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Discovery of the Sun

Posted on July 23rd, 2007 by Admin.
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One of Galileo’s most striking discoveries, when he pointed his
telescope to the heavenly bodies, was that of the irregularly shaped
spots on the sun, with the dark central _umbra_ and the less
dark, but more extensive, _penumbra_ surrounding it, sometimes
with several umbrae in one penumbra. He has left us many drawings of
these spots, and he fixed their period of rotation as a lunar month.

[Illustration: SOLAR SURFACE, As Photographed at the Royal
Observatory, Greenwich, showing sun-spots with umbrae, penumbrae, and
faculae.]

It is not certain whether Galileo, Fabricius, or Schemer was the first
to see the spots. They all did good work. The spots were found to be
ever varying in size and shape. Sometimes, when a spot disappears at
the western limb of the sun, it is never seen again. In other cases,
after a fortnight, it reappears at the eastern limb. The faculae, or
bright areas, which are seen all over the sun’s surface, but specially
in the neighbourhood of spots, and most distinctly near the sun’s
edge, were discovered by Galileo. A high telescopic power resolves
their structure into an appearance like willow-leaves, or rice-grains,
fairly uniform in size, and more marked than on other parts of the
sun’s surface.

Speculations as to the cause of sun-spots have never ceased from
Galileo’s time to ours. He supposed them to be clouds. Scheiner[1]
said they were the indications of tumultuous movements occasionally
agitating the ocean of liquid fire of which he supposed the sun to be
composed.

A. Wilson, of Glasgow, in 1769,[2] noticed a movement of the umbra
relative to the penumbra in the transit of the spot over the sun’s
surface; exactly as if the spot were a hollow, with a black base and
grey shelving sides. This was generally accepted, but later
investigations have contradicted its universality. Regarding the cause
of these hollows, Wilson said:–

  Whether their first production and subsequent numberless changes
  depend upon the eructation of elastic vapours from below, or upon
  eddies or whirlpools commencing at the surface, or upon the
  dissolving of the luminous matter in the solar atmosphere, as clouds
  are melted and again given out by our air; or, if the reader
  pleases, upon the annihilation and reproduction of parts of this
  resplendent covering, is left for theory to guess at.[3]

Ever since that date theory has been guessing at it. The solar
astronomer is still applying all the instruments of modern research to
find out which of these suppositions, or what modification of any of
them, is nearest the truth. The obstacle–one that is perhaps fatal to
a real theory–lies in the impossibility of reproducing comparative
experiments in our laboratories or in our atmosphere.

Sir William Herschel propounded an explanation of Wilson’s observation
which received much notice, but which, out of respect for his memory,
is not now described, as it violated the elementary laws of heat.

Sir John Herschel noticed that the spots are mostly confined to two
zones extending to about 35 degrees on each side of the equator, and that a
zone of equatoreal calms is free from spots. But it was
R. C. Carrington[4] who, by his continuous observations at Redhill, in
Surrey, established the remarkable fact that, while the rotation
period in the highest latitudes, 50 degrees, where spots are seen, is
twenty-seven-and-a-half days, near the equator the period is only
twenty-five days. His splendid volume of observations of the sun led
to much new information about the average distribution of spots at
different epochs.

Schwabe, of Dessau, began in 1826 to study the solar surface, and,
after many years of work, arrived at a law of frequency which has been
more fruitful of results than any discovery in solar physics.[5] In
1843 he announced a decennial period of maxima and minima of sun-spot
displays. In 1851 it was generally accepted, and, although a period of
eleven years has been found to be more exact, all later observations,
besides the earlier ones which have been hunted up for the purpose, go
to establish a true periodicity in the number of sun-spots. But quite
lately Schuster[6] has given reasons for admitting a number of
co-existent periods, of which the eleven-year period was predominant
in the nineteenth century.

In 1851 Lament, a Scotchman at Munich, found a decennial period in the
daily range of magnetic declination. In 1852 Sir Edward Sabine
announced a similar period in the number of “magnetic storms”
affecting all of the three magnetic elements–declination, dip, and
intensity. Australian and Canadian observations both showed the
decennial period in all three elements. Wolf, of Zurich, and Gauthier,
of Geneva, each independently arrived at the same conclusion.

