ASTROPHYSICAL INSTRUMENTS.
So far we have been concerned with instruments which enable us to
ascertain the positions, dimensions, and appearances of the various
orders of heavenly bodies; but we can go further than this, and learn
something of the physical and chemical constitutions of the glittering
orbs by which we are surrounded. We can, for instance, bring
instrumental aid to bear upon the determination of the brightnesses of
the heavenly bodies, and by means of that powerful appliance of modern
astronomy—the spectroscope—we can study the chemistry of all those
bodies which shine by light of their own, and which are not so feebly
luminous as to be out of our range.
PHOTOMETRY.—The naked eye was alone employed in observations of stellar
brightness until quite recently. Each step in the advance of
astronomical research, as in most other branches of science, however,
depends upon the greater precision of observation which can be
introduced, and so we now find the eye to be assisted in these inquiries
by a _photometer_ of some kind or other. The general purpose of
photometry will be familiar to all in connection with such practical
matters as the determination of the illuminating power of coal gas. The
methods here employed, however, are not directly applicable to the
comparatively feeble light-sources which have usually to be dealt with
in astronomical photometry.
As will be more fully explained in another part of this work, the stars
visible to the naked eye are divided into six grades of magnitude. The
brightest of them are classed as first magnitude, while those only just
visible to the naked eye are of the sixth magnitude. Now that telescopes
are used, this division of stars into magnitudes must be continued in
some form or other, so as to include telescopic stars. From photometric
comparisons it has been ascertained that the average star of the first
magnitude may conveniently be reckoned 100 times as bright as a sixth
magnitude star. Hence, the light-ratio corresponding to a difference of
a single magnitude is 2·5. Thus, a star which is 2½ times less bright
than one of the sixth magnitude ranks as seventh magnitude, and so on.
Fractions of magnitudes are also necessary to express the results which
can now be obtained.
LIMITING APERTURES.—For the reason that a large telescope enables us to
see stars which are too dim to be visible in a smaller one, the
brightnesses of stars may be compared with more or less satisfactory
results by reducing the aperture of a telescope until the star in
question ceases to be visible. This is called the method of _limiting
apertures_, and in practice a telescope intended for this work would be
provided with a series of diaphragms, or other arrangement for
conveniently reducing the effective area of its object-glass. A
telescope which has an object-glass 10 inches in diameter should just
show stars of the fourteenth magnitude under favourable conditions; a
star which could just be seen when this aperture was reduced to an inch
would be of the ninth magnitude, and so on.
There are numerous reasons why this method fails to give satisfactory
results, but one of the most important is that the image of a star
becomes more diffuse with each reduction in the aperture of the
telescope. At best it must evidently fail for a comparison of stars
which are visible to the naked eye.
WEDGE PHOTOMETER.—One of the simplest and best methods of estimating
star magnitudes is afforded by the _wedge photometer_. This is a strip
of neutral-tinted glass about six inches in length, and a quarter to
half an inch deep, tapering from one end to the other, so as to present
a gradual reduction in depth of tint from the thick to the thin end. A
similar wedge of clear glass, tapering the opposite way, is cemented to
this, in order to get rid of prismatic action. Compensated in this way,
and mounted in a suitable frame, the wedge is placed in front of the
eye-piece of a telescope, and is pushed along until the star under
examination is just extinguished. A scale is then read off, and from the
results of a previous evaluation of the wedge in the laboratory, the
corresponding star magnitude is easily deduced.
In order to eliminate the effects of differences in the state of the
sky, the position of the wedge at which a standard star, such as
Polaris, ceases to be visible, is determined, and then it is the
difference of wedge readings upon which the final calculation is based.
The great value of the wedge in stellar photometry was demonstrated by
the labours of the late Prof. Pritchard, to whom we owe the catalogue of
the magnitudes of naked eye stars in the northern hemisphere known to
the astronomical world as the “Uranometria Nova Oxoniensis.”
OTHER PHOTOMETERS.—Some photometers depend for their action upon
comparisons with terrestrial sources of light. In some cases, an
artificial star, consisting of a pinhole illuminated by a standard lamp,
is brought into the same field of view as the star to be compared, and
then, by polarising apparatus, the brightnesses of the two images are
equalised. The amount of reduction of either of the stars is determined
by a scale which measures the rotation of the polariscope, and in this
way all the stars are compared with an artificial star of known
brightness.
One of the most notable achievements in this field of astronomical work
is that of Professor Pickering of the Harvard College Observatory, who
invented and made splendid use of the so-called _meridian photometer_.
Here the telescope has two object-glasses of equal aperture side by
side, and in front of each is a silvered flat mirror inclined at an
angle of 45° to the optic axes. The telescope is supported in an east
and west direction, so that one mirror reflects the Pole Star into its
object-glass, while the other can be rotated so as to reflect any other
star which is on the meridian into the second object-glass. Again, by a
polariscope at the eye end of the telescope the images of the two stars
are made of equal brightness, and the readings give the data for
calculating the required magnitude.
Photographs of the stars are also largely employed for the estimation of
magnitudes, stars of different magnitudes being represented on the
photographs by spurious discs of different sizes. If all stars gave out
light of the same quality, the photographic method would be very
trustworthy, but as the colours of the stars vary, the photographic and
visual magnitudes are not invariably in agreement A bright, reddish
star, such as Betelgeuse, would photographically be only equivalent to a
white star which was much less bright to the naked eye.
