ARAGO ON THE SUN.

Lyttelton Times, Volume III, Issue 107, 22 January 1853, Page 9

ARAGO ON THE SUN.

(From Chambers' Journal.)

In the Anmiaire of the Bureau ties Longitudes, recently published in Paris, appears a paper by the distinguished astronomer Arago, *' On the Observations which have made known the Physical Constitution of the sun and of different stars ; and an inquiry into the conjectures of the ancient philosophers, and of the positive ideas of modem astronomers, on the place'that the sun. ought to occupy among- the prodigious number of stars which stud the firmament," in winch all that appertains to the subject is so ably condensed, as to afford material for a popular summary, which wo purpose to convey in the present article. The eclipse of the sun ;vf last July, by enabling observers to repeat former observations and test their accuracy, furnished some of the results which

serve to complete the paper in question, and which may be considered as settled, owing to the improvements continually taking place in the construction of instruments. Although astronomy is the exactest of sciences, its problems are not yet sill fully solved; anil for the determination of some of these, observers have to wait for years—in cerium instances for a century or more—until the circumstances combine for ii favourable observation. From the days of the Epicurean philosopher who, judging from appearances, declared the sun to be no mure than a foot in diameter, to those of livingcalculators, who give to the orb a diameter of 883,000 miles, theie has been a marvellous advance. In these dimensions, we have a sphere one million four hundred thousand limes larger than the earth. ' Numbers so enormous,' says M. Arago, 'not being often employed in ordinary life, and giving us no very precise idea of the magnitudes which they imply. I recall here a remark that will convey a better understanding of the immensity of ihe solar volume. If we imagine the centre of xhe sun to coincide with that of the earth, its surface would not only reach the region in which the moon revolves, but would extend nearly as far again beyond.' By the transit of Venus in 1769, it was demonstrated that the sun is 95,000,000 miles from the earth ; and yet, distant as it is, its physical constitution has been determined; and the history of the successive steps by which this proof has been arrived at, forms one of the most interesting chapters in the progress of science. Jt was in 1611 that Fabiicius,a Dutch astronomer, first observed spots on the eastern edge of the sun, which passed slowly across the disc to the western edge, and disappeared altar a certain number of days. This phenomenon having been often noted subsequently, the conclusion diawn therefrom is, that the sun is a spherical body, having a movement of rotation about its centre, of which the duration is equal to twenty-live days and a half. These dark spots, inegular and variable, but well defined on their edge, are sometimes of considerable dimensions. Some have been seen whose size was iive times that of the earth. They are «c----nerally surrounded by an aureola known as the ■penumbra, and sensibly less luminous than the other portions of the orb. From this penumbra, first observed by Galileo, many apparently singular deductions have been made: namely, 'The sun is a dark body, surrounded at a certain distance by an atmosphere which may be compared to that ol the earth, when the latter is charged with a continuous stratum of opaque and reflecting clouds. To this first atmosphere succeeds a second, luminous in itself, called the photosphere. This photosphere, more or less remote from the inner cloudy atmosphere, would determine by its outline the visible limits of the orb. According to this hypothesis, there would l)e spots on the sun every thus that there occurred in the two concentric atmospheres such corresponding clear spaces as would allow of our seeing the dark central body uncovered.'

This hypothesis is considered by the most competent judges to render a very satisfactory account of the facts. But it has not been universally adopted. Some writers of authority have lately represented the spots as scoria) floating on a liquid surface, and ejected from solar volcanoes, of which the burning mountains of the earlh convey but a feeble idea. Hence observations become necessary as to the nature of the incandescent matter of the sun ; and when we remember the immense distance of that body, such an attempt may well appear to be one of temerity.

The progress of optical science, however, lias given us the means of determining this ■apparently insoluble question. It is well known Unit physicists are enabled at present to distinguish two kinds of light—natural light and polarised light. A ray of the former exhibits the same properties on any part of its form ; not so the latter. A polarised ray is said to have sides, and the different sides have different properties, as demonstrated by many interesling phenomena. Strange as it may seem, these rays thus described as having sides couid pass through the eye of as.ieedlel by hundreds of thousands without disturbing! eauh other. Availing themselves, therefore, of the assistance of polarised light, and an instrument named tho polariscope, or (polarising" telescope, observers obtain a double image of the sun, both alike, and both white; but on reflecting this imago on water, or a glass mirror, [the rays be-

come polarised ; the two images are no longer alike or white, but are intensely coloured, while their form remains unchanged. If one is red, the other is green, or yellow and violet, always producing what are called the complementary colours. With this instrument it becomes possible to tell the difference between natural and polarised light.

Another point for consideration is, that for a long time it was supposed that the light emanating from any incandescent body always came to the eye as natural light, if i:i its passage it had not been refbeted or refracted. But experiments by the polariscops showed that the ray departing from the surface at an angle sufficiently small was polarised ; while at the same time, it was demonstrated that the lightemilted by any gaseous body in flame—that of streetlamps, for instance—is always in the natural state, whatever be its angle of emission. From these remarks, some idea will be formed of the process necessary to prove whether the substance which renders the sun visible is solid, liquid, or gaseous. On looking at the sun in the polariscope, tha image, as before observed, is seen to be purely white—a proof that the medium through which the luminous substance is made visible to us is gaseous. If it were liquid, the light would be coloured ; and as regards solidity, that is out of the question—the rapid change of spots proves that the outer envelope of the sun is not solid. On whatever day of the year we examine, the light is always white. Thus, thesa experiments remove the theory out of the region of simple hypothesis, and give certainty to our conclusions respecting the photosphere.

