THE SUN.
Four years after the death of Herschel, an apothecary in the little
German town of Dessau procured a small telescope, with which he began to
observe the Sun. The name of this apothecary was _Samuel Heinrich
Schwabe_ (1789-1875). In 1826 he commenced to observe the spots on the
Sun’s disc, counting them from day to day, more for self-amusement than
from any hope of discovery; for previous astronomers had agreed that no
law regulated the number of the sun-spots. Every clear day Schwabe
pointed his telescope at the Sun and took his record of the spots; this
he continued for forty-three years, until within a few years of his
death on April 11, 1875. As early as 1843 Schwabe hinted that a possible
period of ten years regulated the distribution of the spots on the Sun,
but no attention was given to his idea. In 1851, however, the result of
his twenty-six years of observation was published in Humboldt’s
‘Cosmos,’ and Schwabe was able to show that the spots increased and
decreased in a period of about ten years. Astronomers at once recognised
the importance of Schwabe’s work, and in 1857 he was rewarded by the
Gold Medal of the Royal Astronomical Society of London.
_Rudolf Wolf_ (1813-1892) of the Zürich Observatory now undertook to
search through the records of sun-spot observation, from the days of
Galileo and Scheiner, to find traces of the solar cycle discovered by
Schwabe. He was successful, and was enabled to correct Schwabe’s
estimate of the length of the period, fixing it as on the average 11·11
years. Additional interest, however, was given to Schwabe’s and Wolf’s
investigations by the remarkable discoveries which followed. In
September 1851 _John Lamont_ (1805-1879), a Scottish astronomer,—born at
Braemar in Aberdeenshire, but employed as director of the Munich
Observatory,—after searching through the magnetic records collected at
Göttingen and Munich, discovered that the magnetic variations indicated
a period of 10⅓ years. Soon after this Sir _Edward Sabine_ (1788-1883),
the English physicist, from a discussion of an entirely different set of
observations, independently demonstrated the same thing, proving
conclusively that once in about ten years magnetic disturbances reached
their height of violence; and Sabine was not slow to notice the
correspondence between the magnetic period and the sun-spot period. In
the same year (1852) Wolf and _Alfred Gautier_ (1793-1881) independently
made the same discovery, which had thus been made by four separate
investigators.
In the same year an English amateur astronomer, _Richard Christopher
Carrington_ (1826-1875), commenced a series of solar observations which
led to some remarkable discoveries. From observations on the spots,
Carrington discovered that while the Sun’s rotation was performed in 25
days at the equator, it was protracted to 27½ days midway between the
equator and the poles. In 1858 Carrington demonstrated the fact that
spots are scarce in the vicinity of the solar equator, but are confined
to two zones on either side, becoming scarce again at thirty-five
degrees north or south of the equator. Contemporary with Carrington was
_Friedrich Wilhelm Gustav Spörer_ (1822-1895), who was born in Berlin in
1822 and died at Giessen, July 7, 1895. He commenced his solar
observations about the same time as Carrington, and independently
discovered the Sun’s equatorial acceleration. From observations at his
little private observatory at Anclam in Pomerania, continued at the
Astrophysical Observatory in Potsdam, Spörer demonstrated a remarkable
law regarding sun-spots. This law is thus described by a well-known
astronomer: “The disturbance which produces the spots of a given
sun-spot period first manifests itself in two belts about thirty degrees
north and south of the Sun’s equator. These belts then draw in toward
the equator, and the sun-spot maximum occurs when their latitude is
about sixteen degrees; while the disturbance gradually and finally dies
out at a latitude of eight or ten degrees. Two or three years before
this disappearance, however, two new zones of disturbance show
themselves. Thus, at the sun-spot minimum there are four well-marked
spot-belts,—two near the equator, due to the expiring disturbance, and
two in high latitudes, due to the newly beginning outbreak.” These
remarkable discoveries, which resulted from the investigations of
Schwabe, Carrington, and Sporer, are a brilliant example of what may be
done by amateurs in astronomy.
