The Story of the Solar System

by George F. Chambers

Besides the spots, streaks of light may frequently be remarked upon the surface of the Sun towards the margin of the disc. These are termed faculæ (torches), and they are often found near the spots, or where spots have previously existed or have afterwards appeared. When quite near the Sun’s limb these faculæ are usually more or less parallel to the limb. They are of irregular form and may be likened to certain kinds of coral. They generally appear to be more luminous than the solar surface immediately adjacent to them, but it is not improbable that this is an optical illusion depending upon the fact that the edges of the Sun always appear much more luminous than the centre. This last-named fact may be readily recognised by the employment of a high magnifying power, and moving the telescope rapidly from the limb to the centre of the disc. If the Sun be projected on a screen, as already mentioned, this degradation of the Sun’s light from centre to circumference becomes particularly manifest.

After having studied the structure and the movement of the spots, one is naturally led to ask if their apparitions at different periods are subject to any general law. This question is one which has much engaged the attention of modern astronomers. The older observers noticed that the number of the spots visible differed in different years. There were said to have been periods when months and even years passed away without any spots being observed. Even allowing that this statement, so far as “years” are concerned, might be exaggerated, and that the absence of spots was due to the want of sufficient care in making the observations, and especially to the want of efficient instruments, it is none the less true that the number of the spots is extremely variable, and that there have been epochs when they were very scarce.

Sir W. Herschel was the first who devoted himself to the question of seeking to establish a relation between the variation of the spots and terrestrial meteorology. For the want of any better object, he compared the annual number of the spots with the price of wheat; but it is easy to see that nothing could result from such a comparison. Without doubt the meteorological phenomena of the globe must depend to some extent on solar changes: but the term of comparison selected by Herschel had no direct bearing on the state of the Sun.

In our time this question has been investigated to its very foundation by Wolf, Director for many years at the Observatory of Zurich. It is to his zeal that we owe a very interesting assemblage of old observations which were buried in archives and chronicles. It was he who endeavoured to reduce them into a systematic form, so as to supply as far as possible the numerous gaps which exist in the different series.

The two most attentive observers at the period when the spots were discovered were Marriott at Oxford and Scheiner at Ingoldstadt, but Scheiner himself has informed us that he did not note down all the spots which he saw; he only recorded those which were likely to assist him in his special task of determining the period of the Sun’s rotation. Several observers after him made isolated series of observations; but some of these have been lost and the others show important gaps. J. G. Staudacher, at Nuremburg, observed the Sun with great perseverance during fifty years from 1749 to 1799. Before him the Cassinis, Maraldi, and others were engaged in the same sort of work, but only in an indirect way: that is to say, they contented themselves, whilst making meridional observations of the Sun, with noting anything in the way of spots which they deemed important. Zucconi and Flaugergues also left behind them a good collection of observations which Wolf utilised, rendering them comparable one with another by applying suitable corrections. The great difficulty herein arises from the fact that the observers were not provided with instruments of equal power; one man, armed with a better telescope than his contemporaries, naturally observed and recorded spots which would escape the others. The numbers entered in their registers are therefore not comparable inter se. Wolf endeavoured to replace these numbers by others which would represent the spots which might have been seen if the observers had all employed telescopes of a given kind and power. The result of his efforts in this direction is an almost continuous series of Sun-spot records from an epoch sufficiently remote, up to the time when this branch of science was taken up with the vigour of modern scientific methods.

The observer who most assiduously devoted himself to this subject in modern times was Schwabe of Dessau. From 1826 to 1868 he never failed to make daily observations when the weather permitted him. His series of records is specially valuable, for Carrington’s fits in with it, and with that in turn Spörer’s is comparable, and the chain is complete by the later photographic and other observations. All these Sun-spot records, though differing in their details, may easily be used together when it is a question of working out relative annual fluctuations.

At the present time there are many Astronomers who are engaged in observing the spots with care; but just as formerly there are few who possess sufficient perseverance. The photographic method is excellent, but it takes much time and is costly. Some have decried, in a very unreasonable manner, a drawing made by hand: such a drawing, of sufficient size, and executed by projection by a skilful draughtsman with a telescope driven by clockwork, may stand comparison with a photograph, and this method has a better chance of being persevered in. The Rev. F. Howlett’s name must be mentioned in this connection as a draughtsman who has accomplished much by hand drawing. Though the once famous Kew observations have been discontinued, they have been replaced by a new series at Greenwich with similar appliances; whilst Janssen at Meudon has also been carrying on for a number of years a splendid course of photographic records.

Schwabe, when he had collected a considerable number of observations, recognised clear indications of periodicity. Very definite epochs of maxima and minima succeeded one another at intervals of 10 or 11 years. It is true that in following out such a study the observations are certain to be in a sense a little defective. At first it was not possible to observe the Sun every day, and the gaps which resulted from bad weather necessarily added to the number of days which had to be set down as being without spots. Moreover, every method of numbering the spots must be a little arbitrary: there are often groups which, in consequence of their sub-divisions, may be counted in different ways: but in a mass of observations so considerable as those of Schwabe’s, such uncertainties will compensate for one another and will disappear in the final result. In fact the law is so striking that it suffices to cast one’s eye over his table[2] to see that.

That table is both interesting and instructive at the same time. The numbers exhibited in it speak for themselves, and it is sufficient to examine them with even a small amount of attention to realise the certainty of the conclusions which have been drawn.

It is therefore now to be deemed an ascertained fact that there are periodical maxima and minima in the display of spots, and that the extent of the period is between 10 and 12 years. In order to determine this value with the utmost exactness, some astronomers have had recourse to early observations. Wolf of Zurich made this the subject of some very interesting inquiries. He was able to establish the chronology of the phases which the Sun has passed through from the time of the first discovery of the spots to the present day—more than 2½ centuries. His calculations led him to a period of 11¹/₉ years. Lamont fixed upon 10.43 years, but this number does not represent the more recent observations with sufficient precision.