Curiosities of Science, Past and Present / A Book for Old and Young

by John Timbs

Before these observations, the dark spots seen on the sun’s surface were supposed to be portions of the solid body of the sun, laid bare to our view by those immense fluctuations in the luminous regions of its atmosphere to which it appears to be subject. It now appears that these dark portions are only an additional and inferior stratum of a very feebly luminous or illuminated portion of the sun’s atmosphere. This again in its turn Mr. Dawes has frequently seen pierced with a smaller and usually much more rounded aperture, which would seem at last to afford a view of the real solar surface of most intense blackness.

M. Schwabe, of Dessau, has discovered that the abundance or paucity of spots displayed by the sun’s surface is subject to a law of periodicity. This has been confirmed by M. Wolf, of Berne, who shows that the period of these changes, from minimum to minimum, is 11 years and 11-hundredths of a year, being exactly at the rate of nine periods per century, the last year of each century being a year of minimum. It is strongly corroborative of the correctness both of M. Wolf’s period and also of the periodicity itself, that of all the instances of the appearance of spots on the sun recorded in history, even before the invention of the telescope, or of remarkable deficiencies in the sun’s light, of which there are great numbers, only two are found to deviate as much as two years from M. Wolf’s epochs. Sir William Herschel observed that the presence or absence of spots had an influence on the temperature of the seasons; his observations have been fully confirmed by M. Wolf. And, from an examination of the chronicles of Zurich from A.D. 1000 to A.D. 1800, he has come to the conclusion “that years rich in solar spots are in general drier and more fruitful than those of an opposite character; while the latter are wetter and more stormy than the former.”

The most extraordinary fact, however, in connection with the spots on the sun’s surface, is the singular coincidence of their periods with those great disturbances in the magnetic system of the earth to which the epithet of “magnetic storms” has been affixed.

These disturbances, during which the magnetic needle is greatly and universally agitated (not in a particular limited locality, but at one and the same instant of time over whole continents, or even over the whole earth), are found, so far as observation has hitherto extended, to maintain a parallel, both in respect of their frequency of occurrence and intensity in successive years, with the abundance and magnitude of the spots in the same years, too close to be regarded as fortuitous. The coincidence of the epochs of maxima and minima in the two series of phenomena amounts, indeed, to identity; a fact evidently of most important significance, but which neither astronomical nor magnetic science is yet sufficiently advanced to interpret.—Herschel’s Outlines.

The signification and connection of the above varying phenomena (Humboldt maintains) can never be manifested in their entire importance until an uninterrupted series of representations of the sun’s spots can be obtained by the aid of mechanical clock-work and photographic apparatus, as the result of prolonged observations during the many months of serene weather enjoyed in a tropical climate.

M. Schwabe has thus distinguished himself as an indefatigable observer of the sun’s spots, for his researches received the Royal Astronomical Society’s Medal in 1857. “For thirty years,” said the President at the presentation, “never has the sun exhibited his disc above the horizon of Dessau without being confronted by Schwabe’s imperturbable telescope; and that appears to have happened on an average about 300 days a-year. So, supposing that he had observed but once a-day, he has made 9000 observations, in the course of which he discovered about 4700 groups. This is, I believe, an instance of devoted persistence unsurpassed in the annals of astronomy. The energy of one man has revealed a phenomenon that had eluded the suspicion of astronomers for 200 years.”

HAS THE MOON AN ATMOSPHERE?

The Moon possesses neither Sea nor Atmosphere of appreciable extent. Still, as a negative, in such case, is relative only to the capabilities of the instruments employed, the search for the indications of a lunar atmosphere has been renewed with fresh augmentation of telescopic power. Of such indications, the most delicate, perhaps, are those afforded by the occultation of a planet by the moon. The occultation of Jupiter, which took place on January 2, 1857, was observed with this reference, and is said to have exhibited no hesitation, or change of form or brightness, such as would be produced by the refraction or absorption of an atmosphere. As respects the sea, if water existed on the moon’s surface, the sun’s light reflected from it should be completely polarised at a certain elongation of the moon from the sun; and no traces of such light have been observed.

MM. Baer and Maedler conclude that the moon is not entirely without an atmosphere, but, owing to the smallness of her mass, she is incapacitated from holding an extensive covering of gas; and they add, “it is possible that this weak envelope may sometimes, through local causes, in some measure dim or condense itself.” But if any atmosphere exists on our satellite, it must be, as Laplace says, more attenuated than what is termed a vacuum in an air-pump.

Mr. Hopkins thinks that if there be any lunar atmosphere, it must be very rare in comparison with the terrestrial atmosphere, and inappreciable to the kind of observation by which it has been tested; yet the absence of any refraction of the light of the stars during occultation is a very refined test. Mr. Nasmyth observes that “the sudden disappearance of the stars behind the moon, without any change or diminution of her brilliancy, is one of the most beautiful phenomena that can be witnessed.”