Fig. 14.—A Sun-spot (after Janssen).
Fig. 15.—Nasmyth’s Drawing of the Willow-leaved Structure of the Sun.
Fig. 16.—Spot nearing the Sun’s Edge.
The spots often last long enough to demonstrate a remarkable fact. We must remember that the sun is a great globe, and that it is poised freely in space. There is nothing to hold it up, and there is nothing to prevent it from turning round. That it does turn round, we can prove by careful observation of the spots. I can best illustrate what I want by [Fig. 17], which shows six imaginary pictures. The first represents the sun on the 1st day of the month; the next shows it five days later, on the 6th; another view is five days later still, on the 11th; and so on until the last picture, which corresponds to the 26th. You see, on the first day there is a spot near the left edge; by the 6th, this spot is near the middle; by the 11th, it is near the right edge; then you do not see it at all on the 16th, or on the 21st; but on the 26th it is back in the same place from which it started. We find other spots to have a similar history. They appear to move across the face, and then to return in a little less than four weeks to the same place where they were originally noticed. These appearances can be illustrated very simply by cutting a small hole through the rind of an orange down to the white interior skin, which may be darkened with ink. Put a knitting needle through the axis of the orange, and then turn it slowly round. The spot will be found to go through the changes that we have seen. We start with the spot near the left, it moves across the face, and then passes to invisibility by moving behind the globe until it reappears again, after having moved round the back. As the same may be observed with every spot which lasts long enough, we learn that the changes in the places must be produced by the turning round of the sun. Here you see is the way in which an astronomical discovery is made. We first observe the fact that the spots do always appear to move. Then we try to account for this, and we find a very simple explanation, by supposing that the whole sun, spots and all, turns steadily round and round. It can also be proved in a very conclusive manner that no other explanation is possible. This rotation of the sun is always going on uniformly, and some curious consequences follow from it. The view of the sun which is turned towards us to-day is quite different from that which was towards us a fortnight ago, or from that which we shall see in a fortnight hence. There is no actual or visible axis about which the sun rotates. In this the sun is like the earth and other celestial bodies.
Fig. 17.—How the Sun turns round.
APPEARANCES SEEN DURING A TOTAL ECLIPSE OF THE SUN.
For a great deal of our knowledge about the sun we are indebted to the moon. It will sometimes happen that the moon comes in between us and the sun, and produces an eclipse. At first you might think that an eclipse would only have the effect of preventing us from seeing anything of the sun, but it really reveals most beautiful and interesting objects, of whose existence we should otherwise be ignorant. The great luminary has curious appendages which are quite hidden under ordinary circumstances. In the full glare of day the dazzling splendor of the sun obliterates and renders invisible these appendages, which only shine with comparatively feeble light. It fortunately happens that the moon is just large enough to intercept the whole of the direct light from the sun, or rather, I should say, from the central parts of the sun. Surrounding that central and more familiar part from which the brilliancy is chiefly derived is a remarkable fringe of delicate and beautiful objects which are self-luminous no doubt, but with a light so feeble that when presented to us amid the full blaze of sunlight they are invisible. When, however, the moon so kindly stops all the stronger beams, then these faint objects spring into visibility, and we have the exquisite spectacle of a total eclipse. The objects that I desire to mention particularly are the corona and the prominences.