At the time when Carrington and Spörer were pursuing these researches,
the spectroscope came into use as an astronomical instrument, and since
1859 solar astronomy has been almost entirely spectroscopic. Before we
can rightly understand the principles of spectroscopic astronomy, we
must go back to the life and work of its founder—Joseph von Fraunhofer.
The son of a poor glazier, _Joseph von Fraunhofer_ was born on March 6,
1787, at Straubing, in Bavaria. His father and mother having died when
their son was quite young, the boy, on account of his poverty, was
apprenticed to a looking-glass manufacturer in Munich named
Weichselberger, who acted tyrannically, keeping him all day at hard
work. Still the lad borrowed some old books, and spent his nights in
study. Young Fraunhofer lodged in an old tenement in Munich, which on
July 21, 1801, collapsed, burying in its ruins its occupants. All were
killed but Fraunhofer, who, though seriously injured, was dug out from
the ruins four hours later. The distressing accident was witnessed by
Prince Maximilian Joseph, Elector of Bavaria. He became interested in
Fraunhofer, and presented him with a sum of money. Of this he made good
use. He was already interested in optics, and he bought some books on
that subject, as well as a glass-polishing machine. The remainder of the
money served to procure his release from his tyrannical master,
Weichselberger.
Fraunhofer became acquainted with prominent scientists at Munich, who
provided him with books on optics and mathematics. Meanwhile the young
optician occupied his time in shaping and finishing lenses. In 1806 he
entered the optical department of the Optical and Physical Institute of
Munich, and the following year, when only twenty years of age, was
appointed to the chief post in that department. In 1814 he commenced his
investigations with the prism, which have made his name famous.
Newton had found that, in passing through a prism, white light is
dispersed into its primary colours, making up the band of coloured light
known as the solar spectrum. But he failed to recognise the existence of
dark lines in the spectrum. Casually seen in 1802 by _William Hyde
Wollaston_ (1786-1828), an English physicist, these lines were first
thoroughly examined by Fraunhofer. Allowing light from the Sun to pass
through a prism attached to the telescope, he was amazed to find several
dark lines in the spectrum. By the year 1814 he had detected no less
than 300 or 400 of these lines. Fraunhofer named the more prominent
lines by the letters of the alphabet, from A in the red to H in the
violet. They are now known as the Fraunhofer lines. At first he was much
perplexed regarding the nature of the dark lines. He suspected that they
might be an optical effect, depending on the quality of the glass used,
and he tried different prisms, but the lines were still to be seen. Then
he turned his prism to bright clouds to see if they were visible in
reflected sunlight, and he found that they were. He examined the Moon
and again perceived them, as moonlight is merely reflected sunlight; and
they were also conspicuous in the spectra of the planets. It was thus
proved that these lines were characteristic of sunlight, whether direct
or reflected. It was, however, still possible that they might be caused
by the passage of the rays of light from the celestial bodies through
the Earth’s atmosphere. In order to test this theory, Fraunhofer
examined the spectra of the brighter stars. He found that the lines
visible in the solar spectrum were not to be seen in the spectra of the
stars, thus proving that the lines were not merely an atmospheric
effect. Each star, Fraunhofer observed, had a different spectrum from
both the Sun and from other stars. These spectra were also characterised
by numerous dark lines, much fainter than those in the solar spectrum.
Although he ascertained the existence of the dark lines in the Sun’s
spectrum, Fraunhofer never really found out what they represented. As
Miss Giberne expresses it, “Although he now saw the lines he could not
understand them: he could not read what they said. They spoke to him
indeed about the Sun, but they spoke to him in a foreign language, the
key to which he did not possess.” However, he expressed the belief that
the pair of lines in the solar spectrum, which he marked D, coincided
with the pair of bright lines emitted by incandescent sodium. Although
he doubtless suspected that the lines conveyed intelligence regarding
the elements in the Sun, he never was able properly to decipher their
meaning. Had he lived, he would probably have made the great discovery;
but these investigations were cut short by his sudden and untimely death
on June 7, 1826.
