Fig. 53.
A tumultuous sun-spot. (Langley.)
Below these clouds, no doubt, the great mass of the sun is in a violent state of heat and commotion, and when from time to time, whether suddenly or steadily, great upheavals and eruptions take place, bright flames dart up and luminous clouds gather and swell, so that long streaks or faculæ surge upon the face of the sun.
Now these hot gases rising up thus on all sides would leave room below for cooler gases to pour down from above, and these, as we know, would cut off, or absorb, much of the light coming from the body of the sun, so that the centre, where the down current was the strongest, would appear black even though some light would pass through. This is the best explanation we have as yet of the formation of a sun-spot, and many facts shown in the spectroscope help to confirm it, as for example the thickening of the dark lines of the spectrum when the slit is placed over the centre of a spot, and the flashing out of bright lines when an uprush of streaks occurs either across the spots or round it.
And now, before you go, I must tell you of one of these wonderful uprushes, which sent such a thrill through our own atmosphere, as to tell us very plainly the power which the sun has over our globe. The year 1859 was remarkable for sun-spots, and on September 1, when two astronomers many miles apart were examining them, they both saw, all at once, a sudden cloud of light far brighter than the general surface of the sun burst out in the midst of a group of spots. The outburst began at eight minutes past eleven in the forenoon, and in five minutes it was gone again, but in that time it had swept across a space of 35,000 miles on the sun! Now both before and after this violent outburst took place a magnetic storm raged all round the earth, brilliant auroras were seen in all parts of the world, sparks flashed from the telegraph wires, and the telegraphic signalmen at Washington and Philadelphia received severe electric shocks. Messages were interrupted, for the storm took possession of the wires and sent messages of its own, the magnetic needles darting to and fro as though seized with madness. At the very instant when the bright outburst was seen in the sun, the self-registering instruments at Kew marked how three needles jerked all at once wildly aside; and the following night the skies were lit up with wondrous lights as the storm of electric agitation played round the earth.
We are so accustomed to the steady glow of sunshine pouring down upon us that we pay very little heed to daylight, though I hope none of us are quite so ignorant as the man who praised the moon above the sun, because it shone in the dark night, whereas the sun came in the daytime when there was light enough already! Yet probably many of us do not actually realise how close are the links which bind us to our brilliant star as he carries us along with him through space. It is only when an unusual outburst occurs, such as I have just described, that we feel how every thrill which passes through our atmosphere, through the life-current of every plant, and through the fibre and nerve of every animal has some relation to the huge source of light, heat, electricity, and magnetism at which we are now gazing across a space of more than 93,000,000 miles. Yet it is well to remember that the sudden storm and the violent eruption are the exceptional occurrences, and that their use to us as students is chiefly to lead us to understand the steady and constant thrill which, never ceasing, never faltering, fulfils the great purpose of the unseen Lawgiver in sustaining all movement and life in our little world.
[1] Fairyland of Science, Chapter II.
[2] Two rare earths, Erbia and Didymium, form an exception to this, but they do not concern us here.
[3] A direct-vision spectroscope is like a small telescope with prisms arranged inside the tube. The object-glass end is covered by two pieces of metal, which slide backwards and forwards by means of a screw, so that a narrow or broad slit can be opened.