Part 18 (1/2)
Born at Chelsea in May, 1826, Richard Christopher Carrington entered Trinity College, Cambridge, in 1844. He was intended for the Church, but Professor Challis's lectures diverted him to astronomy, and he resolved, as soon as he had taken his degree, to prepare, with all possible diligence, to follow his new vocation. His father, who was a brewer on a large scale at Brentford, offered no opposition; ample means were at his disposal; nevertheless, he chose to serve an apprentices.h.i.+p of three years as observer in the University of Durham, as though his sole object had been to earn a livelihood. He quitted the post only when he found that its restricted opportunities offered no farther prospect of self-improvement.
He now built an observatory of his own at Redhill in Surrey, with the design of completing Bessel's and Argelander's survey of the northern heavens by adding to it the circ.u.mpolar stars omitted from their view.
This project, successfully carried out between 1854 and 1857, had another and still larger one superposed upon it before it had even begun to be executed. In 1852, while the Redhill Observatory was in course of erection, the discovery of the coincidence between the sun-spot and magnetic periods was announced. Carrington was profoundly interested, and devoted his enforced leisure to the examination of records, both written and depicted, of past solar observations. Struck with their fragmentary and inconsistent character, he resolved to ”appropriate,” as he said, by ”close and methodical research,” the eleven-year period next ensuing.[410] He calculated rightly that he should have the field pretty nearly to himself; for many reasons conspire to make public observatories slow in taking up new subjects, and amateurs with freedom to choose, and means to treat them effectually, were scarcer then than they are now.
The execution of this laborious task was commenced November 9, 1853. It was intended to be merely a _parergon_--a ”second subject,” upon which daylight energies might be spent, while the hours of night were reserved for cataloguing those stars that ”are bereft of the baths of ocean.” Its results, however, proved of the highest interest, although the vicissitudes of life barred the completion, in its full integrity, of the original design. By the death, in 1858, of the elder Carrington, the charge of the brewery devolved upon his son; and eventually absorbed so much of his care that it was found advisable to bring the solar observations to a premature close, on March 24, 1861.
His scientific life may be said to have closed with them. Attacked four years later with severe, and, in its results, permanent illness, he disposed of the Brentford business, and withdrew to Churt, near Farnham, in Surrey. There, in a lonely spot, on the top of a detached conical hill known as the ”Devil's Jump,” he built a second observatory, and erected an instrument which he was no longer able to use with pristine effectiveness; and there, November 27, 1875, he died of the rupture of a blood vessel on the brain, before he had completed his fiftieth year.[411]
His observations of sun-spots were of a geometrical character. They concerned positions and movements, leaving out of sight physical peculiarities. Indeed, the prudence with which he limited his task to what came strictly within the range of his powers to accomplish, was one of Carrington's most valuable qualities. The method of his observations, moreover, was chosen with the same practical sagacity as their objects.
As early as 1847, Sir John Herschel had recommended the daily self-registration of sun-spots,[412] and he enforced the suggestion, with more immediate prospect of success, in 1854.[413] The art of celestial photography, however, was even then in a purely tentative stage, and Carrington wisely resolved to waste no time on dubious experiments, but employ the means of registration and measurement actually at his command. These were very simple, yet very effective. To the ”helioscope” employed by Father Scheiner[414] two centuries and a quarter earlier, a species of micrometer was added. The image of the sun was projected upon a screen by means of a firmly-clamped telescope, in the focus of which were placed two cross-wires forming angles of 45 with the meridian. The six instants were then carefully noted at which these were met by the edges of the disc as it traversed the screen, and by the nucleus of the spot to be measured.[415] A short process of calculation then gave the exact position of the spot as referred to the sun's centre.
From a series of 5,290 observations made in this way, together with a great number of accurate drawings, Carrington derived conclusions of great importance on each of the three points which he had proposed to himself to investigate. These were: the law of the sun's rotation, the existence and direction of systematic currents, and the distribution of spots on the solar surface.
