The Sun.

Next to the Earth itself the Sun is, to us, the most important body in the Universe, for we depend upon his heat and light for our very existence. It is true that the interior of the Earth is intensely hot; but very little of the heat reaches the surface, and it is certain that, but for the presence of the Sun, this planet of ours would be, to all intents and purposes, a frozen world. The oceans would be a mass of ice from top to bottom, and the land would be so frost-bound as to be incapable of supporting any form of vegetation. Finally, even the very air we now breathe would be reduced to a solid state, so that all animal life would be impossible.

To the philosophers of early times, as to other folk, the Sun was of interest chiefly because he appeared to be unique. To modern astronomers, however, he is of importance for precisely the opposite reason. So far as our observations can inform us, the stars are by far the commonest objects in the Universe, and it is now recognized that our Sun is nothing more than a fairly typical member of this vast class. He is unique only because his nearness enables us to make a more thorough study of him than of the others.

The distances of the stars are so great that they appear to us as mere points of light, even in the most powerful telescopes. But the Sun is near enough to present to us a large disc, which, in angular measure, is about half a degree in diameter. That is to say, there is room for something like 720 Suns, side by side, all along the horizon. As we know the distance of the Sun, we can translate this apparent, or angular, size into real dimensions, with the result that the diameter is found to be 864,000 miles. This is more than a hundred times the diameter of the Earth, so that, if we picture the latter as a large grape about one inch in diameter, the Sun would be represented by a globe nine feet across, such as would almost reach the ceiling of an average room if it rested on the floor. It would take more than a million Earths rolled into one to make a ball as big as the Sun. It is true that our world is one of the smaller planets, but even the largest, Jupiter, is hardly more than one-thousandth of the bulk of the star which occupies the centre of the Solar System.

The surface of the Sun is of an intense brilliance, greater even than that of the glowing carbons of an arc lamp, so that special precautions have to be taken before it can be examined with a telescope. But when the light has been reduced by a dark glass or by some reflecting device, we can obtain a comfortable view. We then find that the surface is not perfectly uniform in brightness, but consists of a very great number of separate cloud-like masses of flame, packed closely together like the pile of a carpet or rug. At a first glance these flames appear to be quiescent, but that is only because they are so large and distant that quite rapid movements seem to take a long time in producing any visible effect. Actually the whole surface is in a constant state of seething activity. The layer of flames is comparatively shallow, and between the bright masses there are much, darker spaces. In certain places these gaps sometimes become so abnormally large that a dusky patch is produced, appearing like a hole in the general surface. Such a patch is known as a "sun-spot," and may often be large enough to be visible to the naked eye.

A sun-spot is only a temporary phenomenon. Small ones often appear and close in again within a few hours, but those of the largest size (often large enough to contain our whole Earth many times over) may persist for weeks or even months. Meanwhile their outlines undergo constant changes, so that it may be quite difficult to identify a spot by its appearance alone after the lapse of a few days. As to the true cause of a sun-spot nothing is very definitely known, except that it represents a disturbance of a cyclonic nature, taking place near, and probably just below, the luminous surface.

Sun-spots had been seen occasionally with the naked eye in ancient and medieval times, though their true nature could not be appreciated. But they soon yielded a most important piece of information to the first users of the telescope, early in the Seventeenth Century. This was the fact that the Sun, like the Earth, is rotating about an axis, the period of rotation being between three and four weeks. But the strange thing about this rotation is that its period is not the same all over the Sun. The parts near the Equator have a period of about twenty-five days, but further North or South the rotation is much slower and is less by four or five days near the poles. This is a proof, if any were needed, that the Sun is not a solid body. It is in fact wholly gaseous, as we have reason to believe is the case with all the stars. The cause is, of course, the high temperature, which does not allow any substance known to us to exist in a solid or even liquid state. This is so even at the surface, which is the coolest part of the Sun. Here the temperature is in the neighbourhood of 6,000° Centigrade, while in the interior it is believed to be many millions of degrees on the same scale.