THE PRISMATIC SPECTROSCOPE.—Reference has already been made in these
pages to the wonderful field of astronomical research which has been
opened up by the discovery of the action of a triangular glass prism
upon rays of light, and the subsequent improvements in the method of
utilising this effect.
A prismatic spectroscope may be regarded as an arrangement which will
enable us to get a pure spectrum, and to observe it to the best
advantage. The light to be analysed is admitted through a narrow
aperture called the _slit_, which is placed at the focus of a double
convex lens. Emerging from this _collimator_, as a parallel beam, the
rays pass through the prism, and after deviation and dispersion they
fall upon another double convex lens, which brings them to a focus in
the form of a spectrum. An eye-piece may then be employed to view the
spectrum, or a sensitive plate may be placed at the focus to photograph
it.
In a simple form of spectroscope the prism is supported at the centre of
a graduated circular plate, to which the collimator is firmly fixed,
while the observing telescope is attached to an arm pivoted at the
centre of the plate. A vernier moving with the telescope indicates the
position, on a scale of degrees, of any colour brought to the centre of
the field of view.
The best results are obtained when the rays of light emerge from the
prism at the same angle at which they enter it, in which case the prism
is said to be at _minimum deviation_, for the reason that the deflection
of the rays from their original path is then the least possible. As
lights of different colours are refracted unequally, it is clear that
the prism can only be at minimum deviation for rays of one particular
colour at any instant. Frequently, however, there is an automatic
arrangement by which, as the observing telescope is moved so as to bring
different colours into the field of view, the prism is turned so as to
be at minimum deviation for the colour actually under observation.
The appearances observed in the spectroscope are a series of images of
the aperture through which the light is admitted. If the source of light
be yellow, such as that of a spirit lamp flame when common salt is
introduced, a yellow image of the aperture will be seen, and so on for
other monochromatic radiations. When a white light is observed, images
of every gradation of colour are formed, and in such a “continuous
spectrum” the separate images cannot be recognised. The form of aperture
most widely adopted is a narrow straight slit with parallel sides. In
this case there is the least possible confusion, because the several
images of the slit appear as so many spectrum “lines.”
For observations of the sun, where the light is so intense, a great
number of prisms, each drawing out the spectrum into a longer band, may
be employed, so that the lines of the spectrum may be widely separated,
and the peculiarities of each more closely investigated. For the fainter
bodies, however, the instrument must generally be one of comparatively
small dispersion, so that the light may not be spread out into
invisibility. It will be evident that the longer the spectrum the
greater will be the chances of accurate measurements.
Another way of obtaining great dispersion is to use prisms of the new
dense Jena glass, one of which is equal to three or four of the flint
glass prisms in general use.
There are various forms of the prismatic spectroscope. In some of them
reflecting prisms are introduced to turn the rays back through the
dispersive train, so as to get increased dispersion without increasing
the number of prisms. In the so-called _direct vision spectroscope_,
prisms of different kinds of glass are combined so that the rays of
light leave them in nearly the same direction that they enter. Here the
collimator and observing telescope are in the same straight line, and
this is a great convenience in certain classes of observation.
THE GRATING SPECTROSCOPE.—Sometimes, especially in instruments designed
for solar observations, the prisms are replaced by what is called a
diffraction grating. Usually this consists of a piece of highly polished
speculum metal, upon which is ruled a great number of equidistant
parallel scratches or lines. A portion of the light falling upon the
grating is simply reflected, while the remainder is spread out into two
series of beautiful spectra, one on each side of the directly reflected
beam. The two nearest to the directly reflected beam are called spectra
of the first order, while following these are spectra of the second,
third, and fourth orders; the length of spectrum increasing in each
case, and all being available for observation if the light dealt with be
sufficiently bright. The production of these spectra is due to the
interference of light waves.
All gratings produce exactly similar spectra, so that the distances
between identical lines as seen with one grating are always strictly
proportional to their distances as seen with any other. With prisms, the
relative separation of colours is by no means constant; a prism made of
one kind of glass may, for example, separate the green and yellow more
than another prism made from different material, while the separation of
yellow and red might be the same in both cases. The grating spectrum
accordingly affords a constant standard of reference, and what is called
the “normal solar spectrum” is the spectrum of the sun mapped with the
various dark lines in the relative positions shown by a grating
spectroscope.
Prof. Rowland, of John Hopkins University, has introduced a form of
grating spectroscope, in which the grating is ruled on a concave
spherical surface of speculum metal. After passing through the slit the
rays of light fall directly upon this concave surface, and are brought
to a focus after reflection, so that no lens except the eye-piece used
for visual observations is required. Several of these gratings, having
mostly a radius of curvature of about 21 feet, and a ruled surface of
about 5½ inches x 2 inches, with 20,000 lines to the inch, are in use at
the present time. Some idea of the difficulties to be faced in making
these magnificent aids to research maybe gathered from the following
remarks of Mr. J. S. Ames:—“It takes months to make a perfect screw for
the ruling engine, but a year may easily be spent in search of a
suitable diamond point.... When all goes well it takes five days and
nights to rule a 6 inch grating having 20,000 lines to the inch.
Comparatively no difficulty is found in ruling 14,000 lines to the
inch.”
With the aid of these wonderful gratings, the solar spectrum can be
photographed with perfect definition, and extending over a total length
of several yards. Thousands of the tell-tale Fraunhofer lines are
rendered visible in this way.
