In the Annuaire of the _Bureau des Longitudes_, recently published in
Paris, appears a paper by the distinguished astronomer Arago--'On the
Observations which have made known the Physical Constitution of the
Sun and of different Stars; and an Inquiry into the Conjectures of the
Ancient Philosophers, and of the Positive Ideas of Modern Astronomers
on the Place that the Sun ought to occupy among the Prodigious Number
of Stars which stud the Firmament'--in which all that appertains to
the subject is so ably condensed, as to afford material for a popular
summary, which we purpose to convey in the present article. The
eclipse of the sun of last July, by enabling observers to repeat
former observations and test their accuracy, furnished some of the
results which serve to complete the paper in question, and which may
be considered as settled, owing to the improvements continually taking
place in the construction of instruments. Although astronomy is the
exactest of sciences, its problems are not yet all fully solved; and
for the determination of some of these, observers have to wait for
years--in certain instances, for a century or more, until all the
circumstances combine for a favourable observation. From the days of
the Epicurean philosopher, who, judging from appearances, declared the
sun to be no more than a foot in diameter, to those of living
calculators, who give to the orb a diameter of 883,000 miles, there
has been a marvellous advance. In these dimensions, we have a sphere
one million four hundred thousand times larger than the earth.
'Numbers so enormous,' says M. Arago, 'not being often employed in
ordinary life, and giving us no very precise idea of the magnitudes
which they imply, I recall here a remark that will convey a better
understanding of the immensity of the solar volume. If we imagine the
centre of the sun to coincide with that of the earth, its surface
would not only reach the region in which the moon revolves, but would
extend nearly as far again beyond.' By the transit of Venus in 1769,
it was demonstrated that the sun is 95,000,000 miles from the earth;
and yet, distant as it is, its physical constitution has been
determined; and the history of the successive steps by which this
proof has been arrived at, forms one of the most interesting chapters
in the progress of science.
It was in 1611 that Fabricius, a Dutch astronomer, first observed
spots on the eastern edge of the sun, which passed slowly across the
disk to the western edge, and disappeared after a certain number of
days. This phenomenon having been often noted subsequently, the
conclusion drawn therefrom is, that the sun is a spherical body,
having a movement of rotation about its centre, of which the duration
is equal to twenty-five days and a half. These dark spots, irregular
and variable, but well defined on their edge, are sometimes of
considerable dimensions. Some have been seen whose size was five times
that of the earth. They are generally surrounded by an aureola known
as the _penumbra_, and sensibly less luminous than the other portions
of the orb. From this penumbra, first observed by Galileo, many
apparently singular deductions have been made: namely, 'The sun is a
dark body, surrounded at a certain distance by an atmosphere which may
be compared to that of the earth, when the latter is charged with a
continuous stratum of opaque and reflecting clouds. To this first
atmosphere succeeds a second, luminous in itself, called the
_photosphere_. This photosphere, more or less remote from the inner
cloudy atmosphere, would determine by its outline the visible limits
of the orb. According to this hypothesis, there would he spots on the
sun every time that there occurred in the two concentric atmospheres
such corresponding clear spaces as would allow of our seeing the dark
central body uncovered.'
This hypothesis is considered by the most competent judges to render a
very satisfactory account of the facts. But it has not been
universally adopted. Some writers of authority have lately represented
the spots as scoriæ floating on a liquid surface, and ejected from
solar volcanoes, of which the burning mountains of the earth convey
but a feeble idea. Hence observations become necessary as to the
nature of the incandescent matter of the sun; and when we remember the
immense distance of that body, such an attempt may well appear to be
one of temerity.
The progress of optical science, however, has given us the means of
determining this apparently insoluble question. It is well known, that
physicists are enabled at present to distinguish two kinds of
light--natural light and polarised light. A ray of the former exhibits
the same properties on any part of its form; not so the latter. A
polarised ray is said to have sides, and the different sides have
different properties, as demonstrated by many interesting phenomena.
Strange as it may seem, these rays thus described as having sides,
could pass through the eye of a needle by hundreds of thousands
without disturbing each other. Availing themselves, therefore, of the
assistance of polarised light, and an instrument named the
polariscope, or polarising telescope, observers obtain a double image
of the sun, both alike, and both white; but on reflecting this image
on water, or a glass mirror, the rays become polarised; the two images
are no longer alike or white, but are intensely coloured, while their
form remains unchanged. If one is red, the other is green, or yellow
and violet, always producing what are called the complementary
colours. With this instrument, it becomes possible to tell the
difference between natural and polarised light.
Another point for consideration is, that for a long time it was
supposed, that the light emanating from any incandescent body always
came to the eye as natural light, if in its passage it had not been
reflected or refracted. But experiment by the polariscope shewed, that
the ray departing from the surface at an angle sufficiently small was
polarised; while at the same time, it was demonstrated that the light
emitted by any gaseous body in flame--that of street-lamps, for
instance--is always in the natural state, whatever be its angle of
emission. From these remarks, some idea will be formed of the process
necessary to prove whether the substance which renders the sun visible
is solid, liquid, or gaseous. On looking at the sun in the
polariscope, the image, as before observed, is seen to be purely
white--a proof that the medium through which the luminous substance is
made visible to us is gaseous. If it were liquid, the light would be
coloured; and as regards solidity, that is out of the question--the
rapid change of spots proves that the outer envelope of the sun is not
solid. On whatever day of the year we examine, the light is always
white. Thus, these experiments remove the theory out of the region of
simple hypothesis, and give certainty to our conclusions respecting
the photosphere.