After the death of Fraunhofer, very little was done to forward the study
of spectrum analysis. Investigations in this branch of research were
made, however, by Sir _John Herschel_ (1792-1871), _William Allen
Miller_ (1817-1870), Sir _David Brewster_ (1781-1868), and others. Two
famous men of science had partly discovered the secret. These were Sir
_George Stokes_ (1819-1903), of Cambridge, and _Anders John Angström_
(1812-1872) of Upsala. Of Angström’s work, published in 1853, it has
been said that it would “have obtained a high celebrity if it had
appeared in French, English, or German, instead of Swedish.”
It was not until 1859 that the principles of spectrum analysis were
fully enunciated by _Gustav Robert Kirchhoff_ (1824-1887), and his
colleague in the University of Heidelberg, _Robert Wilhelm Bunsen_
(1811-1899). Kirchhoff demonstrated that a luminous solid or liquid
gives a continuous spectrum, and a gaseous substance a spectrum of
bright lines. In the words of Miss Clerke, “Substances of every kind are
opaque to the precise rays which they emit at the same temperature. That
is to say, they stop the kinds of light or heat which they are then
actually in a condition to radiate.... This principle is fundamental to
solar chemistry. It gives the key to the hieroglyphics of the Fraunhofer
lines. The identical characters which are written bright in terrestrial
spectra are written dark in the unrolled sheaf of sun-rays.” Kirchhoff
made several determinations of the substances in the Sun, proving the
existence of sodium, iron, calcium, magnesium, nickel, barium, copper,
and zinc. His great map of the solar spectrum was published by the
Berlin Academy in 1860, and represented an enormous amount of labour. It
was succeeded by another map by Angström, published in 1868. But both of
these maps have been recently superseded by the investigations of Sir
_Joseph Norman Lockyer_ (born 1836), and of the American physicist,
_Henry Augustus Rowland_ (1848-1901). Rowland largely increased our
knowledge of the elements in the solar atmosphere.
The spectroscope had become, by 1868, a recognised instrument of
astronomical research, and in that year it was applied during the famous
total eclipse, visible in India. There were many eclipse problems,
arising from the observations made by the eclipse expeditions of 1842,
1851, and 1860. The eclipse of 1851 had finally proved that the red
flames seen surrounding the Sun during total eclipses belonged to the
Sun, and not to the Moon, as many astronomers had believed. At the
eclipse of 1860, visible in Spain, the Italian astronomer, _Angelo
Secchi_ (1818-1878), and the Englishman, _Warren De la Rue_ (1815-1889),
secured photographs of the solar prominences. The problem of 1868 was
the constitution of these prominences.
_Pierre Jules César Janssen_, born in Paris in 1824, was stationed at
Guntoor, in India, to observe the eclipse. He succeeded in observing the
spectrum of the prominences during the progress of totality, and found
it to be one of bright lines, proving the gaseous nature of the
sun-flames. During the progress of the eclipse, Janssen was specially
struck by the brilliancy of the bright lines, and it occurred to him
that the prominence-spectrum could be observed in full daylight, if
sufficient dispersive power was used to enfeeble the ordinary continuous
spectrum. At ten o’clock on the following morning, August 19, 1868,
Janssen applied his spectroscope to the sun, and observed the
prominence-spectrum. After a month’s observation in India, he sent to
the French Academy an account of his success. A short time, however,
before his report arrived, the Academy had received a similar one from
Lockyer, who had independently made the same discovery. Two years
previously, in 1866, the new method had occurred to him, but his
spectroscope was not powerful enough; and although he ordered a more
powerful one at once, it was not until October 16, 1868, that he had the
instrument in his hands. Four days later he observed the
prominence-spectrum in full daylight.
The next advance in the study of the prominences was announced in 1869.
