Part 22
Let us proceed, however, to consider some other conditions necessary for the existence of life on a planet. A suitable temperature is, of course, indispensable, but this is not all. There are other conditions which must be complied with. The planet must have a rotation on its axis, so that every portion shall in turn receive its due share of light and heat. Each point on its surface must have its day and night, the day for work and the night for rest. The axis of rotation must not lie in the plane of the planet’s orbit, but must have a suitable inclination, so that each hemisphere may enjoy its seasons, summer and winter, “seed-time and harvest,” in due course. Further the velocity of rotation on its axis must not be too rapid. If the earth rotated in a period of one and a quarter hours, bodies at the equator would have no weight, and life would be impossible in those regions.
The planet must also possess a mass sufficient to retain bodies on its surface by the force of gravity. In the case of very small bodies, such as the moons of Mars and some of the minor planets between Mars and Jupiter, objects thrown into the air would pass away into space never to return.
The planet should also have a mean density greater than that of water, otherwise the seas would possess no stability, and destructive waves would quickly destroy all life on its surface. All these conditions are fulfilled in the case of Mars as well as on the earth. In the planet Saturn, however, the density is less than that of water, and in Uranus and Neptune only slightly greater.
The planet must also possess a suitable atmosphere. This is an all-important condition for the support of animal life—at least for the existence of man and the higher orders of animals. This atmosphere must consist—so far as we know—of oxygen and nitrogen gases mechanically mixed in proper proportions, and with a small quantity of carbonic acid gas. Were the oxygen in smaller quantity than it exists in the earth’s atmosphere, life could not be supported. On the other hand, were it much in excess of its present amount, a fever would be produced in the blood which would very soon put an end to animal life. The presence of other gases in excessive quantities would also render the air unfit for breathing. We see, therefore, that a comparatively slight change in the composition of a planet’s atmosphere would—so far as our experience goes—render the planet uninhabitable by any of the higher forms of life with which we are familiar.
For the support of life on a planet, water is also absolutely necessary. Without this useful fluid the world would soon become a desert, and life and vegetation would speedily vanish from its surface.
Geological conditions must also be considered. It is clearly necessary for the welfare of human beings at least that the surface soil and rocks should contain coal, iron, lime, and other minerals, substances almost indispensable for the ordinary wants of civilized existence.
[Illustration: Nine Views of the Hour-Glass Sea on Mars
1, Nov. 26, 1864; 2, June 29, 1873; 3, Oct. 28, 1879; 4, June 2, 1888; 5, June 20, 1890; 6, Aug. 6, 1892; 7, Oct., 1894; 8, Dec. 3, 1896; 9, Dec. 7, 1896]
That all or any of the conditions considered would be complied with in the case of a planet revolving round a star it is, of course, impossible to say. But when we find stars showing by their spectra that they contain chemical elements identical with those which exist in the sun and the earth, analogy would lead us to suppose that very possibly a planet resembling our earth may revolve round each of these distant suns. I say _a_ planet, for evidently there would be only _one_ distance from the central luminary—a distance depending on its size—at which the temperature necessary for the support of life would exist, as in the case of the earth, over the whole of the planet’s surface. For other planets of the stellar system, life would be, if it existed at all, most probably confined to restricted regions of the planet’s surface. There would, therefore, be in each system one planet, and only one, _especially_ suitable for the support of animal life as we know it. This is with reference to light and heat. If the other conditions were not complied with, then life would probably not exist even on this one planet. In the case of a star larger than the sun, the planet should be placed at a greater distance than the earth is from the sun, but in this case the length of the year and the seasons would be longer than ours.
The star which more nearly resembles the sun in the character of the light which it emits is the bright star Capella. Arcturus has a somewhat similar spectrum. But these are probably suns of enormous size, if any reliance can be placed on the measures of their distance from the earth. Other bright stars with spectra of the solar type are Pollux, Aldebaran, Beta Andromedæ, Alpha Arietis, Alpha Cassiopeiæ, Alpha Cygni, and Alpha Ursæ Majoris. Another star is Eta Herculis. The magnitude of this star as measured with the photometer is about 3½. A parallax found by Bélopolsky and Wagner places it at a distance of 515,660 times the sun’s distance from the earth. If the sun were placed at this distance, I find that it would be reduced to a star of the third magnitude. This result would imply that Eta Herculis is a slightly smaller sun than ours; and a planet placed a little nearer to the star than the earth is to the sun might, perhaps, fulfil the conditions of a life-bearing world.
