Читать книгу Outlines of the Earth's History: A Popular Study in Physiography - Nathaniel Southgate Shaler - Страница 11

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Before the use of the telescope in astronomical work, which was begun by the illustrious Galileo in 1608, astronomers were unable to approach the problem of the structure of the sun. They could discern no more than can be seen by any one who looks at the great sphere through a bit of smoked glass, as we know this reveals a disklike body of very uniform appearance. The only variation in this simple aspect occurs at the time of a total eclipse, when for a minute or two the moon hides the whole body of the sun. On such occasions even the unaided eye can see that there is about the sphere a broad, rather bright field, of an aspect like a very thin cloud or fog, which rises in streamer like projections at points to a quarter of a million miles or more above the surface of the sphere. The appearance of this shining field, which is called the corona, reminds one of the aurora which glows in the region about either pole of the earth.

One of the first results of the invention of the telescope was the revelation of the curious dark objects on the sun's disk, known by the name of spots from the time of their discovery, or, at least, from the time when it was clearly perceived that they were not planets, but really on the solar body. The interest in the constitution of the sphere has increased during the last fifty years. This interest has rapidly grown until at the present time a vast body of learning has been gathered for the solution of the many problems concerning the centre of our system. As yet there is great divergence in the views of astronomers as to the interpretation of their observations, but certain points of great general interest have been tolerably well determined. These may be briefly set forth by an account of what would meet the eye if an observer were able to pass from the surface of the earth to the central part of the sun.

Lava stream, in Hawaiian Islands, flowing into the sea. Note the "ropy" character of the half-frozen rock on the sides of the nearest rivulet of the lava.

In passing from the earth to a point about a quarter of a million miles from the sun's surface—a distance about that of the moon from our sphere—the observer would traverse the uniformly empty spaces of the heavens, where, but for the rare chance of a passing meteorite or comet, there would be nothing that we term matter. Arriving at a point some two or three hundred thousand miles from the body of the sun, he would enter the realm of the corona; here he would find scattered particles of matter, the bits so far apart that there would perhaps be not more than one or two in the cubic mile; yet, as they would glow intensely in the central light, they would be sufficient to give the illumination which is visible in an eclipse. These particles are most likely driven up from the sun by some electrical action, and are constantly in motion, much as are the streamers of the aurora.

Below the corona and sharply separated from it the observer finds another body of very dense vapour, which is termed the chromosphere, and which has been regarded as the atmosphere of the sun. This layer is probably several thousand miles thick. From the manner in which it moves, in the way the air of our own planet does in great storms, it is not easy to believe that it is a fluid, yet its sharply defined upper surface leads us to suppose that it can not well be a mere mass of vapour. The spectroscope shows us that this chromosphere contains in the state of vapour a number of metallic substances, such as iron and magnesium. To an observer who could behold this envelope of the sun from the distance at which we see the moon, the spectacle would be more magnificent than the imagination, guided by the sight of all the relatively trifling fractures of our earth, can possibly conceive. From the surface of the fiery sea vast uprushes of heated matter rise to the height of two or three hundred thousand miles, and then fall back upon its surface. These jets of heated matter have the aspect of flames, but they would not be such in fact, for the materials are not burning, but merely kept at a high temperature by the heat of the great sphere beneath. They spring up with such energy that they at times move with a speed of one hundred and fifty miles a second, or at a rate which is attained by no other matter in the visible universe, except that strange, wandering star known to astronomers as "Grombridge, 1830," which is traversing the firmament with a speed of not less than two hundred miles a second.

Below the chromosphere is the photosphere, the lower envelope of the sun, if it be not indeed the body of the sphere itself; from this comes the light and heat of the mass. This, too, can not well be a firm-set mass, for the reason that the spots appear to form in and move over it. It may be regarded as an extremely dense mass of gas, so weighed down by the vast attraction of the great sphere below it that it is in effect a fluid. The near-at-hand observer would doubtless find this photosphere, as it appears in the telescope, to be sharply separated from the thinner and more vaporous envelopes—the chromosphere and the corona—which are, indeed, so thin that they are invisible even with the telescope, except when the full blaze of the sun is cut off in a total eclipse. The fact that the photosphere, except when broken by the so-called spots, lies like a great smooth sea, with no parts which lie above the general line, shows that it has a very different structure from the envelope which lies upon it. If they were both vaporous, there would be a gradation between them.