It took many years before this coincidence was accepted as certainly
more than an accident by the old-fashioned astronomers, who want rigid
proof for every new theory. But the last doubts have long vanished,
and a connection has been further traced between violent outbursts of
solar activity and simultaneous magnetic storms.

The frequency of the Aurora Borealis was found by Wolf to follow the
same period. In fact, it is closely allied in its cause to terrestrial
magnetism. Wolf also collected old observations tracing the
periodicity of sun-spots back to about 1700 A.D.

Spoerer deduced a law of dependence of the average latitude of
sun-spots on the phase of the sun-spot period.

All modern total solar eclipse observations seem to show that the
shape of the luminous corona surrounding the moon at the moment of
totality has a special distinct character during the time of a
sun-spot maximum, and another, totally different, during a sun-spot
minimum.

A suspicion is entertained that the total quantity of heat received by
the earth from the sun is subject to the same period. This would have
far-reaching effects on storms, harvests, vintages, floods, and
droughts; but it is not safe to draw conclusions of this kind except
from a very long period of observations.

Solar photography has deprived astronomers of the type of Carrington
of the delight in devoting a life’s work to collecting data. It has
now become part of the routine work of an observatory.

In 1845 Foucault and Fizeau took a daguerreotype photograph of the
sun. In 1850 Bond produced one of the moon of great beauty, Draper
having made some attempts at an even earlier date. But astronomical
photography really owes its beginning to De la Rue, who used the
collodion process for the moon in 1853, and constructed the Kew
photoheliograph in 1857, from which date these instruments have been
multiplied, and have given us an accurate record of the sun’s surface.
Gelatine dry plates were first used by Huggins in 1876.

It is noteworthy that from the outset De la Rue recognised the value
of stereoscopic vision, which is now known to be of supreme
accuracy. In 1853 he combined pairs of photographs of the moon in the
same phase, but under different conditions regarding libration,
showing the moon from slightly different points of view. These in the
stereoscope exhibited all the relief resulting from binocular vision,
and looked like a solid globe. In 1860 he used successive photographs
of the total solar eclipse stereoscopically, to prove that the red
prominences belong to the sun, and not to the moon. In 1861 he
similarly combined two photographs of a sun-spot, the perspective
effect showing the umbra like a floor at the bottom of a hollow
penumbra; and in one case the faculae were discovered to be sailing
over a spot apparently at some considerable height. These appearances
may be partly due to a proper motion; but, so far as it went, this was
a beautiful confirmation of Wilson’s discovery. Hewlett, however, in
1894, after thirty years of work, showed that the spots are not always
depressions, being very subject to disturbance.

The Kew photographs [7] contributed a vast amount of information about
sun-spots, and they showed that the faculae generally follow the spots
in their rotation round the sun.

The constitution of the sun’s photosphere, the layer which is the
principal light-source on the sun, has always been a subject of great
interest; and much was done by men with exceptionally keen eyesight,
like Mr. Dawes. But it was a difficult subject, owing to the rapidity
of the changes in appearance of the so-called rice-grains, about 1″ in
diameter. The rapid transformations and circulations of these
rice-grains, if thoroughly studied, might lead to a much better
knowledge of solar physics. This seemed almost hopeless, as it was
found impossible to identify any “rice-grain” in the turmoil after a
few minutes. But M. Hansky, of Pulkowa (whose recent death is
deplored), introduced successfully a scheme of photography, which
might almost be called a solar cinematograph. He took photographs of
the sun at intervals of fifteen or thirty seconds, and then enlarged
selected portions of these two hundred times, giving a picture
corresponding to a solar disc of six metres diameter. In these
enlarged pictures he was able to trace the movements, and changes of
shape and brightness, of individual rice-grains. Some granules become
larger or smaller. Some seem to rise out of a mist, as it were, and to
become clearer. Others grow feebler. Some are split in two. Some are
rotated through a right angle in a minute or less, although each of
the grains may be the size of Great Britain. Generally they move
together in groups of very various velocities, up to forty kilometres
a second. These movements seem to have definite relation to any
sun-spots in the neighbourhood. From the results already obtained it
seems certain that, if this method of observation be continued, it
cannot fail to supply facts of the greatest importance.