Janssen and Lockyer had shown astronomers how to observe the spectrum of
the prominences; but the researches of other two famous astronomers
enabled observers to see the forms of the prominences. These two men
were _William Huggins_ (born 1824) and _Johann Carl Friedrich Zöllner_.
The latter astronomer, born in Leipzig in 1834, was one of the most
successful students of the solar prominences. He was Professor of
Astrophysics in the University of Leipzig, a position which he filled
with success until his untimely death on April 25, 1882. Independently
of Huggins, he found that by opening the slit of the spectroscope wider,
the forms of the prominences themselves could be seen. The study of the
prominences was at once taken up by the most famous solar observers:
these were Huggins and Lockyer in England, Spörer and Zöllner in
Germany, Janssen in France, Secchi, Respighi, and Tacchini in Italy,
Young in America. To _Charles Augustus Young_ (born 1834) we owe the
careful study of individual prominences. On September 7, 1871, he
observed the most gigantic outburst on the sun ever witnessed, fragments
of an exploded prominence reaching a height of 100,000 miles: Young,
also, made the first attempt to photograph the prominences.
To the Italian school of astronomers, however, we owe the persistent and
systematic study of the prominences. Among them the three greatest names
are _Angelo Secchi_ (1818-1878), _Lorenzo Respighi_ (1824-1889), and
_Pietro Tacchini_ (1838-1905). After the death of Secchi, the recognised
head of spectroscopy in Italy was Pietro Tacchini. Born at Modena in
1838, he was appointed director at Modena in 1859, assistant at Palermo
in 1863, and director at Rome in 1879. In 1870 he commenced to take
daily observations of the prominences, noting their sizes, forms, and
distribution, and these observations were continued for thirty-one
years, until within four years of Tacchini’s death, which took place on
March 24, 1905. Tacchini did for the study of the prominences what
Schwabe did for the spots. The Italian spectroscopists found that the
prominences increased and decreased every eleven years in harmony with
the spots. Tacchini demonstrated that the streamers of the solar corona
originate in regions where the prominences are most numerous, and that
the shape of the corona, on the whole, varies in sympathy with the
prominences.
The researches of Lockyer indicated that the prominences originated in a
shallow gaseous atmosphere which he termed the chromosphere. Formerly
astronomers had to observe only isolated prominences, but in 1892 an
American astronomer, _George Ellery Hale_ (born 1868), formerly director
of the Yerkes Observatory, and now director of the Solar Observatory in
California, succeeded in photographing, by an ingenious process, the
whole of the chromosphere, prominences, and faculæ visible on the solar
surface.
Another solar envelope was discovered in 1870 by Dr Charles Augustus
Young, who from 1866 to 1877 directed the Observatory at Dartmouth, New
Hampshire, and from 1877 to 1905, that at Princeton, New Jersey. During
the eclipse of December 22, 1870, Young was stationed at Tenez de
Frontena, Spain. As the solar crescent grew apparently thinner before
the disc of the Moon, “the dark lines of the spectrum,” he says, “and
the spectrum itself gradually faded away, until all at once, as suddenly
as a bursting rocket shoots out its stars, the whole field of view was
filled with bright lines, more numerous than one could count. The
phenomenon was so sudden, so unexpected, and so wonderfully beautiful,
as to force an involuntary exclamation.” The phenomenon was observed for
two seconds, and the impression was left on the astronomer that a bright
line had taken the place of every dark one in the solar spectrum, the
spectrum being completely reversed. Hence the name which was given to
the hypothetical envelope—“the reversing layer.” For long the existence
of the reversing layer was disputed by numerous astronomers. In 1896
photographs taken during the solar eclipse of that year finally
demonstrated the existence of the “flash spectrum” as seen by Young.
The last of the solar appendages, the corona, can only be seen during
total eclipses. The researches of Young and Janssen indicate that it is
partly gaseous and partly meteoric in its constitution; and various
photographs, taken at the eclipses since 1870, have demonstrated its
variation in shape, which is in harmony with the eleven-year period.