Grave discrepancies were early perceived to exist between determinations of the sun's rotation by different observers. Galileo, with ”comfortable generality,” estimated the period at ”about a lunar month”;[416]
Scheiner, at twenty-seven days.[417] Ca.s.sini, in 1678, made it 2558; Delambre, in 1775, no more than twenty-five days. Later inquiries brought these divergences within no more tolerable limits. Laugier's result of 2534 days--obtained in 1841--enjoyed the highest credit, yet it differed widely in one direction from that of Bohm (1852), giving 2552 days, and in the other from that of Kysaeus (1846), giving 2509 days. Now the cause of these variations was really obvious from the first, although for a long time strangely overlooked. Scheiner pointed out in 1630 that different spots gave different periods, adding the significant remark that one at a distance from the solar equator revolved more slowly than those nearer to it.[418] But the hint was wasted. For upwards of two centuries ideas on the subject were either retrograde or stationary. What were called the ”proper motions” of spots were, however, recognised by Schroter,[419] and utterly baffled Laugier,[420] who despaired of obtaining any concordant result as to the sun's rotation except by taking the mean of a number of discordant ones.
At last, in 1855, a valuable course of observations made at Capo di Monte, Naples, in 1845-6, enabled C. H. F. Peters[421] to set in the clearest light the insecurity of determinations based on the a.s.sumption of fixity in objects plainly affected by movements uncertain both in amount and direction.
Such was the state of affairs when Carrington entered upon his task.
Everything was in confusion; the most that could be said was that the confusion had come to be distinctly admitted and referred to its true source. What he discovered was this: that the sun, or at least the outer sh.e.l.l of the sun visible to us, has _no single period of rotation_, but drifts round, carrying the spots with it, at a rate continually accelerated from the poles to the equator. In other words, the time of axial revolution is shortest at the equator and lengthens with increase of lat.i.tude. Carrington devised a mathematical formula by which the rate or ”law” of this lengthening was conveniently expressed; but it was a purely empirical one. It was a concise statement, but implied no physical interpretation. It summarised, but did not explain the facts.
An a.s.sumed ”mean period” for the solar rotation of 2538 days (twenty-five days nine hours, very nearly), was thus found to be _actually_ conformed to only in two parallels of solar lat.i.tude (14 north and south), while the equatorial period was slightly less than twenty-five, and that of lat.i.tude 50 rose to twenty-seven days and a half.[422] These curious results gave quite a new direction to ideas on solar physics.
The other two ”elements” of the sun's rotation were also ascertained by Carrington with hitherto unattained precision. He fixed the inclination of its axis to the ecliptic at 82 45'; the longitude of the ascending node at 73 40' (for the epoch 1850 A.D.). These data--which have scarcely yet been improved upon--suffice to determine the position in s.p.a.ce of the sun's equator. Its north pole is directed towards a star in the coils of the Dragon, midway between Vega and the Pole-star; its plane intersects that of the earth's...o...b..t in such a way that our planet finds itself in the same level on or about the 3rd of June and the 5th of December, when any spots visible on the disc cross it in apparently straight lines. At other times, the paths pursued by them seem curved--downward (to an observer in the northern hemisphere) between June and December, upward between December and June.
A singular peculiarity in the distribution of sun-spots emerged from Carrington's studies at the time of the minimum of 1856. Two broad belts of the solar surface, as we have seen, are frequented by them, of which the limits may be put at 6 and 35 of north and south lat.i.tude.
Individual equatorial spots are not uncommon, but nearer to the poles than 35 they are a rare exception. Carrington observed--as an extreme instance--in July, 1858, one in south lat.i.tude 44; and Peters, in June, 1846, watched, during several days, a spot in 50 24' north lat.i.tude.
But beyond this no true macula has ever been seen; for Lahire's reported observation of one in lat.i.tude 70 is now believed to have had its place on the solar globe erroneously a.s.signed; and the ”veiled spots”
described by Trouvelot in 1875[423] as occurring within 10 of the pole can only be regarded as, at the most, the same kind of disturbance in an undeveloped form.