On the surface of the photosphere, almost altogether within thirty degrees of the equator of the sun, a field corresponding approximately to the tropical belt of the earth, there appear from time to time the curious disturbances which are termed spots. These appear to be uprushes of matter in the gaseous state, the upward movement being upon the margins of the field and a downward motion taking place in the middle of the irregular opening, which is darkened in its central part, thus giving it, when seen by an ordinary telescope, the aspect of a black patch on the glowing surface. These spots, which are from some hundred to some thousand miles in diameter, may endure for months before they fade away. It is clear that they are most abundant at intervals of about eleven years, the last period of abundance being in 1893. The next to come may thus be expected in 1904. In the times of least spotting more than half the days of a year may pass without the surface of the photosphere being broken, while in periods of plenty no day in the year is likely to fail to show them.

Fig. 6.—Ordinary Sun-spot, June 22, 1885.

It is doubtful if the closest seeing would reveal the cause of the solar spots. The studies of the physicists who have devoted the most skill to the matter show little more than that they are tumults in the photosphere, attended by an uprush of vapours, in which iron and other metals exist; but whether these movements are due to outbreaks from the deeper parts of the sun or to some action like the whirling storms of the earth's atmosphere is uncertain. It is also uncertain what effect these convulsions of the sun have on the amount of the heat and light which is poured forth from the orb. The common opinion that the sun-spot years are the hottest is not yet fully verified.

Below the photosphere lies the vast unknown mass of the unseen solar realm. It was at one time supposed that the dark colour of the spots was due to the fact that the photosphere was broken through in those spaces, and that we looked down through them upon the surface of the slightly illuminated central part of the sphere. This view is untenable, and in its place we have to assume that for the eight hundred and sixty thousand miles of its diameter the sun is composed of matter such as is found in our earth, but throughout in a state of heat which vastly exceeds that known on or in our planet. Owing to its heat, this matter is possibly not in either the solid or the fluid state, but in that of very compressed gases, which are kept from becoming solid or even fluid by the very high temperature which exists in them. This view is apparently supported by the fact that, while the pressure upon its matter is twenty-seven times greater in the sun than it is in the earth, the weight of the whole mass is less than we should expect under these conditions.

As for the temperature of the sun, we only know that it is hot enough to turn the metals into gases in the manner in which this is done in a strong electric arc, but no satisfactory method of reckoning the scale of this heat has been devised. The probabilities are to the effect that the heat is to be counted by the tens of thousands of degrees Fahrenheit, and it may amount to hundreds of thousands; it has, indeed, been reckoned as high as a million degrees. This vast discharge is not due to any kind of burning action—i.e., to the combustion of substances, as in a fire. It must be produced by the gradual falling in of the materials, due to the gravitation of the mass toward its centre, each particle converting its energy of position into heat, as does the meteorite when it comes into the air.

It is well to close this very imperfect account of the learning which relates to the sun with a brief tabular statement showing the relative masses of the several bodies of the solar system. It should be understood that by mass is meant not the bulk of the object, but the actual amount of matter in it as determined by the gravitative attraction which it exercises on other celestial bodies. In this test the sun is taken as the measure, and its mass is for convenience reckoned at 1,000,000,000.

Table of Relative Masses of Sun and Planets. [2]
The sun	1,000,000,000
Mercury	200
Venus	2,353
Earth	3,060
Mars	339
Asteroids	?
Saturn	285,580
Jupiter	954,305
Uranus	44,250
Neptune	51,600
Combined mass of the four inner planets	5,952
Combined mass of all the planets	1,341,687

It thus appears that the mass of all the planets is about one seven hundredth that of the sun.

Those who wish to make a close study of celestial geography will do well to procure the interesting set of diagrams prepared by the late James Freeman Clarke, in which transparencies placed in a convenient lantern show the grouping of the important stars in each constellation. The advantage of this arrangement is that the little maps can be consulted at night and in the open air in a very convenient manner. After the student has learned the position of a dozen of the constellations visible in the northern hemisphere, he can rapidly advance his knowledge in the admirable method invented by Dr. Clarke.