Several attempts have been made to observe the corona without an
eclipse. In 1882 Huggins made the attempt, but failed, and Hale, with
his photographic process, had no better success. More recently, in 1904,
a Russian astronomer, _Alexis Hansky_, observing from the top of Mont
Blanc, secured some photographs on which he believes the corona is
represented, but so far his observations have not been confirmed by
other astronomers.
The application of the spectroscope to the motions on the solar surface
is perhaps one of the most wonderful triumphs in astronomical science.
In 1842 _Christian Doppler_ (1803-1853), Professor of Mathematics at
Prague, had expressed the view that the colour of a luminous body must
be changed by its motion of approach or recession. It was obvious to
Doppler that if the body was approaching, a larger number of light waves
must be entering the eye of the observer than if it were retreating.
Miss Clerke thus illustrates Doppler’s principle: “Suppose shots to be
fired at a target at fixed intervals of time. If the marksman advances,
say, twenty paces between each discharge of his rifle, it is evident
that the shots will fall faster on the target than if he stood still;
if, on the contrary, he retires by the same amount, they will strike at
correspondingly longer intervals.” It occurred to various astronomers
that it would be possible to measure cyclones and hurricanes in the Sun,
not by change of colour in the spectrum, but by the shifting of the
lines; and in 1870 this was successfully done by Lockyer. In the next
few years efforts to measure the solar rotation were made by Young,
Zöllner, and others, who succeeded in measuring the displacement of the
lines, but not the time of rotation. This was reserved for the famous
Swedish astronomer, Dunér.
_Nils Christopher Dunér_, born in 1839 in Scania, was employed as an
assistant at Lund Observatory from 1858 to 1888, when he was appointed
director of the Observatory at Upsala. In that year he commenced a study
of the solar rotation, measuring it by means of Doppler’s principle. He
confirmed the telescopic work of Carrington and Spörer on the equatorial
acceleration, and measured the displacement up to within fifteen degrees
of the poles. He brought out the surprising fact that the rotation
period of the Sun is there protracted to 38½ days. These remarkable
researches were published in 1891.
In 1873 the Astronomer-Royal of England commenced at Greenwich
Observatory to photograph the Sun daily. This work has been carried on
there by _Edward Walter Maunder_ (born 1851), and Greenwich Observatory
possesses a photographic record of sun-spots. At the Meudon
Astrophysical Observatory, near Paris, Janssen has, since 1876, secured
photographs of the solar surface, which were comprised in a great atlas,
published by him in January 1904. These photographs have revealed a
remarkable phenomenon—the “réseau photospherique,” the distribution over
the solar surface of blurred patches of light, which Janssen considers
are inherent in the Sun. The Greenwich records of sun-spots and of
magnetic disturbances have been made use of by Maunder in his remarkable
studies, promulgated in 1904, of the connection between sun-spots and
terrestrial magnetism. Maunder finds that on the average magnetic storms
are dependent on the presence of sun-spots, and on the size of the spot.
The magnetic action, he finds, does not radiate equally in all
directions from the sun-spots, but along definite and restricted lines.
Herschel’s hypothesis of a dark and cool globe beneath the solar
photosphere was seen to be untenable after the introduction of the
spectroscope. The first important theory as to the solar constitution
was that advanced in 1865 by the French astronomer, _Hervé Faye_
(1814-1902). Numerous other theories were afterwards advanced by Secchi,
Zöllner, Young, and others, but a complete description of the various
developments in solar theorising cannot be given here. There is no
complete “theory” of the exact constitution of every part of the Sun,
but the unpretentious “Views of Professor Young on the Constitution of
the Sun,” which appeared in April 1904 in ‘Popular Astronomy,’ represent
the latest ideas of the foremost solar investigator. Professor Young
regards the reversing layer and the chromosphere as “simply the
uncondensed vapours and gases which form the atmosphere in which the
clouds of the photosphere are suspended.” He says that the contraction
theory of Helmholtz,—explained in another chapter,—advanced to explain
the maintenance of the Sun’s heat, is true so far as it goes; but that
it is all the truth is now made doubtful by the discovery of radium,
which “suggests that other powerful sources of energy may co-operate
with the mechanical in maintaining the Sun’s heat.”