But the novelty of Carrington's observations consisted in the detection of certain changes in distribution concurrent with the progress of the eleven-year period. As the minimum approached, the spot-zones contracted towards the equator, and there finally vanished; then, as if by a fresh impulse, spots suddenly reappeared in high lat.i.tude, and spread downwards with the development of the new phase of activity. Scarcely had this remark been made public,[424] when Wolf[425] found a confirmation of its general truth in Bohm's observations during the years 1833-36; and a perfectly similar behaviour was noted both by Sporer and Secchi at the minimum epoch of 1867. The ensuing period gave corresponding indications; and it may now be looked upon as established that the spot-zones close in towards the equator with the advance of each cycle, their activity culminating, as a rule, in a mean lat.i.tude of about 16, and expiring when it is reduced to 6. Before this happens, however, a completely new disturbance will have manifested itself some 35 north and south of the equator, and will have begun to travel over the same course as its predecessor. Each series of sun-spots is thus, to some extent, overlapped by the succeeding one; so that while the average interval from one maximum to the next is eleven years, the period of each distinct wave of agitation is twelve or fourteen.[426] Curious evidence of the r.e.t.a.r.ded character of the maximum of 1883-4 was to be found in the unusually low lat.i.tude of the spot-zones when it occurred.
Their movement downward having gone on regularly while the crisis was postponed, its final symptoms were hence displaced locally as well as in time. The ”law of zones” was duly obeyed at the minima of 1890[427] and 1901, and Sporer found evidence of conformity to it so far back as 1619.[428] His researches, however, also showed that it was in abeyance during some seventy years previously to 1716, during which period sun-spots remained persistently scarce, and auroral displays were feeble and infrequent even in high northern lat.i.tudes. An unaccountable suspension of solar activity is, in fact, indicated.[429]
Gustav Sporer, born at Berlin in 1822, began to observe sun-spots with the view of a.s.signing the law of solar rotation in December, 1860. His a.s.siduity and success with limited means attracted attention, and a Government endowment was procured for his little solar observatory at Anclam, in Pomerania, the Crown Prince (afterwards Emperor Frederick) adding a five-inch refractor to its modest equipment. Unaware of Carrington's discovery (not made known until January, 1859), he arrived at and published, in June, 1861,[430] a similar conclusion as to the equatorial quickening of the sun's movement on its axis. Appointed observer in the new Astrophysical establishment at Potsdam in 1874, he continued his sun-spot determinations there for twenty years, and died July 7, 1895.
The time had now evidently come for a fundamental revision of current notions respecting the nature of the sun. Herschel's theory of a cool, dark, habitable globe, surrounded by, and protected against, the radiations of a luminous and heat-giving envelope, was shattered by the first _dicta_ of spectrum a.n.a.lysis. Traces of it may be found for a few years subsequent to 1859,[431] but they are obviously survivals from an earlier order of ideas, doomed to speedy extinction. It needs only a moment's consideration of the meaning at last found for the Fraunhofer lines to see the incompatibility of the new facts with the old conceptions. They implied not only the presence near the sun, as glowing vapours, of bodies highly refractory to heat, but that these glowing vapours formed the relatively cool envelope of a still hotter internal ma.s.s. Kirchhoff, accordingly, included in his great memoir ”On the Solar Spectrum,” read before the Berlin Academy of Sciences, July 11, 1861, an exposition of the views on the subject to which his memorable investigations had led him. They may be briefly summarised as follows:
Since the body of the sun gives a continuous spectrum, it must be either solid or liquid,[432] while the interruptions in its light prove it to be surrounded by a complex atmosphere of metallic vapours, somewhat cooler than itself. Spots are simply clouds due to local depressions of temperature, differing in no respect from terrestrial clouds except as regards the kinds of matter composing them. These _sun-clouds_ take their origin in the zones of encounter between polar and equatorial currents in the solar atmosphere.