Here an example occurs of the aids and confirmations which science may
derive from apparently trivial circumstances. Complaint was made at a
large warehouse in Paris, that the gas-fitters had thrown the light on
the goods from the narrow, and not from the broad side of the flame.
Experiments were instituted, which proved that the amount of light was
the same whether emitted from the broad or narrow surface. It was
shewn also, that a gaseous substance in flame appears more luminous
when seen obliquely than perpendicular, which explains what are known
as _faculæ_ and _lucules_, being those parts of the solar disk that
shew themselves brighter than other portions of the surface. These are
due to the presence of clouds in the solar atmosphere; the inclined
portions of the clouds appearing brightest to the spectator. The
notion, that there were thousands on thousands of points
distinguishing themselves from the rest by a greater accumulation of
luminous matter, is thus disposed of.
Still, there remained something more to be determined. The existence
of the photosphere being proved, the question arose--was there nothing
beyond? or did it end abruptly? and this could only be determined at
the period of a total eclipse, at the very moment when the obscuration
of the sun being greatest, our atmosphere ceases to be illuminated.
Hence the interest felt in an eclipse of the sun of late years.
In July 1842, at a total eclipse of the sun visible in several parts
of the continent, the astronomers noticed, just as the sun was hidden
by the moon, certain objects, in the form of rose-coloured
protuberances, about two or three minutes high, astronomically
speaking, projected from the surface of the moon. These appearances
were variously explained: some supposed them to be lunar mountains;
others saw in them effects of refraction or diffraction; but no
precise explanation could be given; and mere guesses cannot be
accepted as science. Others, again, thought them to be mountains in
the sun, the summits stretching beyond the photosphere; but at the
most moderate calculation, their height would have been about 60,000
miles--an elevation which, as is said, the solar attraction would
render impossible. Another hypothesis was, that they were clouds
floating in a solar, gaseous atmosphere.
M. Arago considers the last as the true explanation: it remained the
great point to be proved. If it could be ascertained, that these red
protuberances were not in actual contact with the moon, the
demonstration would be complete. Speculation was busy, but nothing
could be done in the way of verification until another eclipse took
place. There was one in August 1850 total to the Sandwich Islands, at
which, under direction of the French commandant at Tahiti,
observations were made, the result being that the red prominences were
seen to be separated by a fine line from the moon's circumference.
Here was an important datum. It was confirmed by the observations of
July 1851, by observers of different nations at different localities,
who saw that the coloured peaks were detached from the moon; thus
proving that they are not lunar mountains.
If it be further ascertained, that these luminous phenomena are not
produced by the inflexion of rays passing over the asperities of the
moon's disk, and that they have a real existence, then there will be a
new atmosphere to add to those which already surround the sun; for
clouds cannot support themselves in empty space.
We come next to that part of the subject which treats of the true
place of the sun in the universe. In the year 448 B.C., Archelaüs, the
last of the Ionian philosophers, without having made any measurements,
taught that the sun was a star, but only somewhat larger than the
others. Now, the nearest fixed star is 206,000 times further from us
than the sun: 206,000 times 95,000,000 of miles--a sum beyond all our
habits of thought. The light from the star _Alpha_ of the Centaur is
three years in its passage to the earth, travelling at the rate of
192,000 miles per second; and there are 86,400 seconds in a day, and
365 days in a year. Astounding facts! If the sun, therefore, were
removed to the distance of a Centauri, its broad disk, which takes a
considerable time in its majestic rising and setting above and below
the horizon, would have no sensible dimensions, even in the most
powerful telescopes; and its light would not exceed that of stars of
the third magnitude--facts which throw the guess of Archelaüs into
discredit. If our place in the material universe is thus made to
appear very subordinate, we may remember, as M. Arago observes, that
man owes the knowledge of it entirely to his own resources, and
thereby has raised himself to the most eminent rank in the world of
ideas. Indeed, astronomical investigations might not improperly excuse
a little vanity on our part.'
Among the stars, Sirius is the brightest; but twenty thousand millions
of such stars would be required to transmit to the earth a light equal
to that of the sun. And if it were difficult to ascertain the nature
and quality of the sun, it would appear to be still more so to
determine these points with regard to the stars; for the reason, that
the rays, coming from all parts of their disk, at once are
intermingled, and of necessity produce white. This difficulty did not
exist in similar investigations on the sun, because its disk is so
large, that the rays from any one part of it may be examined while the
others are excluded. Under these circumstances, further proof might
seem to be hopeless; but advantage was taken of the fact, that there
are certain stars which are sometimes light, sometimes dark, either
from having a movement of rotation on their own axis, or because they
are occasionally eclipsed by a non-luminous satellite revolving around
them. It is clear, that while the light is waxing or waning, it comes
from a part only of the star's disk; consequently, the neutralisation
of rays, which takes place when they depart from the whole surface at
once, cannot then occur; and from the observations on the portion of
light thus transmitted, and which is found to remain white under all
its phases, we are entitled to conclude, in M. Arago's words, that
'our sun is a star, and that its physical constitution is identical
with that of the millions of stars strewn in the firmament.'