PHOTOGRAPH OF A GROUP OF SUN-SPOTS.
(From a photograph taken at the Royal Observatory, Greenwich, April, 1882, 20 d. 10 h. 6 m.)
The clearest connection between the magnetic movements and the sun-spot changes is seen when we take the mean values of either for considerable periods of time, as, for instance, year by year. But occasionally we have much more special instances of this connection. Some three or four times within the last twenty years an enormous spot has broken out on the sun, a spot so vast that worlds as great as our own could lie in it like peas in a breakfast saucer, and in each case there has been an immediate and a threefold answer from the earth. One of the most remarkable of these occurred in November, 1882. A great spot was then seen covering an area of more than three thousand millions of square miles. The weather in London happened to be somewhat foggy, and the sun loomed, a dull red ball, through the haze, a ball it was perfectly easy to look at without specially shading the eyes. So large a spot under such circumstances was quite visible to the naked eye, and it caught the attention of a great number of people, many of whom knew nothing about the existence of spots on the sun.
This great disturbance, evidently something of the nature of a storm in the solar atmosphere, stretched over one hundred thousand miles on the surface of the sun. The disturbance extended farther still, even to nearly one hundred millions of miles. For simultaneously with the appearance of the spot the magnetic needles at Greenwich began to suffer from a strange excitement, an excitement which grew from day to day until it had passed half-way across the sun's disc. As the twitchings of the magnetic needle increased in frequency and violence, other symptoms were noticed throughout the length of the British Isles. Telegraphic communication was greatly interfered with. The telegraph lines had other messages to carry more urgent than those of men. The needles in the telegraph instruments twitched to and fro. The signal bells on many of the railway lines were rung, and some of the operators received shocks from their instruments. Lastly, on November 17, a superb aurora was witnessed, the culminating feature of which was the appearance, at about six o'clock in the evening, of a mysterious beam of greenish light, in shape something like a cigar, and many degrees in length, which rose in the east and crossed the sky at a pace much quicker than but nearly as even as that of sun, moon, or stars, till it set in the west two minutes after its rising.
So far we have been dealing only with effects. Their causes still rest hidden from us. There is clearly a connection between the solar activity as shown by the spots and the agitation of the magnetic needles. But many great spots find no answer in any magnetic vibration, and not a few considerable magnetic storms occur when we can detect no great solar changes to correspond.
Thus even in the simplest case before us we have still very much to explain. Two far more difficult problems are still offered us for solution. What is the cause of these mysterious solar spots? and have they any traceable connection with the fitful vagaries of earthly weather? It was early suggested that probably the first problem might find an answer in the ever-varying combinations and configurations of the various planets, and that the sun-spots in their turn might hold the key of our meteorology. Both ideas were eagerly followed up—not that there was much to support either, but because they seemed to offer the only possible hope of our being able to foretell the general current of weather change for any long period in advance. So far, however, the first idea may be considered as completely discredited. As to the second, there would appear to be, in the case of certain great tropical and continental countries like India, some slight but by no means conclusive evidence of a connection between the changes in the annual rainfall and the changes in the spotted surface of the sun. Dr. Meldrum, the late veteran Director of the great Meteorological Observatory in Mauritius, has expressed himself as confident that the years of most spots are the years of most violent cyclones in the Indian Ocean. But this is about as far as real progress has been made, and it may be taken as certain that many years more of observation will be required, and the labours of many skilful investigators, before we can hope to carry much farther our knowledge as to any connection between storm and sun.
A further relation of great interest has come to light within the last few years. The year 1868 opened a new epoch in the study of eclipses of the sun. These, perhaps, scarcely lie within the scope of a book on the Royal Observatory, since Greenwich has seen but one in all its history. That fell in the year 1715; for the next it must wait many centuries. Yet, as the late Astronomer Royal conducted three expeditions to see total eclipses, and as the present Astronomer Royal has undertaken a like number, and members of the staff have been sent on other occasions, it may not be deemed quite a digression to refer to one feature which they have brought to light.
When the dark body of the moon has entirely hidden the sun, we have revealed to us, there and then only, that strange and beautiful surrounding of the sun which we call the corona. The earlier observations of the corona seem to reveal it as a body of the most weird and intricate form, a form which seemed to change quite lawlessly from one eclipse to another. But latterly it has been abundantly clear that the forms which it assumes may be grouped under a few well-defined types. In 1878 the corona was of a particularly simple and striking character. Two great wings shot out east and west in the direction of the sun's equator; round either pole was a cluster of beautiful radiating 'plumes.' It was then recollected that the corona of 1867 had been of precisely the same character, both years being years when sun-spots were at their fewest. The coronæ, on the other hand, seen at times when sun-spots are more abundant, were of an altogether different character, the streamers being irregularly distributed all round the sun. Other types also have been recognized, and it is perfectly apparent that the corona changes its shape in close accordance with the eleven-year period. The eclipses of 1889 and 1900, for example, showed coronæ that bore the very closest resemblance to those of 1878 and 1866, the interval of eleven years bringing a return to the same form.
The further problem, therefore, now confronts us: Does the corona produce the sun-spots, or do the sun-spots produce the corona, or are both the result of some mysterious magnetic action of the sun, an action powerful enough on occasion to thrill through and through this world of ours, ninety-three millions of miles away?