This explanation was liable to all the objections urged against the ”c.u.mulus theory” on the one hand, and the ”trade-wind theory” on the other. Setting aside its propounder, it was consistently upheld perhaps by no man eminent in science except Sporer; and his advocacy of it proved ineffective to secure its general adoption.
M. Faye, of the Paris Academy of Sciences, was the first to propose a coherent scheme of the solar const.i.tution covering the whole range of new discovery. The fundamental ideas on the subject now in vogue here made their first connected appearance. Much, indeed, remained to be modified and corrected; but the transition was finally made from the old to the new order of thought. The essence of the change may be conveyed in a single sentence. The sun was thenceforth regarded, not as a mere heated body, or--still more remotely from the truth--as a cool body unaccountably spun round with a coc.o.o.n of fire, but as a vast _heat-radiating machine_. The terrestrial a.n.a.logy was abandoned in one more particular besides that of temperature. The solar system of circulation, instead of being adapted, like that of the earth, to the distribution of heat received from without, was seen to be directed towards the transportation towards the surface of the heat contained within. Polar and equatorial currents, tending to a purely superficial equalisation of temperature, were replaced by vertical currents bringing up successive portions of the intensely heated interior ma.s.s, to contribute their share in turn to the radiation into s.p.a.ce which might be called the proper function of a sun.
Faye's views, which were communicated to the Academy of Sciences, January 16, 1865,[433] were avowedly based on the anomalous mode of solar rotation discovered by Carrington. This may be regarded either as an acceleration increasing from the poles to the equator, or as a r.e.t.a.r.dation increasing from the equator to the poles, according to the rate of revolution we choose to a.s.sume for the unseen nucleus. Faye preferred to consider it a r.e.t.a.r.dation produced by ascending currents continually left behind as the sphere widened in which the matter composing them was forced to travel. He further supposed that the depth from which these vertical currents rose, and consequently the amount of r.e.t.a.r.dation effected by their ascent to the surface, became progressively greater as the poles were approached, owing to the considerable flattening of the spheroidal surface from which they started;[434] but the adoption of this expedient has been shown to involve inadmissible consequences.
The extreme internal mobility betrayed by Carrington's and Sporer's observations led to the inference that the matter composing the sun was mainly or wholly gaseous. This had already been suggested by Father Secchi[435] a year earlier, and by Sir John Herschel in April, 1864;[436] but it first obtained general currency through Faye's more elaborate presentation. A physical basis was afforded for the view by Cagniard de la Tour's experiments in 1822,[437] proving that, under conditions of great heat and pressure, the vaporous state was compatible with a very considerable density. The position was strengthened when Andrews showed, in 1869,[438] that above a fixed limit of temperature, varying for different bodies, true liquefaction is impossible, even though the pressure be so tremendous as to retain the gas within the same s.p.a.ce that enclosed the liquid. The opinion that the ma.s.s of the sun is gaseous now commands a very general a.s.sent; although the gaseity admitted is of such a nature as to afford the consistence rather of honey or pitch than of the aeriform fluids with which we are familiar.
On another important point the course of subsequent thought was powerfully influenced by Faye's conclusions in 1865. Arago somewhat hastily inferred from experiments with the polariscope the wholly gaseous nature of the visible disc of the sun. Kirchhoff, on the contrary, believed (erroneously, as we now know) that the brilliant continuous spectrum derived from it proved it to be a white-hot solid or liquid. Herschel and Secchi[439] indicated a cloud-like structure as that which would best harmonise the whole of the evidence at command.
The novelty introduced by Faye consisted in regarding the photosphere no longer ”as a defined surface, in the mathematical sense, but as a limit to which, in the general fluid ma.s.s, ascending currents carry the physical or chemical phenomena of incandescence.”[440] Uprus.h.i.+ng floods of mixed vapours with strong affinities--say of calcium or sodium and oxygen--at last attain a region cool enough to permit their combination; a fine dust of solid or liquid compound particles (of lime or soda, for example) there collects into the photospheric clouds, and descending by its own weight in torrents of incandescent rain, is dissociated by the fierce heat below, and replaced by ascending and combining currents of similar const.i.tution.