Sir John Herschel describes these phenomena, when watched from day to
day, or even from hour to hour, as appearing to enlarge or contract,
to change their forms, and at length disappear altogether, or to break
out anew in parts of the surface where none were before. Occasionally
they break up or divide into two or more. The scale on which their
movements takes place is immense. A single second of angular measure,
as seen from the earth, corresponds on the sun’s disc to 461 miles; and
a circle of this diameter (containing therefore nearly 167,000 square
miles) is the least space which can be distinctly discerned on the sun
as a _visible area_. Spots have been observed, however, whose linear
diameter has been upwards of 45,000 miles; and even, if some records
are to be trusted, of very much greater extent. That such a spot should
close up in six weeks time (for they seldom last much longer), its
borders must approach at the rate of more than 1000 miles a-day.
The same astronomer saw at the Cape of Good Hope, on the 29th March
1837, a solar spot occupying an area of near _five square minutes_,
equal to 3,780,000,000 square miles. “The black centre of the spot of
May 25th, 1837 (not the tenth part of the preceding one), would have
allowed the globe of our earth to drop through it, leaving a thousand
miles clear of contact on all sides of that tremendous gulf.” For such
an amount of disturbance on the sun’s atmosphere, what reason can be
assigned?
The Rev. Mr. Dawes has invented a peculiar contrivance, by means of
which he has been enabled to scrutinise, under high magnifying power,
minute portions of the solar disc. He places a metallic screen, pierced
with a very small hole, in the focus of the telescope, where the image
of the sun is formed. A small portion only of the image is thus allowed
to pass through, so that it may be examined by the eye-piece without
inconveniencing the observer by heat or glare. By this arrangement,
Mr. Dawes has observed peculiarities in the constitution of the sun’s
surface which are discernible in no other way.
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.”