As we shall see hereafter, in the chapters which are devoted to terrestrial phenomena, the physical condition of the sun determines the course of all the more important events which take place on the surface of the earth. It is therefore fit that in this preliminary study of the celestial bodies, which is especially designed to make the earth more interpretable to us, we should give a somewhat special attention to what is known under the title of "Solar Physics."
The reader has already been told that the sun is one of many million similar bodies which exist in space, and, furthermore, that these aggregations of matter have been developed from an original nebulous condition. The facts indicate that the natural history of the sun, as well as that of its attendant spheres, exhibits three momentous stages: First, that of vapour; second, that of igneous fluidity; third, that in which the sphere is so far congealed that it becomes dark. Neither of these states is sharply separated from the other; a mass may be partly nebulous and partly fluid; even when it has been converted into fluid, or possibly into the solid state, it may still retain on the exterior some share of its original vaporous condition. In our sun the concentration has long since passed beyond the limits of the nebulous state; the last of the successively developed rings has broken, and has formed itself into the smallest of the planets, which by its distance from the sun seems to indicate that the process of division by rings long ago attained in our solar system its end, the remainder of its nebulous material concentrating on its centre without sign of any remaining tendency to produce these planet-making circles.
The Constitution of the Sun.
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.