It is quite impossible to do justice here to the work of all those who
are engaged on astronomical physics. The utmost that can be attempted
is to give a fair idea of the directions of human thought and
endeavour. During the last half-century America has made splendid
progress, and an entirely new process of studying the photosphere has
been independently perfected by Professor Hale at Chicago, and
Deslandres at Paris.[8] They have succeeded in photographing the sun’s
surface in monochromatic light, such as the light given off as one of
the bright lines of hydrogen or of calcium, by means of the
“Spectroheliograph.” The spectroscope is placed with its slit in the
focus of an equatoreal telescope, pointed to the sun, so that the
circular image of the sun falls on the slit. At the other end of the
spectroscope is the photographic plate. Just in front of this plate
there is another slit parallel to the first, in the position where the
image of the first slit formed by the K line of calcium falls. Thus is
obtained a photograph of the section of the sun, made by the first
slit, only in K light. As the image of the sun passes over the first
slit the photographic plate is moved at the same rate and in the same
direction behind the second slit; and as successive sections of the
sun’s image in the equatoreal enter the apparatus, so are these
sections successively thrown in their proper place on the photographic
plate, always in K light. By using a high dispersion the faculae which
give off K light can be correctly photographed, not only at the sun’s
edge, but all over his surface. The actual mechanical method of
carrying out the observation is not quite so simple as what is here
described.

By choosing another line of the spectrum instead of calcium K–for
example, the hydrogen line H(3)–we obtain two photographs, one
showing the appearance of the calcium floculi, and the other of the
hydrogen floculi, on the same part of the solar surface; and nothing
is more astonishing than to note the total want of resemblance in the
forms shown on the two. This mode of research promises to afford many
new and useful data.

The spectroscope has revealed the fact that, broadly speaking, the sun
is composed of the same materials as the earth. Angstrom was the first
to map out all of the lines to be found in the solar spectrum. But
Rowland, of Baltimore, after having perfected the art of making true
gratings with equidistant lines ruled on metal for producing spectra,
then proceeded to make a map of the solar spectrum on a large scale.

In 1866 Lockyer[9] threw an image of the sun upon the slit of a
spectroscope, and was thus enabled to compare the spectrum of a spot
with that of the general solar surface. The observation proved the
darkness of a spot to be caused by increased absorption of light, not
only in the dark lines, which are widened, but over the entire
spectrum. In 1883 Young resolved this continuous obscurity into an
infinite number of fine lines, which have all been traced in a shadowy
way on to the general solar surface. Lockyer also detected
displacements of the spectrum lines in the spots, such as would be
produced by a rapid motion in the line of sight. It has been found
that both uprushes and downrushes occur, but there is no marked
predominance of either in a sun-spot. The velocity of motion thus
indicated in the line of sight sometimes appears to amount to 320
miles a second. But it must be remembered that pressure of a gas has
some effect in displacing the spectral lines. So we must go on,
collecting data, until a time comes when the meaning of all the facts
can be made clear.

_Total Solar Eclipses_.–During total solar eclipses the time is so
short, and the circumstances so impressive, that drawings of the
appearance could not always be trusted. The red prominences of jagged
form that are seen round the moon’s edge, and the corona with its
streamers radiating or interlacing, have much detail that can hardly
be recorded in a sketch. By the aid of photography a number of records
can be taken during the progress of totality. From a study of these
the extent of the corona is demonstrated in one case to extend to at
least six diameters of the moon, though the eye has traced it
farther. This corona is still one of the wonders of astronomy, and
leads to many questions. What is its consistency, if it extends many
million miles from the sun’s surface? How is it that it opposed no
resistance to the motion of comets which have almost grazed the sun’s
surface? Is this the origin of the zodiacal light? The character of
the corona in photographic records has been shown to depend upon the
phase of the sun-spot period. During the sun-spot maximum the corona
seems most developed over the spot-zones–i.e., neither at the
equator nor the poles. The four great sheaves of light give it a
square appearance, and are made up of rays or plumes, delicate like
the petals of a flower. During a minimum the nebulous ring seems to
be made of tufts of fine hairs with aigrettes or radiations from both
poles, and streamers from the equator.