The number of stars visible in our largest telescopes is usually estimated at 100,000,000. Of these we may perhaps assume that 10,000,000 have a spectrum of the solar type, and therefore closely resemble our sun in their chemical constitution. If we suppose that only one in ten of these is similar in size to the sun, and has a habitable planet revolving round it, we have a total of 1,000,000 worlds in the visible universe fitted for the support of animal life.
We may therefore conclude, with a high degree of probability, that among the “multitudinous” stellar hosts there are probably many stars having life-bearing planets revolving round them.
THE SUN—WHAT WE LEARN FROM IT.—RICHARD A. PROCTOR
The Sun, the central and ruling body of the planetary system, and the source of light and heat to our earth and all the members of that system, is a globe about 852,900 miles in diameter. So far as observation extends, his figure is perfectly spherical, no difference having been observed between his polar and spherical diameters. It has been well remarked, indeed, by Sir G. Airy, that if any observer could by ordinary modes of measurement satisfy himself that a real difference existed between the diameters, that observer would have proved the inexactness of his own work; for the absence of any measurable compression comes out as the result of comparisons between thousands of observations of the sun’s limbs made at Greenwich and other leading observatories. The volume of the sun exceeds the earth’s 1,252,700 times. His mean density is almost exactly one-fourth of the earth’s, and his mass exceeds hers about 316,000 times. Gravity at the surface of the sun exceeds terrestrial gravity about 27.1 times, so that a body dropped from rest near the sun’s surface would fall through 436 feet in the first second, and have acquired a velocity of 872 feet per second.
Let the reader consider a terrestrial globe three inches in diameter, and search out on that globe the tiny triangular speck which represents Great Britain. Then let him endeavor to picture the town in which he lives as represented by the minutest pin-mark that could possibly be made upon this speck. He will then have formed some conception, though but an inadequate one, of the enormous dimensions of the earth’s globe, compared with the scene in which his daily life is cast. Now, on the same scale, the sun would be represented by a globe about twice the height of an ordinary sitting-room. A room about twenty-six feet in length, and height, and breadth, would be required to contain the representation of the sun’s globe on this scale, while the globe representing the earth could be placed in a moderately large goblet.
Such is the body which sways the motions of the Solar System. The largest of his family, the giant Jupiter, though of dimensions which dwarf those of the earth or Venus almost to nothingness, would yet only be represented by a thirty-two inch globe, on the scale which gives to the sun the enormous volume I have spoken of. Saturn would have a diameter of about twenty-eight inches, his ring measuring about five feet in its extreme span. Uranus and Neptune would be little more than a foot in diameter, and all the minor planets would be less than the three-inch earth. It will thus be seen that the sun is a worthy centre of the great scheme he sways, even when we merely regard his dimensions.
[Illustration: Fig. 29.—Sun Spot seen in 1870]
The sun outweighs fully seven hundred and forty times the combined mass of all the planets which circle around him, so that, when we regard the energy of his attraction, we still find him a worthy ruler of the planetary scheme.
Viewed with the naked eye, the sun appears only as a luminous mass of intense and uniform brightness; but when examined with the telescope, his surface is frequently observed to be mottled over with a number of dark spots, of irregular and ill-defined forms, constantly varying in appearance, situation, and magnitude. These spots are occasionally of immense size, so as to be visible even without the aid of the telescope; and their number is frequently so great that they occupy a considerable portion of the sun’s surface. Sir W. Herschel observed one in 1779 the diameter of which exceeded 50,000 miles, more than six times the diameter of the earth; and Scheiner affirms that he has seen no less than fifty on the sun’s disk at once. Most of them have a deep black nucleus, surrounded by a fainter shade, or _umbra_, of which the inner part, nearest to the nucleus, is brighter than the exterior portion. The boundary between the nucleus and umbra is in general tolerably well defined; and beyond the umbra a stripe of light appears more vivid than the rest of the sun.
[Illustration: Fig. 30.—Phase of Spot]
The discovery of the sun’s spots has been attributed to Fabricius, Galileo, and Scheiner, and has been claimed for the English astronomer Harriot. Amid these conflicting pretensions it is perhaps impossible to arrive at the truth; but the matter is of little importance; the discovery is one which followed inevitably that of the telescope, and an accidental priority of observation can hardly be considered as establishing any claim to merit.
