Sun-Spots

Abstract

RECORDS concerning phenomena are considerably enhanced in value if they include accurate determinations of the times of occurrence. This appears specially applicable to solar phenomena, and particularly to sun-spots, of which there must be many thousands of exact delineations without precise record of the times when the spots first appeared on the visible surface of the sun. No doubt there are several reasons to account for this unavoidable absence of valuable information; amongst others the intense brightness and heat of the sun make it an exceedingly disagreeable object for protracted telescopic inspection; nor yet are we in a position at present to photograph it continuously, so that we are necessarily content to compare photographs taken at intervals of perhaps many hours, and to assume, or at any rate, not to dispute, that events of great importance have not occurred in the intervals. This is the more to be regretted because a knowledge of solar events is comparatively of little importance, unless it helps us to ascertain what influence those events exercise on the earth and its inhabitants; and it is obvious that in comparing solar and terrestrial phenomena the times of occurrences are of essential importance, if only to avoid ascribing an undue effect to a given cause. It thus follows that even an approximate time of the appearance of sun-spots is not without value. On these grounds, as well as on the score of magnitude, I communicate the following particulars of a recent appearance, or outburst, of sun-spots, which occurred within certain moderate limits of time. I premise briefly that a photoheliograph is in daily use at the Trigonometrical Survey Office, Dehra Doon, India, of which I have executive charge. At present the instrument yields but a 4-inch negative, which is merely a microscopic delineation of 11/4 million millions square miles of solar surface; however, as surely as the sun shines, so surely are at least two negatives taken of it daily. Interruptions, even in a land of sunshine like this, sometimes occur; notably at the bursting of the monsoons, which occurred here last month (July), when the photographer was compelled to take the sun whenever visible, rather than not take him at all. Under this choice of alternations the first negative (or say N₁) on July 25, 1881, was taken at 3.58 p.m. of local apparent time; it exhibited several sun-spots, as is now usual, and of which therefore little need be said, since solar observers are well aware that the sun has for some months past resumed a state of considerable energy in respect to development of features; the second negative, or N₂, was taken at 4.47 p.m. On comparing N₁ and N₂ it was at once seen that in the interval of 49m. a considerable group of spots had appeared in the neighbourhood of the sun's centre. It is exceedingly difficult to exhibit an exact delineation of spots when the negative is on so minute a scale; I however inclose a silver print, as well as a hand-tracing of N₂, from which the position and magnitude of the group, i. e. the new group, may be nearly inferred. This new group consists of sixteen spots, of which no individual spot is notably large, but there is this peculiarity about them all, that they exhibit hardly any penumbra, but consist almost entirely of well-defined umbra; what penumbra appears, is confined chiefly to two spots, where it is seen only to the southeast; imagine a round, straight hole bored through a stratum of sand sufficiently adhesive for the sides to remain erect for a time, and after this suppose that the sand begins to fall inwards, creating a partial cone around to the south-east side; this is the sort of progress that these two spots convey. As to magnitude, the spots are scattered over an area of some 6000 millions of square miles; while the collective area of the spots themselves is about 630 millions of square mile, or, say, six times the area presented by the earth to the sun. Remembering that of solar change “a little goes a long way”, so far as we are concerned, who shall say that changes of this magnitude are inappreciable on the earth, however ineffectual the instruments we can now command may be able to measure them? But was this sudden change inappreciable? that is now the question. Unhappily the sun remained invisible till July 30, when two negatives were taken, i.e. after an interval of just five days; so far as solar rotation could effect, the so-called new group of N₂ should have been visible not far from the sun's western edge; but the entire group had vanished, leaving no trace behind. In the interim of five days two new spots had come out; of one of these I may add that the umbra is about 200 millions of square miles, and the penumbra some 700 millions, presenting in all a single feature of more than 900 millions of square miles, or say nine times the area exhibited by the earth to a distant spectator. This ends the purport of my letter. But I cannot help adding that I believe the bright solar features or faculæ will eventually prove to be more effective exponents than the dark features or spots; as a matter of fact, faculse commonly appear in abundance, covering considerable areas and branching out from one another like coral reefs; and it is a mistake to suppose that faculæ exist only in the vicinity of spots; the former may abound where the latter are quite absent, not only in a 4-inch negative, but in a very fair 5-inch equatorial. But I suppose the world will be better informed some day. Meanwhile, surely the sun is worthy of more earnest attention, not only from points of attack already so ably occupied, but from others none the less important, though at present greatly neglected: need I name solar radiation and photography? Physicians are alarmed for the safety of our bodies on detection of even a trifling change in temperature; but what do we know of fluctuations in the source of all terrestrial heat, though it be measurable with an actinometer? Again, land surveys are often made on huge scales; but for the solar survey of 11/4 million millions of square miles, what is our largest delineation, and at how many spots round the world is the required daily record made? If a survey of London pays, depend on it surveys of the sun will pay all nations infinitely better.