Once a Week (magazine)/Series 1/Volume 3/The parentage of a sunbeam

THE PARENTAGE OF A SUNBEAM.


Of all the heads the sun shines down upon, the most are far too busy or too idle to think much about him, except vaguely as the great source of light and heat, whose morning appearance sets the world astir, and evening disappearance stills the din, and leaves the world to rest. What is it that so beneficently rules over us, subduing and enriching our earth with the shining host of sunbeams? Is it a great globe of fire, or disc of light, put there solely for the benefit of the earth and her companion planets? or has it a life of its own, so to speak, movement, change, ceaselessly active forces?

If we want to know the size of a distant object, we must first find how far off it is. We know familiarly that its apparent position depends upon the situation of the observer; that if we walk half a mile, the church steeple which was in one direction at starting, seems in quite another now. If it be a very near object, a small change of position will displace it; but if of such magnitude—a distant mountain, say—as to be seen a long way off, we may shift our own place considerably without its apparent position being altered. This simple fact lies at the root of all the knowledge attained respecting the distance, size, and motion of the heavenly bodies; and goes scientifically by the name of parallax. By means of it the sun’s distance has been ascertained with as much certainty as if a rule could be laid along to measure it. To arrive, indeed, at perfect accuracy, more recondite means have been and still are adopted. Witness the recent astronomical expedition to Chili, sent by the Americans to verify and rectify the calculation by a series of observations of Venus. But these are niceties important only to astronomers. It has been said that the Condor eagle could fly round the earth in a week if helped by favouring winds. That kingly bird would have to spread his wings for nearly seventy-three years to reach the sun, for the journey is ninety-five millions of miles long. A sunbeam does it in about eight minutes.

The distance known, we can understand that it was a comparatively easy task to find out the actual size of the great parent who sends his bright offspring to vivify we know not yet how many worlds. It is as large as fourteen hundred thousand globes like ours rolled into one. This includes the atmospheres which surround the sun; and it is not at present known whether these form so fractional a part of the entire bulk as does the atmosphere of the earth. The size of the solid globe within that wonderful light or photosphere, which latter alone is what our eyes behold, we do not know. There is one fact which may perhaps indicate that the sun’s atmospheres do occupy a vast depth; namely, that in proportion to its bulk the sun is four times lighter than the earth.

Each science has its own special class of difficulties to contend with: formidable dragons guarding the magician’s castle. The chemist, the meteorologist, still more the physiologist, are baffled by the silence and secresy with which nature prepares her effects, and by the multitude of causes conspiring to or modifying one result. The astronomer has, above all other students of nature, to contend with the confused evidence the senses give in regard to motion and position, furnishing us not with facts at all, but only with the materials out of which facts have slowly to be elaborated. It took five or six thousand years to ascertain whether our earth move or the sun move; astronomy being throughout the whole period more or less cultivated by one nation or another.

We have not now to account for the sun’s varying apparent position. The great circle he with irregular speed seems to describe, is the result of the earth’s shiftings, and belongs therefore to a study of the earth as a planet. But has he any movements of his own? A Dutchman, Fabricius, was the first to find the answer; by help of those remarkable appearances, the spots on the sun, the discovery of which may be reckoned one of the first fruits of the telescope. For though they had occasionally been seen by persons gifted with rare powers of sight as early as the time of Charlemagne, and before that by the Chinese, and perhaps the Peruvians, no suspicion of their real nature had been aroused; nor could they be observed long enough to deduce any kind of conclusion from them.

Armed with their new invention, the telescope, Fabricius, Galileo, and others, saw some of the spots appear on the eastern limb of the sun, reach the centre in six or seven days, disappear at the western edge in seven more, and, after an interval of nearly fourteen lays, reappear at the east, to repeat their course. Now these appearances could be accounted for in two ways: either that the spots are a part of the sun, and revolve with him on his axis, or that they are dark bodies at a very short distance from the surface, travelling round a motionless sun. Happily, besides these dark spots, there are spots of light, of especial brightness. These, if they were independent bodies revolving round the sun, would not disappear immediately after passing the edge, lost in the light of the photosphere, as the dark spots would: they would be seen a little longer. And that this does not happen, is conclusive that they are something belonging to the sun. Whilst the earth takes only twenty-four hours, the sun takes twenty-five days to revolve upon his axis, or thereabouts,—for, owing to changes which take place in the actual size and movements of the spots themselves, the learned are not quite agreed as to the exact period of rotation.

What are the spots? Astronomers have watched them as anxiously as a mother watches her child’s face, in the hope they would reveal something of what goes on beneath that mantle of flame which envelopes the dark and solid globe. Every spot is carefully mapped down, its course followed, its minutest change noted. They do not appear on all parts of the disc, but in two parallel zones on both sides of the sun’s equator, in a position, in fact, which nearly corresponds with those regions of the earth in which the trade-winds prevail. In duration they vary greatly. Some disappear in the course of a single revolution. Others—but this is rare—have been known to last six or seven months. Some years, the sun is scarcely a single day free from them: in others, there will be none on perhaps a hundred out of the three hundred and sixty-five. These variations are periodical. For five or six years the spots progressively increase in number and size, and then for five or six more diminish; after which they again begin increasing. They have been on the increase for the last five years, and will reach the maximum this year. Not the least remarkable feature is their enormous size. The earth might be flung through some of them without touching. Nay, last summer, there were spots sixty thousand miles across. Yet they disappear with rapidity, closing up at the rate of a thousand miles a day.

