The Story of the Solar System

by George F. Chambers

CHAPTER II.
THE SUN.

There was once a book published, the title of which was “The Sun, Ruler, Fire, Light and Life of the Planetary System.” The title was by no means a bad one, for without doubt the Sun may fairly be said to represent practically all the ideas conveyed by the designations quoted.

There is certainly no one body in creation which is so emphatically pre-eminent as the Sun. Whether or no there are stars which are suns—centres of systems serving in their degree the purposes served by our Sun, I need not now pause to enquire, though I think the idea is a very probable one; but of those celestial objects with which our Earth has a direct relationship, beyond doubt the Sun is unquestionably entitled to the foremost place. It is, as it were, the pivot on which the Earth and all the various bodies comprising the Solar System revolve in their annual progress. It is our source of light and heat, and therefore may be called the great agent by which an Almighty Providence wills to sustain animal and vegetable life. The consideration of all the complicated questions which arise out of these functions of the Sun belongs to the domain of Physics rather than that of Astronomy; still these matters are of such momentous interest that an allusion to them must be made, for they ought not to be lost sight of by the student of Astronomy. Half a century ago the actual state of our knowledge respecting the Sun might without difficulty be brought within the compass of a single chapter in any book on Astronomy, but so enormous has been the development of knowledge respecting the Sun of late years, that it is no longer a question of getting the materials properly into one chapter, but it is a matter of a whole volume being devoted to the Sun, or even, as in the case of Secchi, of two large octavo volumes of 500 pages each being required to cover the whole ground exhaustively. The reader will therefore easily understand that in the space at my disposal in this little work nothing but a passing glimpse can possibly be obtained of this great subject. It is great not only in regard to the vast array of purely astronomical facts which are at a writer’s command, but also on account of the extensive ramifications which the subject has into the domains of chemistry, photography, optics and cognate sciences. I shall therefore endeavour to limit myself generally to what an amateur can see for himself with a small telescope, and can readily understand, rather than attempt to say a little something about everything, and fail in the effort.

The mean distance of the Earth from the Sun may be taken to be about 93 millions of miles, and this distance is employed by Astronomers as the unit by which most other long celestial distances are reckoned. The true diameter of the Sun is about 866,000 miles. The surface area exceeds that of the Earth 11,946 times, and the volume is 1,305,000 times greater. The mass or weight of the Sun is 332,000 times that of the Earth, or about 700 times that of all the planets put together. Bulk for bulk the Sun is much lighter than the Earth: whilst a cubic foot of the Earth on an average weighs rather more than 5 times as much as a cubic foot of water, a cubic foot of Sun is only about 3½ times the weight of the same bulk of water. This consideration of the comparative lightness of the Sun (though in his day the Sun was thought to be lighter than it is now supposed to be) led Sir J. Herschel to infer that an intense heat prevails in its interior, independent it may be of its surface heat, so to speak, of which alone we are directly cognizant by the evidence of our senses.

The Sun is a sphere, and is surrounded by an extensive but attenuated envelope, or rather series of envelopes, which taken together bear some analogy to the atmosphere surrounding the Earth. These envelopes, which we shall have to consider more in detail presently, throw out rays of light and heat to the confines of the Solar System, though as to the conditions and circumstances under which that light and heat are generated we are entirely ignorant. Of the potency of the Sun’s rays we can form but a feeble conception, for the amount received by the Earth is, it has been calculated, but one 2300-millionth of the whole. Our annual share would, it is supposed, be sufficient to melt a layer of ice spread uniformly over the Earth to a depth of 100 feet, or to heat an ocean of fresh water 60 feet deep from freezing point to boiling point. The illuminating power of the Sun has to be expressed in language of similar profundity. Thus it has been calculated to equal that which would be afforded by 5563 wax candles concentrated at a distance of one foot from the observer. Again, it has been concluded that no fewer than half a million of full moons shining all at once would be required to make up a mass of light equal to that of the Sun. I present all these conclusions to the reader as they are furnished by various physicists who have investigated such matters, but it is rather uncertain as to how much reliance can safely be placed on such calculations in detail.

Fig. 5.—Ordinary Sun-spot, June 22, 1885.

To an amateur possessed of a small telescope, the Sun offers (when the weather is above the English average of recent years) a very great and constant variety of matters for studious scrutiny in its so-called “spots.” To the naked eye, or even on a hasty telescopic glance, the Sun presents the appearance of a uniform disc of yellowish white colour, though often a little attention will soon result in the discovery of a few, or it may be many, little black, or blackish patches, scattered here and there over the disc seemingly without order or method. We shall presently find out, however, that this last-named suggestion is wholly inaccurate. Though commonly called “spots,” these dark appearances are not simple spots, as the word might imply, for around the rather black patch which constitutes generally the main feature of the spot there is almost invariably a fringe of paler tint; whilst within the confines of the black patch which first catches the eye there is often a nucleus or inner portion of far more intense depth of shade. The innermost and darkest portion being termed the nucleus, the ordinary black portion is known as umbra, whilst the encompassing fringe is the penumbra. It is not always the case that each individual umbra has a penumbra all to itself, for several spots are occasionally included in one common penumbra. And it may further be remarked that cases of an umbra without a penumbra and the contrary are on record, though these may be termed exceptional, often having relation to material organic changes either just commencing or just coming to a conclusion. A marked contrast subsists in all cases between the luminosity of the penumbra and that of the general surface of the Sun contiguous. Towards its exterior edge the penumbra is usually darker than at its inner edge, where it comes in contact with the umbra. The outline of the penumbra is usually very irregular, but the umbra, especially in the larger spots, is often of regular form (comparatively speaking of course) and the nucleus (or nuclei) of the umbra still more noticeably partakes of a compactness of outline.

Spots are for the most part confined to a zone extending 35° or so on each side of the solar equator; and they are neither permanent in their form nor stationary in their position. In their want of permanence, they are subject, apparently, to no definite laws, for they frequently appear and disappear with great suddenness.

Their motions are evidently of a two-fold nature; the Sun itself rotates on its axis, and the spots collectively participate in this movement of rotation; but over and above this it has been conclusively proved that sometimes a spot has a proper motion of translation of its own independently of the motion which it has in consequence of the Sun’s axial rotation. Curiously enough, spots are very rare immediately under the Sun’s equator. It is in the zone extending from 8° to 20° North or South, as the case may be, that they are most abundant; or, to be more precise still, their favourite latitude seems to be 17° or 18°. They are often more numerous and of a greater general size in the northern hemisphere, to which it may be added that the zone between 11° and 15° North is particularly noted for large and enduring spots. A gregarious tendency is often very obvious, and where the groups are very straggling an imaginary line joining the extreme ends of the group will generally be found more or less parallel to the solar equator; and not only so, but extending a long way, or sometimes almost entirely, across the whole of the visible disc. With respect to the foregoing matters Sir John Herschel remarked:—“These circumstances ... point evidently to physical peculiarities in certain parts of the Sun’s body more favourable than in others to the production of the spots, on the one hand; and on the other, to a general influence of its rotation on its axis as a determining cause in their distribution and arrangement, and would appear indicative of a system of movements in the fluids which constitute its luminous surface; bearing no remote analogy to our trade-winds—from whatever cause arising.” More often than not when a main spot has a train of minor spots as followers that train will be found extending eastwards from the east side of the spot, rather than in any other direction.