The study of solar physics may be said to have commenced with the discovery of the sun spots, about two hundred and sixty years ago. These spots were presently found to traverse the solar disk in such a way as to indicate that the sun turns upon an axis once in about twenty-six days. Nor will this rotation appear slow, when we remember that it implies a motion of the equatorial parts of the sun’s surface at a rate exceeding some seventy times the motion of our swiftest express train.
Next came the discovery that the solar spots are not surface stains, but deep cavities in the solar substance. The changes of appearance presented by the spots as they traverse the solar disk led Dr. Wilson to form this theory so far back as 1779; but, strangely enough, it is only in comparatively recent times that the hypothesis has been finally established, since even within the last ten years a theory was put forward which accounted satisfactorily for most of the changes of appearance observed in the spots, by supposing them to be due to solar clouds hanging suspended at a considerable elevation above the true photosphere.
Sir William Herschel, reasoning from terrestrial analogies, was led to look on the spot-cavities as apertures through a double layer of clouds. He argued that, were the solar photosphere of any other nature, it would be past comprehension that vast openings should form in it, to remain open for months before they close up again. Whether we consider the enormous rapidity with which the spots form and with which their figure changes, or the length of time that many of them remain visible, we find ourselves alike perplexed, unless we assume that the solar photosphere resembles a bed of clouds. Through a stratum of terrestrial clouds openings may be formed by atmospheric disturbances, but while undisturbed the clouds will retain any form once impressed upon them, for a length of time corresponding to the weeks and months during which the solar spots endure.
And because the solar spots present two distinct varieties of light, the faint penumbra and the dark umbra or nucleus, Herschel saw the necessity of assuming that there are two beds of clouds, the outer self-luminous and constituting the true solar photosphere, the inner reflecting the light received from the outer layer, and so shielding the real surface of the sun from the intense light and heat which it would otherwise receive.
But while recent discoveries have confirmed Sir William Herschel’s theory about the solar cloud-envelopes, they have by no means given countenance to his view that the body of the sun may possibly be cool. The darkness of the nucleus of a spot is found, on the contrary, to give proof that in that neighborhood the sun is hotter, because it parts less readily with its heat. We shall see presently how this is. Meantime let it be noticed, in passing, that a close scrutiny of large solar spots has revealed the existence of an intensely black spot in the midst of the umbra. This black spot must be regarded as the true nucleus.
The circumstance that the spots appear only on two bands of the sun’s globe, corresponding to the sub-tropical zones on our own earth, led the younger Herschel to conclusions as important as those which his father had formed. He reasoned, like his father, from terrestrial analogies. On our own earth the sub-tropical zones are the regions where the great cyclonic storms have their birth, and rage with their chief fury. Here, therefore, we have the analogue of the solar spots, if only we can show reason for believing that any causes resembling those which generate the terrestrial cyclone operate upon those regions of the sun where the solar spots make their appearance.
We know that the cyclone is due to the excess of heat at the earth’s equator. It is true that this excess of heat is always in operation, whereas cyclones are not perpetually raging in sub-tropical climates. Ordinarily, therefore, the excess of heat does not cause tornadoes. Certain aerial currents are generated whose uniform motion suffices, as a rule, to adjust the conditions which the excess of heat at the equator would otherwise tend to disturb. But when through any cause the uniform action of the aerial currents is either interfered with or is insufficient to maintain equilibrium, then cyclonic or whirling motions are generated in the disturbed atmosphere, and propagated over a wide area of the earth’s surface.
Now we recognize the reason of the excess of heat at the earth’s equator in the fact that the sun shines more directly upon that part of the earth than on the zones which lie in higher latitudes. Can we find any reason for suspecting that the sun, which is not heated from without as the earth is, should exhibit a similar peculiarity? Sir John Herschel considers that we can. If the sun has an atmosphere extending to a considerable distance from his surface, then there can be little doubt that, owing to his rotation upon his axis, this atmosphere would assume the figure of an oblate spheroid, and would be deepest over the solar equator. Here, then, more of the sun’s heat would be retained than at the poles, where the atmosphere is shallowest. Thus, that excess of heat at the solar equator which is necessary to complete the analogy between the sun spots and terrestrial cyclones seems satisfactorily established.
It must be remarked, however, that this reasoning, so far as the excess of heat at the sun’s equator is concerned, only removes the difficulty a step. If there were indeed an increased depth of atmosphere over the sun’s equator sufficing to retain the requisite excess of heat, then the amount of heat we receive from the sun’s equatorial regions ought to be appreciably less than the amount emitted from the remaining portions of the solar surface. This is not found to be the case, so that either there is no such excess of absorption, or else the solar equator gives out more heat, in other words, is essentially hotter, than the rest of the sun. But this is just the peculiarity of which we want the interpretation.