The important question of the distance of the Sun was thoroughly
investigated in 1824 by _Johann Franz Encke_ (1791-1865), then of
Seeberg, near Gotha, who, from a discussion of the transits of Venus in
1761 and 1769, found a parallax of 8″·571, corresponding to a mean
distance of 95,000,000 miles. This value was accepted for thirty years,
until _Peter Andreas Hansen_ (1795-1874), in 1854, and _Urban Jean
Joseph Le Verrier_ (1811-1877), in 1858, found from mathematical
investigations that the distance indicated was too great. Preparations
were accordingly made for the observation of the transits of Venus,
which took place respectively on December 8, 1874, and December 6, 1882.
On the first occasion many expeditions were sent to view the transit,
consisting of French, German, American, English, Scottish, Italian,
Russian, and Dutch astronomers, and it was hoped that the solar parallax
would be accurately measured once for all. However, the transit,
although favoured with good weather, was not successful, owing to the
difficulty of making exact measurements, by reason of the illumination
and refraction in the atmosphere of Venus. Accordingly the values
deduced for the parallax were far from unanimous. The transit of 1882
was not observed so extensively, as astronomers had found the transit of
Venus to be by no means the best method. In 1877 Sir _David Gill_ (born
1843), the great Scottish astronomer, determined the solar parallax
successfully from measures of the parallax of Mars in opposition. His
value was 8″·78, corresponding to 93,080,000 miles. Some years previous
to this _Johann Gottfried Galle_ (born 1812), the German astronomer,
had, from measurements of the parallax of the asteroid Flora, deduced a
solar parallax of 8″·87. Gill’s work at the Cape in 1888, on the
Asteroids, was successful in giving a more accurate value than the
transit of Venus: in 1900 and 1901 measures of the parallax of the
asteroid Eros, the nearest minor planet, were made by many different
observatories, and agree with the other results. The values which have
been derived from the velocity of light, and from the constant of
aberration, are fairly in agreement with those derived from direct
measurement. On the whole, the most probable value of the parallax is
about 8″·8, indicating a mean distance of about 92,700,000 miles, with a
“probable error” of about 150,000 miles.
What a different picture the sun presents to us at the beginning of the
twentieth century from that which it presented to Herschel and his
contemporaries at the beginning of the nineteenth! To Herschel, the Sun
was a cool dark globe, surrounded by a luminous atmosphere. As the
outcome of the researches and discoveries outlined in this chapter, the
Sun is now seen to be a vast central world, which is over a million
times larger than the Earth. In the words of an able writer, “It is most
probably a world of gases, where most of the metals and metallic gases
that we know exist only as vapours, even at the Sun’s surface, hotter
than any furnace on earth, and getting a still fiercer heat for every
mile of descent lower. Of that heat in the Sun’s interior we can form no
conception. The pressure within the Sun is equally inconceivable. A
cannon-ball weighing 100 lb. on earth would weigh 2700 on the Sun. Thus
a mighty conflict goes on unceasingly between imprisoned and expanding
gases and vapours struggling to burst out, and massive pressures holding
them down. For reasons we cannot fully understand, no equilibrium is
reached. For millions of years up-rushes and down-rushes of the
white-hot materials have been proceeding on that bright photosphere
which gives us light, and looks a picture of calm and quiescence. Above
that is a comparatively thin rose-coloured layer, the chromosphere,
agitated with fiery ‘prominences,’ and outside all these the coronal
glory—all alike pointing to immeasurable activities.”
The following remark of Professor Newcomb shows our inability to realise
the solar activity. “Suppose,” he says, “every foot of space in a whole
country covered with 13-inch cannon, all pointed upward, and all
discharged at once. The result would compare with what is going on
inside the photosphere about as much as a boy’s popgun compares with the
cannon.”