[Illustration: SOLAR ECLIPSE, 1882. From drawing by W. H. Wesley,
Secretary R.A.S.; showing the prominences, the corona, and an unknown
comet.]

On September 19th, 1868, eclipse spectroscopy began with the Indian
eclipse, in which all observers found that the red prominences showed
a bright line spectrum, indicating the presence of hydrogen and other
gases. So bright was it that Jansen exclaimed: “_Je verrai ces
lignes-la en dehors des eclipses_.” And the next day he observed the
lines at the edge of the uneclipsed sun. Huggins had suggested this
observation in February, 1868, his idea being to use prisms of such
great dispersive power that the continuous spectrum reflected by our
atmosphere should be greatly weakened, while a bright line would
suffer no diminution by the high dispersion. On October 20th
Lockyer,[10] having news of the eclipse, but not of Jansen’s
observations the day after, was able to see these lines. This was a
splendid performance, for it enabled the prominences to be observed,
not only during eclipses, but every day. Moreover, the next year
Huggins was able, by using a wide slit, to see the whole of a
prominence and note its shape. Prominences are classified, according
to their form, into “flame” and “cloud” prominences, the spectrum of
the latter showing calcium, hydrogen, and helium; that of the former
including a number of metals.

The D line of sodium is a double line, and in the same eclipse (1868)
an orange line was noticed which was afterwards found to lie close to
the two components of the D line. It did not correspond with any known
terrestrial element, and the unknown element was called “helium.” It
was not until 1895 that Sir William Ramsay found this element as a gas
in the mineral cleavite.

The spectrum of the corona is partly continuous, indicating light
reflected from the sun’s body. But it also shows a green line
corresponding with no known terrestrial element, and the name
“coronium” has been given to the substance causing it.

A vast number of facts have been added to our knowledge about the sun
by photography and the spectroscope. Speculations and hypotheses in
plenty have been offered, but it may be long before we have a complete
theory evolved to explain all the phenomena of the storm-swept
metallic atmosphere of the sun.

The proceedings of scientific societies teem with such facts and
“working hypotheses,” and the best of them have been collected by Miss
Clerke in her _History of Astronomy during the Nineteenth Century_. As
to established facts, we learn from the spectroscopic researches (1)
that the continuous spectrum is derived from the _photosphere_ or
solar gaseous material compressed almost to liquid consistency; (2)
that the _reversing layer_ surrounds it and gives rise to black
lines in the spectrum; that the _chromosphere_ surrounds this, is
composed mainly of hydrogen, and is the cause of the red prominences
in eclipses; and that the gaseous _corona_ surrounds all of
these, and extends to vast distances outside the sun’s visible
surface.

FOOTNOTES:

[1] _Rosa Ursina_, by C. Scheiner, _fol_.; Bracciani, 1630.

[2] _R. S. Phil. Trans_., 1774.

[3] _Ibid_, 1783.

[4] _Observations on the Spots on the Sun, etc.,_ 4 degrees; London and
Edinburgh, 1863.

[5] _Periodicitat der Sonnenflecken. Astron. Nach. XXI._, 1844,
P. 234.

[6] _R.S. Phil. Trans._ (ser. A), 1906, p. 69-100.

[7] “Researches on Solar Physics,” by De la Rue, Stewart and Loewy;
_R. S. Phil. Trans_., 1869, 1870.

[8] “The Sun as Photographed on the K line”; _Knowledge_, London,
1903, p. 229.

[9] _R. S. Proc._, xv., 1867, p. 256.

[10] _Acad. des Sc._, Paris; _C. R._, lxvii., 1868, p. 121.