“Closing up:” the expression implies that they are an opening in something. That curtain of flame, the photosphere, which surrounds the sun as flame does the wick of a candle, is, by an unknown cause, powerful currents or atmospheric disturbances of some kind, reft asunder; and it is the body of the sun we have a glimpse of in that dark spot. There it is, dark and mysterious, yet solid, actual. Little enough can be made out from this glimpse; but it is something to set eyes upon the very core of the solar system.

The black spot is generally surrounded by a fringe brighter than the nucleus, though dull compared with the adjoining surface. This fringe (or penumbra) is also something seen through an opening in the sun’s outer luminous covering. It is part of a dense, cloudy atmosphere, situated at a vast depth below the surface of the photosphere: a great cavity, in fact, with a floor of cloud.

But there is still another, a third covering of the sun’s; the existence of which is revealed during a total eclipse of the sun. A circlet of pale light is then seen surrounding the two orbs, and in the midst of this sometimes rosy peaks of enormous height,—more than 40,000 miles high. This circlet, or corona, must be something aërial, belonging either to the sun or the moon. But the moon has no appreciable atmosphere. It is then the sun’s outermost covering,—a transparent atmosphere with no light of its own, but freely transmitting that of the photosphere: and the crimson mountains are clouds in it. This summer there has been an opportunity of observing these and other interesting phenomena in the eclipse that took place on the 18th of July.

Dependent as our world is upon the sun, it is not unreasonable to suppose we might feel some effect from those solar disturbances of which the spots are an evidence. Are our summers hotter, or our winters colder, crops more abundant, or falls of rain heavier, when spots prevail? These are points busily investigated, not yet cleared up. But that there is a connection between the spots and the magnetic state of the earth, General Sabine, the able and energetic leader in this field of inquiry (Terrestrial Magnetism), has established beyond doubt. Last September, a very remarkable fact was observed by Mr. Carrington, of Reigate. He saw a spot of intense brightness on the sun, which endured ten minutes; and, a week later, going to the Kew Observatory, found that during those same ten minutes the magnets had experienced most extraordinary deviations.

But not only does the sun, like his dependent worlds, revolve upon an axis. Like them, too, he moves obedient to a mighty influence from without, which draws him along at the rate, it is believed, of about 400,000 miles a day,—little more than a quarter of the speed with which the earth travels round him. Is he travelling in company with other suns round some great central sun? M. Maedler, the Prussian astronomer, has devoted many years of his life to this abstruse inquiry. He holds that the sun, with its attendant planets, is advancing towards a point in the constellation Hercules. The solar system, then, is not flung aside into some corner of the universe, “a law unto itself.” It is bound up with other systems, obeys the influence of other vaster centres of force, and—how can we believe otherwise?—of life; and is to visit inconceivably remote regions of space.

“If you ask me whether the sun is inhabited,” said Arago, “I am bound to reply, I know nothing about the matter. But if you say, can the sun be inhabited? Yes, certainly: and that too by beings of an organisation not wholly unlike our own, is my answer.” That dense and cloudy inner atmosphere we have spoken of may effectually protect it from the dazzling light of the photosphere, and conduct but little of its heat. Besides, though we have called it an ocean of flame, it is possible that the intensity of the light and heat given out may be due to the enormous depth of luminous matter; so that the vividness of any one particular film might not surpass that of an Aurora Borealis. So said Sir William Herschel.

Such, then, is the parentage of the sunbeam. But what are the sunbeams? What do they bring us on their radiant wings? Not light alone. Heat, chemical force (actinism), perhaps electric force, are in them, linked together in close, but not indissoluble union. And when they reach man’s domain, he has to some extent power over them. By cunningly-devised experiments, he dissolves the union, that he may search more thoroughly into the nature of each, and through this better knowledge find out perhaps something more about their birthplace. Thus even light, besides what we may call its direct revelations, has yielded to subtle modes of questioning a fragment of knowledge as to the nature of the photosphere.

Light, we may remind the reader, is of two kinds—natural and polarised. Polarisation is a hard word. It means the modification a ray of light undergoes in certain circumstances, through which it acquires different properties on one side to what it has on the other. And as it is ascertained under what circumstances light becomes polarised; so, vice versâ, if light be polarised, the circumstances under which it became so, the nature of its source, may be arrived at with tolerable certainty. There is a wonderful little instrument,—a blackened tube, with a plate of rock-crystal at one end, and of Iceland spar at the other,—called a Polariscope, which tests the two kinds of light. Look at something through this, and you will see two images of it in part overlapping one another. If the light reflected by this something be polarised, the two images will be of different colours,—complementary colours, one red, the other green, and so on. If it be natural light, both images will be white. Light that is emitted at a very small angle from a burning solid or liquid body is polarised. But from a burning gaseous substance, however small the angle at which it issues, it is natural light. Examined by this test, the sun’s luminous covering is concluded to be gaseous, (and flame is neither more nor less than burning air or gas): for it forms only white images in the Polariscope, though of course the rays from the edges do come at a very small angle. In what manner made luminous is unknown, though there are weighty reasons for suspecting electricity to be the agent.

Merely to sketch in outline what is already known of the work accomplished by the sun’s rays, would lead us within the precincts of almost every science. Herschel has told us that they are the primary source of all motion on the earth. Like the Prince whose kiss awoke the Sleeping Beauty, their touch rouses the germ of every green thing to put forth the life that is in it. They are the presiding genii in nature’s grand chemical laboratory. They set the winds in motion; draw up out of the earth mere watery vapour, which, shaped by those winds into a canopy of cloud, they paint with varying hues.

Anne Gilchrist.