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] Cassini, in 1678, made it 25·58; Delambre, in 1775, no more than twenty-five days. Later inquiries brought these divergences within no more tolerable limits. Laugier's result of 25·34 days—obtained in 1841—enjoyed the highest credit, yet it differed widely in one direction from that of Böhm (1852), giving 25·52 days, and in the other from that of Kysæus (1846), giving 25·09 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 Schröter,[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 assumption 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 shell 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 latitude. 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 assumed "mean period" for the solar rotation of 25·38 days (twenty-five days nine hours, very nearly), was thus found to be actually conformed to only in two parallels of solar latitude (14° north and south), while the equatorial period was slightly less than twenty-five, and that of latitude 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 space 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 orbit 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 latitude. 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 latitude 44°; and Peters, in June, 1846, watched, during several days, a spot in 50° 24′ north latitude. But beyond this no true macula has ever been seen; for Lahire's reported observation of one in latitude 70° is now believed to have had its place on the solar globe erroneously assigned; 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 latitude, 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 Böhm's observations during the years 1833-36; and a perfectly similar behaviour was noted both by Spörer 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 latitude 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 retarded character of the maximum of 1883-4 was to be found in the unusually low latitude 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 Spörer 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 latitudes. An unaccountable suspension of solar activity is, in fact, indicated.[429]
Gustav Spörer, born at Berlin in 1822, began to observe sun-spots with the view of assigning the law of solar rotation in December, 1860. His assiduity 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 analysis. 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 mass. 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 "cumulus 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 Spörer; 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 constitution 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 cocoon of fire, but as a vast heat-radiating machine. The terrestrial analogy 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 mass, to contribute their share in turn to the radiation into space which might be called the proper function of a sun.