It may be taken for granted, however, that there is an analogy between the sun spots and terrestrial cyclonic storms, though as yet we are not very well able to understand its nature.
Then next we come to one of the most interesting discoveries ever made respecting the sun—the discovery that the spots increase and diminish in frequency in a periodic manner. We owe this discovery to the laborious and systematic observations made by Herr Schwabe of Dessau.
Schwabe found, in the course of about ten and a half years, the solar spots pass through a complete cycle of changes. They become gradually more and more numerous up to a certain maximum, and then as gradually diminish. At length the sun’s face becomes not only clear of spots, but a certain well-marked darkening around the border of his disk disappears altogether for a brief season. At this time the sun presents a perfectly uniform disk. Then gradually the spots return, become more and more numerous, and so the cycle of changes is run through again.
The astronomers who have watched the sun from the Kew Observatory have found that the process of change by which the spots sweep in a sort of “wave of increase” over the solar disk is marked by several minor variations. As the surface of a great sea wave will be traversed by small ripples, so the gradual increase and diminution in the number of the solar spots are characterized by minor gradations of change, which are sufficiently well marked to be distinctly cognizable.
[Illustration: Fig. 31.—Ptolemaic System]
There seems every reason for believing that the periodic changes thus noticed are due to the influence of the planets upon the solar photosphere, though in what way that influence is exerted is not at present perfectly clear. Some have thought that the mere attraction of the planets tends to produce tides of some sort in the solar envelopes. Then, since the height of a tide so produced varies as the cube or third power of the distance, it has been thought that a planet when in perihelion would generate a much larger solar tide than when in aphelion. So that, as Jupiter has a period nearly equal to the sun-spot period, it has been supposed that the attractions of this planet are sufficient to account for the great spot period. Venus, Mercury, the Earth, and Saturn have, in a similar manner, been rendered accountable for the shorter and less distinctly marked periods.
Without denying that the planets may be, and probably are, the bodies to whose influence the solar-spot periods are to be ascribed, I yet venture to express very strong doubts whether the attraction of Jupiter is so much greater in perihelion than in aphelion as to account for the fact that, whereas at one season the face of the sun shows many spots, at another it is wholly free from them.[23]
However, we are not at present concerned so much with the explanation of facts as with the facts themselves. We have to consider rather what the sun is and what he does for the Solar System than why these things are so.
Let us note, before passing to other circumstances of interest connected with the sun, that the variable condition of his photosphere must cause him to change in brilliancy as seen from vast distances. If Herr Schwabe, for instance, instead of observing the sun’s spots from his watch-tower at Dessau, could have removed himself to a distance so enormous that the sun’s disk would have been reduced, even in the most powerful telescope, to a mere point of light, there can be no doubt that the only effect which he would have been able to perceive would have been a gradual increase and diminution of brightness, having a period of about ten and a half years.
Our sun, therefore, viewed from the neighborhood of any of the stars, whence undoubtedly he would simply appear as one among many fixed stars, would be a “variable,” having a period of ten and a half years. And further, if an observer, viewing the sun from so enormous a distance, had the means of very accurately measuring its light, he would undoubtedly discover that, while the chief variation of the sun takes place in a period of ten and a half years, its light is subjected to minor variations having shorter periods.
The discovery that the periodic changes of the sun’s appearance are associated with the periodic changes in the character of the earth’s magnetism is the next that we have to consider.
It had long been noticed that, during the course of a single day, the magnetic needle exhibits a minute change of direction, taking place in an oscillatory manner. And, when the character of this vibration came to be carefully examined, it was found to correspond to a sort of effort on the needle’s part to turn toward the sun. For example, when the sun is on the magnetic meridian, the needle has its mean position. This happens twice in a day, once when the sun is above the horizon and once when he is below it. Again, when the sun is midway between these two positions—which also happens twice in the day—the needle has its mean position, because the northern and the southern ends make equal efforts (so to speak) to direct themselves toward the sun. Four times in the day, then, the needle has its mean position, or is directed toward the magnetic meridian. But, when the sun is not in one of the four positions considered, that end of the needle which is nearest to him is slightly turned away from its mean position toward him. The change of position is very minute, and only the exact modes of observation made use of in the present age would have sufficed to reveal it. There it is, however, and this minute and seemingly unimportant peculiarity has been found to be full of meaning.