MEASUREMENT OF SPECTRA.—The spectra of many substances, including
hydrogen and iron, are so characteristic as to be recognisable at a
glance by an experienced observer, but one must as a rule resort to
measurement for the identification of lines, or for the purpose of
locating unknown lines for future reference. One of the simplest methods
of measurement is that of reading the position of the observing
telescope upon a graduated circle, when the line is seen at the centre
of the field. If supplemented by a micrometer eye-piece, for
differential measures with regard to known spectra, this method is
extremely convenient. As recorded on arbitrary scales of this character,
the position of the same line would be represented by a number which
would be different for every instrument, and it is therefore necessary
to reduce all measurements to a common scale; that now universally
adopted is the natural one of wave-lengths. The position of a line in
the spectrum depends upon the length of the waves constituting the rays
of light which produce it, so that a measure of wave-length completely
specifies the situation of a line whatever spectroscope maybe employed.
Light waves are excessively minute, but by the use of the diffraction
grating they can be measured with great accuracy. So small are they,
that the most convenient unit of wave-length is the ten-millionth part
of a millimetre[5]—or tenth metre, as it is technically named. Expressed
in this way, the wave-length of the glorious red line seen in the
spectrum of hydrogen is 6563·07, while that of the blue line
characteristic of the same gas is 4861·51.
When the positions of a certain number of lines of known wave-length
have been read off on the scale of any spectroscope, the required
wave-lengths of other lines are ascertained by a graphical
interpolation, or by calculation. Elaborate tables of the wave-lengths
of the lines in the spectra of the sun and chemical elements have been
prepared by various investigators, and these are in constant demand by
all workers in the field of astrophysics.
THE TELESPECTROSCOPE.—For the examination of the spectra of the heavenly
bodies, a spectroscope is attached to the eye end of a telescope from
which the eye-piece has been removed, such a combination forming a
_telespectroscope_. The slit is placed at the principal focus of the
object-glass of the main telescope, and an image of the object to be
observed is thus produced upon it. If the sun be under observation, any
special part of it, such as a sun-spot or the chromosphere, may be
separately investigated by bringing the corresponding part of the image
upon the slit.
In the case of the sun, moon, comets, planets, or nebulæ, the image is
one of sensible size and the spectrum lines have a perceptible length.
With a star, however, the image is only an illuminated dot upon the
slit, and the spectrum would have no appreciable breadth, so that all
but the strongest lines would in general fail to show themselves.
Accordingly, when observing star spectra, a cylindrical lens is placed
in front of the slit, so that the stellar image is drawn out into a
bright line, and the necessary breadth of spectrum and length of the
spectrum lines are secured.
For photographing the spectra of the heavenly bodies it is simply
necessary to replace the eye-piece by a small camera, and to expose a
sensitive plate for a length of time dependent on the brightness of the
spectrum. The spectrum of a terrestrial substance, such as hydrogen or
iron, photographed in juxtaposition, is always a great convenience, and
is essential for the investigation of stellar movements by the
displacement of spectrum lines.
THE LICK STAR SPECTROSCOPE.—Among the most complete and perfect
spectroscopes adapted for use with the telescope is that designed by
Prof. Keeler for the great refractor of the Lick Observatory. It is
illustrated in Fig. 55, and it will be at once evident that there are
ample means for keeping the instrument under control. Towards the upper
part of the diagram, on the left, is the eye end of the telescope,
without the eye-piece. Two stout brass rods 3 inches in diameter and 6
feet long are attached by clamps to a revolving jacket which surrounds
the end of the telescope tube, and on these the spectroscope is
supported by clamps which allow of it being moved inwards or outwards
from the focus of the telescope. The collimator of the spectroscope lies
midway between the rods, and in order to facilitate the focussing of the
image upon the slit, it has a small longitudinal movement independently
of that of the whole spectroscope. The observing telescope is seen on
the left of the diagram, while the grating rests on the circular
graduated plate over which the observing telescope can be moved. The
grating has 14,438 lines to the inch.
Three prisms can also be used with the spectroscope, two of them being
single prisms of 30° and 60° refracting angles respectively, and the
third a compound prism giving a very high dispersion. Two observing
telescopes are provided, one being of extra power for use with the
grating in solar spectroscopy
The instrument is generously supplied with the small refinements which
contribute so largely to easy and successful manipulation. Among these
are a diagonal eye-piece for viewing the image of the object on the slit
plate, electrical illumination of the graduated scale and wires of the
micrometer eye-piece, and an automatic arrangement for keeping the
prisms at minimum deviation.
There is a small totally-reflecting prism covering half of the slit, by
which the light from an electric spark, or other source of luminosity,
can be made to pass through the spectroscope so as to produce a series
of known reference lines which serve as so many mile-posts for the
measurement of the spectrum of the celestial body under observation. The
induction coil, seen to the right of the diagram, is for the purpose of
producing these electrical sparks.
In mounting the spectroscope, which weighs no less than 200 pounds, the
eye end of the great telescope tube is first supported by a prop, and
the long rods are inserted. The spectroscope is then placed on the rods,
and balancing weights equivalent to the weight of the spectroscope are
removed from the lower part of the telescope tube.
[Illustration:
FIG. 55.—_The Spectroscope adapted to the Eye End of the Lick
Telescope._
]
THE OBJECTIVE PRISM.—It is a very remarkable fact that many of the
recent advances in our knowledge of the spectra of stars have followed
from the revival of a method first employed by Fraunhofer in 1814, in
which the slit and collimating lens, forming part of an ordinary
spectroscope, are dispensed with. The rays coming from a star being
already parallel, and the star itself being a virtual slit without
length, a large prism placed in front of the object-glass of a telescope
makes a complete stellar spectroscope. A prism employed in this way is
known as an _objective prism_.