Having learned the constellations, the student may well proceed to find the several planets, and to trace them in their apparent path across the fixed stars. It will be well for him here to gain if he can the conception that their apparent movement is compounded of their motion around the sun and that of our own sphere; that it would be very different if our earth stood still in the heavens. At this stage he may well begin to take in mind the evidence which the planetary motion supplies that the earth really moves round the sun, and not the sun and planets round the earth. This discovery was one of the great feats of the human mind; it baffled the wits of the best men for thousands of years. Therefore the inquirer who works over the evidence is treading one of the famous paths by which his race climbed the steeps of science.

The student must not expect to find the evidence that the sun is the centre of the solar system very easy to interpret; and yet any youth of moderate curiosity, and that interest in the world about him which is the foundation of scientific insight, can see through the matter. He will best begin his inquiries by getting a clear notion of the fact that the moon goes round the earth. This is the simplest case of movements of this nature which he can see in the solar system. Noting that the moon occupies a different place at a given hour in the twenty-four, but is evidently at all times at about the same distance from the earth, he readily perceives that it circles about our sphere. This the people knew of old, but they made of it an evidence that the sun also went around our sphere. Here, then, is the critical point. Why does the sun not behave in the same manner as the moon? At this stage of his inquiry the student best notes what takes place in the motions of the planets between the earth and the sun. He observes that those so-called inferior planets Mercury and Venus are never very far away from the central body; that they appear to rise up from it, and then to go back to it, and that they have phases like the moon. Now and then Venus may be observed as a black spot crossing the disk of the sun. A little consideration will show that on the theory that bodies revolve round each other in the solar system these movements of the inner planets can only be explained on the supposition that they at least travel around the great central fire. Now, taking up the outer planets, we observe that they occasionally appear very bright, and that they are then at a place in the heavens where we see that they are far from the solar centre. Gradually they move down toward the sunset and disappear from view. Here, too, the movement, though less clearly so, is best reconcilable with the idea that these bodies travel in orbits, such as those which are traversed by the inner planets. The wonder is that with these simple facts before them, and with ample time to think the matter over, the early astronomers did not learn the great truth about the solar system—namely, that the sun is the centre about which the planets circled. Their difficulty lay mainly in the fact that they did not conceive the earth as a sphere, and even after they attained that conception they believed that our globe was vastly larger than the planets, or even than the sun. This misconception kept even the thoughtful Greeks, who knew that the earth was spherical in form, from a clear notion as to the structure of our system. It was not, indeed, until mathematical astronomy attained a considerable advance, and men began to measure the distances in the solar system, and until the Newtonian theory of gravitation was developed, that the planetary orbits and the relation of the various bodies in the solar system to each other could be perfectly discerned.

Care has been taken in the above statements to give the student indices which may assist him in working out for himself the evidence which may properly lead a person, even without mathematical considerations of a formal kind, to construct a theory as to the relation of the planets to the sun. It is not likely that he can go through all the steps of this argument at once, but it will be most useful to him to ponder upon the problem, and gradually win his way to a full understanding of it. With that purpose in mind, he should avoid reading what astronomers have to say on the matter until he is satisfied that he has done as much as he can with the matter on his own account. He should, however, state his observations, and as far as possible draw the results in his note-book in a diagrammatic form. He should endeavour to see if the facts are reconcilable with any other supposition than that the earth and the other planets move around the sun. When he has done his task, he will have passed over one of the most difficult roads which his predecessors had to traverse on their way to an understanding of the heavens. Even if he fail he will have helped himself to some large understandings.

The student will find it useful to make a map of the heavens, or rather make several representing their condition at different times in the year. On this plot he should put down only the stars whose places and names he has learned, but he should plot the position of the planets at different times. In this way, though at first his efforts will be very awkward, he will soon come to know the general geography of the heavens.

Although the possession or at least the use of a small astronomical telescope is a great advantage to a student after he has made a certain advance in his work, such an instrument is not at all necessary, or, indeed, desirable at the outset of his studies. An ordinary opera-glass, however, will help him in picking out the stars in the constellations, in identifying the planets, and in getting a better idea as to the form of the moon's surface—a matter which will be treated in this work in connection with the structure of the earth.