These then are some of the facts revealed to us by our magic glasses. I scarcely expect you to remember all the details I have given you, but you will at least understand now how astronomers actually penetrate into the secrets of the sun by bringing its image into their observatory, as we brought it to-day on the card-board, and then making it tell its own tale through the prisms of the spectroscope; and you will retain some idea of the central light of the sun with its surrounding atmosphere of cooler gases and its layer of luminous lambent gases playing round it beyond.
Of the corona I cannot tell you much, except that it is far more subtle than anything we have spoken of yet; that it is always strongest when the sun is most spotted; that it is partly made up of self-luminous gases whose bright lines we can see, especially an unknown green ray; while it also shines partly by reflected light from the sun, for we can trace in it faint dark lines; lastly it fades away into the mysterious zodiacal light, and so the sun ends in mystery at its outer fringe as it began at its centre.
And now at last, having learnt something of the material of the sun, we can come back to the spots and ask what is known about them. As I have said, they are not always the same on the sun's face. On the contrary, they vary very much both in number and size. In some years the sun's face is quite free from them, at others there are so many that they form two wide belts on each side of the sun's equator, with a clear space of about six degrees between. No spots ever appear near the poles. Herr Schwabe, who watched the sun's face patiently for more than thirty years, has shown that it is most spotted about every eleven years, then the spots disappear very quickly and reappear slowly till the full-spot time comes round again.
Some spots remain a very short time and then break up and disappear, but others last for days, weeks, and even months, and when we watch these, we find that a spot appears to travel slowly across the face of the sun from east to west and then round the western edge so that it disappears. It is when it reaches the edge that we can convince ourselves that the spot is really part of the sun, for there is no space to be seen between them, the edge and the spot are one, as the last trace of the dark blotch passes out of sight. In fact, it is not the spot which has crossed the sun's face, but the sun itself which has turned, like our earth, upon its axis, carrying the spot round with it. As some spots remain long enough to reappear, after about twelve or thirteen days, on the opposite edge, and even pass round two or three times, astronomers can reckon that the sun takes about twenty-five days and five hours in performing one revolution. You will wonder why I say only about twenty-five, but I do so because all spots do not come round in exactly the same time, those farthest from the equator lag rather more than a day behind those nearer to it, and this is explained by the layer of gases in which they are formed, drifting back in higher latitudes as the sun turns.
It is by watching a spot as it travels across the sun, that we are able to observe that the centre part lies deeper in the sun's face than the outer rim. There are many ways of testing this, and you can try one yourselves with a telescope if you watch day after day. I will explain it by a simple experiment. I have here a round lump of stiff dough, in which I have made a small hollow and blackened the bottom with a drop of ink. As I turn this round, so that the hollow facing you moves from right to left, you will see that after it passes the middle of the face, the hole appears narrower and narrower till it disappears, and if you observe carefully you will note that the dark centre is the first thing you lose sight of, while the edges of the cup are still seen, till just before the spot disappears altogether. But now I will stick a wafer on, and a pea half into, the dough, marking the centre of each with ink. Then I turn the ball again. This time you lose sight of the foremost edge first, and the dark centre is seen almost to the last moment. This shows that if the spots were either flat marks, or hillocks, on the sun's face, the dark centre would remain to the last, but as a fact it disappears before the rim. Father Secchi has tried to measure the depth of a spot-cavity, and thinks they vary from 1000 to 3000 miles deep. But there are many difficulties in interpreting the effects of light and shadow at such an enormous distance, and some astronomers still doubt whether spots are really depressions.
For many centuries now the spots have been watched forming and dispersing, and this is roughly speaking what is seen to happen. When the sun is fairly clear and there are few spots, these generally form quietly, several black dots appearing and disappearing with bright streaks or faculæ round their edge, till one grows bigger than the rest, and forms a large dark nucleus, round which, after a time, a half-shadow or penumbra is seen and we have a sun-spot complete, with bright edges, dark shadow, and deep black centre (Fig. 52). This lasts for a certain time and then it becomes bridged over with light streaks, the dark spot breaks up and disappears, and last of all the half-shadow dies away.
Fig. 52.
A quiet sun-spot. (Secchi.)
But things do not always take place so quietly. When the sun's face is very troubled and full of spots, the bright faculæ, which appear with a spot, seem to heave and wave, and generally several dark centres form with whirling masses of light round them, while in some of them tongues of fire appear to leap up from below (Fig. 53). Such spots change quickly from day to day, even if they remain for a long time, until at last by degrees the dark centres become less distinct, the half-shadows disappear, leaving only the bright streaks, which gradually settle down into luminous points or light granules. These light granules are in fact supposed by astronomers to be the tips of glowing clouds heaving up everywhere, while the dark spaces between them are cooler currents passing downwards.