In place of the image of a star, which would be seen in the absence of
the prism, a spectrum without appreciable width appears at the focus of
the telescope, and the spectrum lines will be represented by mere dots.
To turn these dots into lines so that they may be better visible, a
cylindrical lens must be employed in conjunction with the eye-piece.
It is to the application of photography, however, that we owe so much,
and in this case the cylindrical lens is removed, while a small camera
replaces the eye-piece of the telescope. In this form the instrument is
often called a _prismatic camera_.
The prism is so arranged that the spectrum lies along the meridian
passing through the star, and it is then only necessary to allow the
driving clock to be slightly in error in order that the spectrum may
trail a short distance perpendicular to its own length, and in this way
broaden the photographed spectrum. On the proper regulation of the clock
rate, and consequent “trail” of the spectrum across the plate parallel
to itself, depends very largely the success of the photograph obtained.
The spectrum of a bright star must obviously be made to travel more
quickly than that of a fainter one, and a short exposure suffices. For
the same clock rate, and in the same time, a star near the Pole will
give a shorter trail than one nearer the Equator, and declination must
therefore be taken into account in adjusting the clock error for this
method of photography.
One great advantage of the objective prism in the photography of stellar
spectra depends upon the fact that all the light passing through the
object-glass is utilised in the production of the spectrum, whereas in
an ordinary telespectroscope a large percentage of the light is lost in
the jaws of the slit. The large focal length of the telescope also
enables a long spectrum to be obtained even with a single prism of small
angle.
When the dispersion is only small, the spectra of stars as faint as the
tenth or eleventh magnitude can be photographed by this method, so that
sometimes as many as 200 spectra are registered with a single exposure.
Here, again, the objective prism has an immense advantage over the
telespectroscope.
Professor Pickering, of Harvard College, was among the first to
recognise the value of the objective prism for the photography of
stellar spectra, and the munificent endowment of this research, by Mrs.
Draper, as a memorial to Dr. Henry Draper, has enabled him to produce
the Draper catalogue of stellar spectra, giving the chief
characteristics of the spectra of over 10,000 stars.
Professor Norman Lockyer, at South Kensington, has also been
conspicuously successful in this department of astrophysical research.
The chief instrument he employs is a photographic telescope of only six
inches aperture, with an objective prism of 45° refracting angle. The
spectra thus obtained show hundreds of lines in such stars as Arcturus,
with very fine definition, so that they bear almost unlimited
enlargement.
An objective prism of twenty-four inches aperture will form one of the
accessories of the fine telescope which is now being erected at the
expense of Dr. Frank McClean, for the Cape Observatory, and there can be
no doubt that the use of this gigantic prism will add greatly to our
knowledge of the chemistry of the fainter stars.
As yet there is no very practicable method of employing the objective
prism for determining the velocities of stars in the line of sight from
the displacement of spectrum lines, and herein lies its one great
disadvantage as compared with the telespectroscope. The difficulty is to
ensure that the spectrum always falls absolutely in the same position
with respect to the terrestrial spectrum, which must be photographed
alongside for purposes of measurements. It is true that the spectrum of
an approaching star is somewhat shorter, and of a receding star slightly
longer than that of one at rest relatively to the observer, but these
changes are so small as to little more than indicate the direction of
movement even when a large instrument is employed.
Under the direction of Professor Norman Lockyer, the objective prism was
very successfully used for photographing the spectra of the solar
surroundings during the total eclipses of 1893 and 1896. In place of the
picture of the solar corona, which would appear in the absence of the
prism, the prismatic camera shows a spectrum consisting of bright rings.
If, for instance, the corona were wholly composed of hydrogen, there
would be a picture of it in red, blue-green, blue, and violet,
corresponding to the lines ordinarily seen in the spectrum of that gas.
These rings thus indicate the chemical nature of the corona, and at the
same time show, by their differing forms, the distribution of different
gases throughout its extent. The spectra of the solar prominences and
chromosphere are also depicted during the brief time of their
visibility, during an eclipse, with such distinctness that a series of
“snap shots” is all that is required to give a lasting record.
THE SPECTROHELIOGRAPH.—A special form of spectroscope—called the
_spectroheliograph_—has been devised by Prof. Hale, of Chicago, for
photographing the sun in monochromatic light. It consists of a
spectroscope, arranged for photography, in which the slit can be made to
travel by clock-work across the sun’s image, which is projected upon it
by the telescope to which the instrument is attached. In front of the
photographic plate there is a secondary slit, so that only a very
restricted part of the spectrum reaches the sensitive film. The
secondary slit is connected by mechanism with the primary one, so that
as the latter traverses the sun’s image, the former exposes different
parts of the photographic plate to the light which passes through it,
and in this way builds up an image of the sun in monochromatic light,
matters being so arranged that light of the same wave-length always
falls upon the secondary slit. By utilising the brightest lines which
appear in the spectrum of the solar prominences, monochromatic images of
those interesting appendages to our luminary have been successfully
photographed without waiting for a total solar eclipse.
THE BOLOMETER.—Besides the luminous effects of the spectrum, there are
heating effects which can be measured by the _bolometer_, an instrument
invented by Prof. Langley. A very thin strip of metal is connected with
a delicate galvanometer, and is arranged so that it can be passed a long
the whole spectrum. The electrical resistance of the strip varies
according to its temperature, and the galvanometer at once signals any
fluctuations which may occur. If, for instance, the strip comes to the
place occupied by a dark line, there will be a notable fall of
temperature. In this way, the bolometer is used to map the solar
spectrum in the “infra-red” region—a part of the spectrum invisible to
the eye, and of which we should otherwise have remained in ignorance.