Here an example occurs of the aids and confirmations which science may derive from apparently trivial circumstances. Complaint was made at a large warehouse in Paris, that the gas-fitters had thrown the light on the goods from the narrow, and not fro in the broad side of the flame. Experiments were instituted, which proved that the amount of light was the same whether emitted from the broad or narrow surface. It was shown also that a gazeous substance in flame appears more luminous when seen obliquely than perpendicular, which explains what are known as faculcß and lucules, being those parts of the solar disc that show themselves brighter than other portions of the surface. These are due to the presence of clouds in the solar atmosphere ; the inclined portions of the clouds appearing brightest to the spectator. The notion, that there were thousands oji thousands of points distinguishing themselves from the rest by a greater accumulation of luminous matter, is thus disposed of.

Still, there remained something1 more to be determined. The existence of the photosphere being proved, the question arose—was there nothing' beyond? or did it end abruptly? and this could only be determined at the period of a total eclipse, at the very moment when the obscuration of the sun beiutv greatest, our atmosphere ceases to be illuminated. Hence the interest felt in an eclipse of the sun of late years.

In July, 1542, at a total eclipse of the sun, visible in several ports of the continent, the astronomers noticed, just as the sun was hidden by the moon, certain objects in the form of rosecoloured protuberances, about two or three minutes high, astronomically speaking, projected from the surface of the moon. These appearances were variously explained : some supposed them \o be lunar mountains ; others saw in them effects of refraction or defracUon ; but no precise explanation could be given : and mere guesses cannot be accepted as science. Others, again, thought them to be mountains in the sun, the summits stretching beyond the photosphere ; but at the most moderate calculation, their height would have been about (iO,OOO miles—an elevation which, as is said, the solar attraction would render impossible. Another hypothesis was, that they were clouds floating- in a solar gaseous atmosphere. M. Arago considers the last as the true explanation; it remained the great point to be proved. If it could be ascertained that these red protuberances were not in actual contact with the moon, the demonstration would be complete. Speculation was busy, but nothing could be done in the wav of verification until another eclipse took place. There was one in August, 1850, total to the Sandwich Islands, at which, under direction of the French commandant at Tahiti, observations were made, the result being* that, the red prominences were seen to be sepu.

rated by a fine line from the moon's circumference. Here was an important datum. It was confirmed by the observations of July, 1851, by the observers of different nations at 'different localities, who saw that the coloured peaks were detached from the moon ; thus proving that they are not lunar mountains.

If it be further ascertained that these luminous phenomena are not produced by the inflexion of rays passing over the asperities of the moon's disk, and that they have a real existence, then there will be a new atmosphere to add to those which already surround the sun ; for clouds cannot support themselves in empty space. We come next to that of the subject which treats of the true place of the sun in the universe. In the year 448 8.C., Archelaus, the last of the lonian philosophers, without having made any measurements, taught that the sun was a star, but only somewhat larger than the others. Now the nearest fixed star is 206,000 ■times further from us than the sun: 206,000 times 95,000,000 of miles—a sum beyond all our habits of thought. The light from* the star Alpha of the Centaur is three years in its passage to the earth, travelling *at the rate of 192,000 miles per second ; and there are 86,400 seconds in a day, and 365 days in a year. Astounding facts ! If the sun, therefore, were removed to the distance of the Centaur, its broad disc, which takes a considerable time in its majestic rising and setting above and below the horizon, would have no sensible dimensions, even in the most powerful telescopes; and its light would not exceed that of stars of the third magnitude—facts which throw the guess of Archelaus into discredit. If our place in the material universe is thus made to appear very subordinate, we may remember, as M. Arago observes, that man owes the knowledge of it entirely to his own resources, and thereby has raised himself to the most eminent rank in the world of ideas.

Among the stars, Sirias is the brightest j but twenty thousand millions of such stars would be required to transmit to the earth a light equal to that of the sun, Ind if it were difficult to ascertain the nature and quality of the sun, it would appear to be still more so to determine these points with regard to the stars; for the reason, that the rays, coming from all parts of their disc at once, are intermingled, and of necessity produce white. This difficulty did not exist in similar investigations on the sun, because its disk is so large that the rays from any one part of it may be examined while the others are excluded. Under these circumstances, further proof might seem to be hopeless; -but advantage was taken of the fact, that there are certain stars which are sometimes light, sometimes dark, either from having a movement of rotation on their own axis, or because they are occasionally eclipsed by a non-luminous satellite revolving around them. It is clear that, while the light is waxing or waning, it comes from a part only of the star's disc; consequently, the neutralization of rays, which takes place when they depart from the whole surface at once, cannot then occur; and from the observations on the portion of light thus transmitted, and which is found to remain white under all its phases, we are entitled to conclude, in M. Arago's words, that 'f our sun is a star, and that its physical constitution is identical with that of the millions of stars strewn in ' the firmament." ' '