Smoothed Sunspot Curve (Wolf) compared with the number of turns made in each year by the Osler Anemometer Vane of the Royal Observatory, Greenwich (the excess of the direct turns (D) over the retrograde turns (R) or vice versa.)

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The upper curve is in each case the Sunspot Curve, the lower the Vane Curve. The break in 1882 in the Vane Curve is due to the omission of evidently accidental turns from that date.

Plate XV.

The chromosphere, from which shoot out the prominences or “red flames,” can now be observed without an eclipse if we employ the beautiful instrument above-mentioned, the spectroheliograph; and Professor Hale has succeeded in photographing spots, faculæ, and prominences all on the same plate. But although many have made the attempt (and Professor Hale, perhaps, a more determined attempt than any man living), no one has yet succeeded in obtaining any picture or evidence of the existence of the corona excepting on the occasion of a total solar eclipse.

Eclipses of sun.

Now these occasions are very rare. There are two or three eclipses of the sun every year, but they are generally of the kind known as partial; when the moon does indeed come between us and the sun to some extent, but only cuts off a portion of his light—a clean-cut black disc is seen to encroach more or less on the surface of the sun. Most of us have had an opportunity of seeing a partial eclipse, probably more than once; but few have seen a total eclipse. For this the moon must come with great exactness centrally between us and the sun; and the spot where this condition is fulfilled completely only covers a few hundred miles of the earth’s surface at one moment. As the earth turns round, and as the moon revolves in its orbit, this patch from which the sun is totally eclipsed travels over the earth’s surface, marking out a track some thousands of miles in length possibly, but still not more than 200 miles wide;Total eclipses rare. and in order to see the sun totally eclipsed even on the rare occasions when it is possible at all (for, as already remarked, in the majority of cases the eclipse is only partial), we must occupy some station in this narrow belt or track, which often tantalisingly passes over either the ocean or some regions not easily accessible to civilised man. Moreover, if we travel to such favoured spots the whole time during which the sun is totally eclipsed cannot exceed a few minutes, and hence observations are made under rather hurried and trying conditions. In these modern days of photography it is easier to take advantage of these precious moments than it used to be when there was only the eye and memory of an excited observer to rely upon. It is perhaps not surprising that some of the evidence collected on these earlier occasions was conflicting; but nowadays the observers, generally speaking, direct their energies in the first place to mounting accurately in position photographic apparatus of different kinds, each item of it specially designed to settle some particular problem in the most feasible way; secondly, to rehearsing very carefully the exact programme of exposures necessary during the critical few minutes; and finally, to securing these photographs with as few mistakes as possible when the precious moments actually arrive. Even then the whole of their efforts are quite likely to be rendered unavailing by a passing cloud; and bitter is the disappointment when, after travelling thousands of miles, and spending months in preparation, the whole enterprise ends in nothing owing to some caprice of the weather.

Hence it will easily be imagined that our knowledge of the corona, the part of the sun which we can still only study on occasions of a total solar eclipse, advances but slowly. During the last twenty years there has been altogether scarcely half-an-hour available for this research, though it may fairly be said that the very best possible use has been made of that half-hour. And, what is of importance for our immediate purpose, it has gradually been established by comparing the photographs of one eclipse with those of another,Corona follows spots. that the corona itself undergoes distinct changes in form in the same period which governs the changes of sun-spots. When there are many sun-spots the corona spreads out in all directions from the edge of the sun’s disc; when there are few sun-spots the corona extends very much further in the direction of the sun’s equator, so that at sun-spot minimum there is an appearance of two huge wings. Although the evidence is necessarily collected in a scrappy manner, by this time there is sufficient to remove this relationship out of the region of mere suspicion, and to give it a well-established place in our knowledge of the sun’s surroundings.