ASTRONOMY
[Illustration:
DONATI’S COMET, OCTOBER 9, 1858. (FROM LANGLEY’S “NEW ASTRONOMY”.)
]
SECTION III.—THE SOLAR SYSTEM.
BY AGNES M. CLERKE.
CHAPTER I.
THE SOLAR SYSTEM AS A WHOLE.
The solar system consists of one supereminent body, with a train of
miscellaneous attendants. By its immense gravitative power, their
movements are so governed that they not only revolve round it as a
common centre, but accompany its march through space; they are, in
various degrees, warmed and enlightened by its copious emissions of heat
and light; they are linked with it by origin and destiny. Some, indeed,
much more closely than others. Planets, satellites, and asteroids belong
to the immediate family of the sun; periodical comets and revolving
meteoric rings have been adopted into it. The planets are eight in
number; the six nearest the sun—Mercury, Venus, the Earth, Mars,
Jupiter, and Saturn—have been known immemorially; Uranus and Neptune
were discovered respectively in 1781 and 1846. Mercury, Venus, and Mars
form, with the Earth, a group of “terrestrial planets,” so-called
because they differ not very greatly in scale from our globe, and are
constructed on nearly the same lines. The outer quartette of planets are
giants by comparison, and show obvious symptoms of being in a very
different physical condition. And it is noteworthy that the zone of
asteroids, lying between Mars and Jupiter, divides the planetary
classes.
The asteroids are sometimes designated minor planets; but the former
term is preferable, as accentuating their distinctive character. For
they are not simply diminutive planets. A planet revolves in solitary
state within its own broad domain. The asteroids traverse intercrossing
and entangled paths, indefinitely numerous, ranging widely in celestial
latitude, and covering with their network nearly the entire chasm of
space between Mars and Jupiter. The small bodies moving in them have
doubtless been formed in a manner totally different from that by which
the single body they seem to replace would have taken shape.
Satellites bear in many respects the same relation to planets that
planets bear to the sun. They are united with them into secondary
systems, one of which is particularly well known to us, since it is
constituted by the earth and the moon. The existence of twenty-one
satellites has been ascertained, and many more possibly remain to be
detected. Their apportionment is singularly unequal. Only three of the
twenty one belong to the four small interior planets, while eighteen are
attached to the four exterior giants. Moreover, both Mercury and Venus
are solitary; so that the solar neighbourhood appears to be a region
unpropitious to the development of subordinate systems.
Seventeen comets certainly, and many more probably, are domiciled in the
solar kingdom. And even these preserve traces of an alien origin. They
revolve round the sun in closed orbits, and are hence periodical in
their apparitions; but their periodicity has to be qualified by a saving
clause. They come up to time _barring accidents_. For their orbits, not
being adjusted to stability, are liable to violent changes through the
influence of the powerful masses, the tracks of which they intersect. In
running up to, or back from perihelion, comets have to cross many
railroads, so to speak, and do not always escape disturbing or
destructive encounters with passing trains. Thus, many are entered in
our astronomical visitor’s book as lost or strayed. Halley’s is the only
well-secured cometary prisoner of the sun of imposing magnitude; the
rest are of little spectacular, although of very high theoretic,
interest. Comets are the only self-luminous members of the solar system.
Meteorites, besides being intrinsically obscure, reflect, owing to their
minuteness, so little sunlight that they remain invisible until ignited
in our atmosphere. They travel round the sun in annular systems, each
mote-like component of which pursues its way, independently of the
others, under the strict regimen of gravitational law. The number of
these meteoric rings must be prodigious. Some hundreds have been brought
to our acquaintance, which can only include such as cut the earth’s
orbit; and these must be an insignificant fraction of the whole. The
innumerable closely-related orbits grouped into each ring are
ill-regulated for the safety of the bodies moving in them, since they
conform in no way to the rules of planetary circulation. Hence the
numerous encounters with the earth announced by the luminous trails of
shooting stars.
Our system, as at present known, is 5,585 millions of miles in diameter.
It is limited by the orbit of Neptune. But no less than three
trans-Neptunian planets have been, on some show of evidence, alleged to
exist. One of them, held by Professor Todd of Amherst College, U.S., to
be responsible for some outstanding perturbations of Uranus, was placed
by him in 1877 at a distance from the sun fifty-two times that of the
earth (the radius of Neptune’s orbit being measured by thirty of the
same units); the two others, called into existence by Professor Forbes
of Edinburgh in 1880, to account for the formation of two groups of
comets with aphelia respectively at one hundred, and three hundred
astronomical units, were believed to occupy those enormously remote
positions. Although none of the three, in spite of telescopic and
photographic search, has yet been found, the possibility is not excluded
that the appearance on a long-exposed sensitive plate of a line in lieu
of a dot as the representative of a seeming star, may in the future
announce the annexation by the sun of a further immense slice of
territory out in the depths of space. The boundaries of our system are
thus only provisionally fixed.
