable that active and intelligent beings, salamanders, might exist there, but they could not properly be said to live. In Jupiter and Saturn the same condition of high temperature prevails, and probably also in Ura.n.u.s and Neptune, so that it seems highly improbable that any of these planets should be the home of life.

Of the inner planets, Mercury and the moon seem dest.i.tute of any considerable atmospheres, and are therefore lacking in the supply of water necessary for life, and the same is almost certainly true of all the asteroids. There remain Venus, Mars, and the satellites of the outer planets, which latter, however, we must drop from consideration as being too little known. On Venus there is an atmosphere probably containing vapor of water, and it is well within the range of possibility that liquid water should exist upon the surface of this planet and that its temperature should fall within the prescribed limits. It would, however, be straining our actual knowledge to affirm that such is the case, or to insist that if such were the case, life would necessarily exist upon the planet.

On Mars we encounter the fundamental difficulty of temperature already noted in -- 152. If in some unknown way the temperature is maintained sufficiently high for the polar caps to be real snow, thawing and forming again with the progress of the seasons, the necessary conditions of life would seem to be fulfilled here and life if once introduced upon the planet might abide and flourish. But of positive proof that such is the case we have none.

On the whole, our survey lends little encouragement to the belief in planetary life, for aside from the earth, of all the hundreds of bodies in the solar system, not one is found in which the necessary conditions of life are certainly fulfilled, and only two exist in which there is a reasonable probability that these conditions may be satisfied.

CHAPTER XII

COMETS AND METEORS

158. VISITORS IN THE SOLAR SYSTEM.--All of the objects--sun, moon, planets, stars--which we have thus far had to consider, are permanent citizens of the sky, and we have no reason to suppose that their present appearance differs appreciably from what it was 1,000 years or 10,000 years ago. But there is another cla.s.s of objects--comets, meteors--which appear unexpectedly, are visible for a time, and then vanish and are seen no more. On account of this temporary character the astronomers of ancient and mediaeval times for the most part refused to regard them as celestial bodies but cla.s.sed them along with clouds, fogs, Jack-o'-lanterns, and fireflies, as exhalations from the swamps or the volcano; admitting them to be indeed important as harbingers of evil to mankind, but having no especial significance for the astronomer.

The comet of 1618 A. D. inspired the lines--

"Eight things there be a Comet brings, When it on high doth horrid range: Wind, Famine, Plague, and Death to Kings, War, Earthquakes, Floods, and Direful Change,"

which, according to White (History of the Doctrine of Comets), were to be taught in all seriousness to peasants and school children.

It was by slow degrees, and only after direct measurements of parallax had shown some of them to be more distant than the moon, that the tide of old opinion was turned and comets were transferred from the sublunary to the celestial sphere, and in more recent times meteors also have been recognized as coming to us from outside the earth. A meteor, or shooting star as it is often called, is one of the commonest of phenomena, and one can hardly watch the sky for an hour on any clear and moonless night without seeing several of those quick flashes of light which look as if some star had suddenly left its place, dashed swiftly across a portion of the sky and then vanished. It is this misleading appearance that probably is responsible for the name shooting star.

[Ill.u.s.tration: FIG. 101.--Donati's comet.--BOND.]

159. COMETS.--Comets are less common and much longer-lived than meteors, lasting usually for several weeks, and may be visible night after night for many months, but never for many years, at a time. During the last decade there is no year in which less than three comets have appeared, and 1898 is distinguished by the discovery of ten of these bodies, the largest number ever found in one year. On the average, we may expect a new comet to be found about once in every ten weeks, but for the most part they are small affairs, visible only in the telescope, and a fine large one, like Donati's comet of 1858 (Fig. 101), or the Great Comet of September, 1882, which was visible in broad daylight close beside the sun, is a rare spectacle, and as striking and impressive as it is rare.