Intra-Mercurian planets have proved equally recalcitrant to
prediction; and it may safely be said that no globe of the superficial
dimensions of an English county lies concealed in the comparatively
narrow space available for its circulation. The necessity for the
presence of “Vulcan” was deduced by Leverrier from an unexplained
displacement of Mercury’s perihelion, and a transit of the required
body, supposed to have been observed March 26, 1859, was thereupon, in
all good faith, brought forward by Dr. Lescarbault of Orgères. Another
pseudo-discovery—this time of a pair of Vulcans—was made during the
total eclipse of July 29, 1878; but neither on nor off the sun has the
body needed to satisfy the French mathematician’s theory been
genuinely seen, and few believe that it will ever be forthcoming.
Professor Titius of Wittenberg pointed out in 1772 that the relative
distances of the planets from the sun could be expressed by adding 4 to
the series 0, 3, 6, 12, 24, 48, etc. Thus, if the distance of Mercury
were called 4, those of Venus, the Earth, Mars, and so on, would
severally be 7, 10, 16. The validity of this relation—known as “Bode’s
Law”—was strengthened by the conformity to it of Uranus and Ceres,
neither of which had been discovered when it was enunciated; Neptune,
however, proved to be much nearer to the sun than he should have been,
and the formula hence ranks as an empirical one, not grounded in the
nature of things.
Yet the grand outlines of the solar system are traced on a visibly
symmetrical plan. The larger bodies composing it move nearly in the same
plane, in orbits nearly circular, and at regulated intervals, augmenting
rapidly outward. All revolve from west to east, or “counter clockwise,”
and this fundamental current of motion carries with it, besides the
asteroids, all the periodical comets, save Halley’s. Among secondary
systems only the Uranian and Neptunian escape from its sway; there being
a visible tendency towards deviations from rule towards the confines of
the solar domain. These deviations, however, are not of a subversive
character.
The planetary machine may continue working forever without a hitch. Such
irregularities as would be likely to throw it out of gear are found only
in parts of almost evanescent mass and negligeable influence. Two modes
of action which should, in the long run, bring about a collapse, are
non-existent or insensible. These destructive agencies are a resisting
medium, and the progressive transmission of gravity. The presence of
either should prove fatal in the same ultimate fashion. Along slowly
narrowing tracks, the planets would descend, one after the other, into
the ample lap of the sun. Their circulation is, however, to the best of
our present knowledge, unimpeded and undeflected; the disturbances
affecting it are self-compensatory.
But while the mechanical stability of the system is assured, its
physical state is continually changing. And the change is always in the
same direction. A degradation of energy steadily progresses. The sun is,
in fact, spending his capital, and even with a millionaire of his stamp
this cannot last. The time must come, if science is to be believed, when
his radiative powers will have become exhausted. Five millions of years
hence they will, in all probability, be much less efficacious than they
are now. Within twice or thrice that interval they may have become
almost extinct.
Planetary globes, too, grow old through the wasting of their internal
heat. The moon seems in a measure to prefigure the future condition of
all, should their decay not be arrested. Possibly the lunar stage is not
the last. Death may, in the long ages to come, be succeeded by
disintegration, when a ring of rubbish will be substituted for our
“wan-faced” companion. To what purpose, then, our readers will ask, the
mechanical perfections of a system destined eventually to be involved in
darkness and destruction? To what purpose its exquisite balance, the
nicely-adjusted relations of its members, its self-righting faculty, its
compensatory springs? We can reply only by recalling that the extreme
conclusions of science are invariably pessimistic, because they are
reached without taking any account of the intelligent control
perpetually, though insensibly, overruling the workings of blind forces.
If, in one sense, heaven and earth pass away, we still know that, in
good time, “a new heaven and a new earth” shall inscrutably arise. Not
“faintly,” then, but boldly and ardently, we “trust the larger hope”
that renovation will succeed, or anticipate subversion.
Whatever _can_ have an end _must_ have had a beginning, and the origins
of things have an especial fascination for our minds. As regards the
history of the planetary world, we are not altogether in the dark. The
problem of the maintenance of the sun’s heat was satisfactorily solved
by Helmholtz in 1854. Its radiative supplies, as he showed all but
conclusively, are derived from gravitative power. As they are diffused
into space, the cooled particles from which they proceed, clash
together, and their arrested motion is converted into a fresh thermal
stock. This implies a steady diminution, although to a surprisingly
slight extent, in the bulk of the solar globe. It has been computed that
a shortening of the sun’s diameter by 380 feet yearly would suffice to
keep this grand heat-producing machine in full working order; and at
least ten thousand years should elapse before the contraction became
measurable by any instrumental means at our command. Its progress
should, nevertheless, eventually reduce our glowing luminary to an
obscure, inert mass.
Now, evidently, its shining in the past was sustained in the same way as
at present. The globe that blazes in our summer skies is, accordingly,
but the shrunken remnant of what it once was. It is shrunken in
proportion to the vast quantity of its former emissions. Hence, the
farther we look back into the ages, the more voluminous its dimensions.
And, sounding the utmost profundities of time, we arrive at an epoch
when all the planets were swallowed up in a sphere girdled by the
present orbit of Neptune.
The tenuity of this distended body was unimaginable. At ninety miles of
altitude, our air is one hundred million times rarer than it is at
sea-level; yet the primitive solar “nebula” was considerably more
attenuated still. This aerial mass had, doubtless, been in some way
impressed with a slow movement of rotation, which, by mechanical
necessity, quickened as condensation progressed. The planets represent a
few fragments detached during the process; nearly the whole of its
substance being compacted into the sun. How the fragments came to be
detached is the crux of cosmogonists. According to Laplace’s famous
hypothesis, equatorial rings of matter separated successively from the
parent nebula at certain critical epochs when gravity was overcome by
the gaining centrifugal tendency due to accelerating rotation. These
rings drew together into planets, from which satellites were generated
by a repetition of their own birth-process. Many incongruities are,
however, involved in this _modus operandi_. Only two need here be
mentioned. Reason and experience teach us that globes of small interior
consistence easily break up into rings, while cosmic rings show not the
slightest tendency to collect into globes. Again, Laplace supposed that
the production of each planet relieved a long antecedent strain. But
nebulous stuff is almost absolutely incoherent. Hence it _cannot be
stretched or strained_. As the nebula condensed and whirled, it would,
accordingly, have left behind innumerable disaggregated particles, but
no massive rings.
M. Faye of the French Academy has attempted to remedy these defects. The
planets, he considers, were not abandoned, but formed at centres of
condensation within the nebular matrix. The order of their formation
would thus have been quite different from that assigned by Laplace, in
whose theory the exterior globes were necessarily the earliest to take
shape. M. Faye, on the contrary, argues Uranus and Neptune, from their
retrograde rotation, to be the _youngest_ instead of the _oldest_
members of the solar system, while the terrestrial group belong to the
first era of planetary development.
Astronomers are now virtually agreed that “The world was once a fluid
haze of light,” but by what precise means, in what succession, under
what compulsion, its constituent bodies were set wheeling in the void,
they are less ready to pronounce than were their predecessors, who,
dazzled with the analytical triumphs of the eighteenth century, accepted
unquestioningly the plan of creation it complacently transmitted to
them. The complexities of world-making have, besides, been instructively
illustrated by Professor G. H. Darwin’s discovery that tidal friction
was essentially concerned in the process. By an able mathematical
investigation, he showed, in 1879, that it was particularly effective in
modelling the earth-moon system, owing to the fact that our satellite,
comparatively to its primary, is by far the largest in the solar system.
Tidal friction may be regarded under a two-fold aspect. Its effect in
grinding down the speed of rotation has been explained in Section II.
(page 166). The energy, however, thus apparently destroyed is only
transformed. The rotational momentum subtracted from the earth is added
to the orbital momentum of the moon, which thus travels (setting aside
other causes of change) along continually widening spires. This retreat
from the earth is even now going on, although with elusive slowness,
amid the rise and fall of secular change. Its effects in past ages,
nevertheless, coupled with those due to the slackening of rotation by
the friction of the tidal wave—the two forming, as it were, the obverse
and reverse of one medal—must have been of overruling importance. Laying
hold of the clue they offer, Professor Darwin succeeded in tracing back
the history of the moon through a “corridor of time” nearly a hundred
million years long. It was then spinning at a vertiginous rate, round,
and nearly in contact with the earth, which must have been fluid or
plastic, while of about its present size. The _month_ of that epoch was
three or four hours in duration; the _day_ was shorter still. The actual
existence of the moon convinces us of this latter fact. Otherwise, the
huge tidal wave raised by the moon upon the earth should have lagged,
however slightly. Its attraction would have pulled the moon backwards at
the decisive moment of its emergence into separate being, and led
infallibly to its re-engulfment.
The origin of the moon has been, by Professor Darwin’s analysis, made
clearer than that of any other heavenly body. Certainty regarding such
remote events is unattainable; but it is highly probable that our globe,
at a late stage of its development, gave birth, amid the throes of
disruption, to its solitary offspring. But the case is unique. The
terrestrial system presents conditions not repeated elsewhere.
Generalisations founded upon them are sure to be misleading. We have
indeed gained, from all recent inquiries into cosmogony, the profound
conviction that no single scheme will account for everything; that the
utmost variety prevailed in the circumstances under which the heavenly
bodies attained their present status; and that a rigidly constructed
hypothesis can only misrepresent the boundless diversity of nature.
CHAPTER II.
THE SUN.
The sun is an immense reservoir of radiant energy. For our daily uses we
have no other store worth mentioning to draw upon, our fuel being the
embalmed sun-heat of former ages; and all the physical and vital
operations carried on over the whole globe derive their motive power
from the same copious source. Yet only 1/2,128,000,000th part of the sum
total of solar radiations strike its comparatively diminutive surface;
while all the planets combined intercept no more than 1/234,000,000th of
that inconceivable effluence.
The sun gives as much light as 600,000 full moons, or two and a half
billions of the most powerful electric lights, or as 1,575 billions of
billions of standard candles. And since his disc is the projection of a
hemisphere, and is thus equivalent only to one-fourth the globular
surface, these vast numbers must be quadrupled to represent the whole
luminous emissions of this surpassing body. Their amazing profusion is
the combined result of immensity of shining area, and vivid intrinsic
brilliancy. Each square inch of the sun’s surface has been estimated to
integrate the lustre of twenty-five electric arcs,[6] and Professor
Langley, by direct experiment, proved it to be 5,300 times brighter, and
87 times hotter, area for area, than the white-hot “pour” from a
Bessemer converter; notwithstanding that the circumstances of the
comparison were exceedingly “unfair to the sun.”[7]
Radiant heat and light do not indeed differ in themselves, but only in
their effects. The sun sends out into space ethereal waves of various
lengths, but all of the same kind, subject to the same laws, and
travelling with the same velocity of 186,000 miles a second. They
appear, however, under diverse forms of energy according to the
qualities of the substances upon which they impinge. Thus a small
section of this long range of undulations affects our eyes as light, the
human retina being so fashioned as to be able to _see_ with their help.
There is nothing in the nature of the rays themselves to make them
visible, and it is in fact more than probable that other living
creatures perceive vibrations to which we are blind. Our eyes are
sensitive over nearly two octaves; from waves measuring about 760
millionths of a millimetre, to those of less than 400 millionths. In the
solar spectrum the limits are roughly marked at one end by a great dark
band in the deep red—Fraunhofer’s “A,”—and at the other by “H,” in the
extreme violet. Beyond H extend undulations so short as to be visually
imperceptible, while photographically active. This means that certain
salts of silver are capable of taking up the energy they bring from the
sun, and of using it to break their chemical bonds; while on differently
prepared plates similar effects can be produced by rays in all parts of
the spectrum, even in the ultra-red, where the undulations, too long to
be sensible as light, are mainly felt as heat. Here, as Professor
Langley has shown by “bolometric”[8] explorations, reside three-fourths
of the energy distributed throughout the solar spectrum; nor is it
impossible that this great stretch of heat waves may merge, without
interruption, into electrical _rollers_, measured, not by millionths of
a millimetre, but by metres, or even by kilometres. The important point
to be borne in mind, however, is that the solar energy is diffused
abroad by means of ethereal vibrations of a single type, but immensely
varied size and frequency, and hence susceptible of dispersion into a
spectrum.
The “solar constant” expresses the quantity of heat received by the
earth from the sun. Its value, according to the most trustworthy
determinations, is three calories per square centimetre per minute. This
means that a vertical sun pours down upon each square centimetre of the
globe heat enough (supposing the atmosphere out of the way) to raise the
temperature of three grams of water by one degree centigrade in a
minute. Putting it otherwise, the energy imparted would suffice to keep
an engine of three-horse power continually at work on every square yard
of the terrestrial surface. Or, if the heat were distributed uniformly
in all latitudes, it would annually melt a complete ice-jacket one
hundred and seventy feet thick.
The temperature of the body lavishing heat at this tremendous rate must
obviously be very high; but enquiries on the point are necessarily
limited to the actual emitting shell, or “photosphere.” Their success is
testified to by a noteworthy reduction of late in the range of
uncertainty. The difficulty attending them consists mainly in our
ignorance of any systematic relation between temperature and radiation.
Excessively hot bodies lose heat much more rapidly, under the same
conditions, than moderately hot ones; and empirical “laws of radiation”
have been, over and over again, arrived at as the upshot of long series
of laboratory experiments. But such laws are only too apt to turn
traitors if trusted without control; and since the thermal power of the
sun vastly exceeds that of any terrestrial source, they are precarious
guides in this particular research. Nevertheless, as the outcome of
various improvements and refinements, it has, within the last few years,
been prosecuted with excellent results. That obtained in 1894 by Messrs.
Wilson and Gray deserves particular confidence. The _effective_
temperature of the sun was by them fixed at 8,000°, or allowing for
absorption in the solar atmosphere (measured by Wilson and Rambaud), at
8,800° centigrade. This estimate, which makes the sun’s surface more
than twice as hot as the carbons of the electric arc, is unlikely to be
widely erroneous. The word “effective” signifies the condition that the
photosphere is equivalent in radiative power to a stratum of lampblack;
if it fall short of this standard, as appears probable, then the
temperature must be raised by a corresponding amount.
The solar atmosphere, of which the absorptive effects have just been
alluded to, is a shallow envelope, stopping predominantly the shorter
wave-lengths of the light transmitted through it. Hence, if it were
removed, the sun would appear, not only much brighter, but also much
_bluer_ than it does at present. The general darkening of the limb due
to its action is apparent to visual, and conspicuous in photographic,
observations. By its aid, “faculæ”—brilliant and elevated portions of
the photosphere—were early detected. Invisible on or near the middle of
the disc, they stand out in relief against its dusky edges as they are
brought round, and carried off again by the sun’s rotation.
The magnitude of this astonishing luminary fairly baffles our
conceptions. Its mass is 745 times that of all the planets taken
together. Its volume is such, that if Jupiter were located centrally
within it, two of his Galilean moons, besides the lately discovered
inner satellite, would have “ample room and verge enough” to revolve
round him, keeping well inside the photosphere. The entire Uranian
system could be easily accommodated in the same way; while Neptune and
his satellite, and the earth and moon, could very nearly perform their
evolutions side by side in the sun’s excavated interior.
The sun is 865,000 miles in diameter, and in figure is sensibly
spherical. Its surface is 12,000 times, its volume 1,300,000 times that
of the earth. In mass it is equal to 332,000 earths. Its mean density,
then, is only one-quarter that of the earth, or 1·4 times that of water.
In other words, the terrestrial globe, if equally bulky, would contain
four times the quantity of matter contained in the solar globe. Yet we
know that it is largely made up of iron and still heavier metals; while
gravity at its surface is 27·6 more powerful than it is here. Thus, the
sun’s materials are weighed down by an inconceivable pressure, and would
be of a density utterly transcending our experience but for the
counteracting agency of heat. The comparative insubstantiality of such a
globe gives us some faint notion of the violent molecular agitation
affecting every particle of its mass. Contrasted with the fires raging
within, the surface temperature of 8,000° or 9,000° might perhaps be
deemed moderate or cool. There is much evidence that it is throughout
gaseous, although of a consistence approaching more nearly that of pitch
or treacle than can easily be reconciled with established ideas as to
the qualities proper to an aerial substance. Yet the laws governing the
gaseous state are plainly those obeyed in the sun.
Its function, as a great thermal engine, is to produce and diffuse heat
For these purposes it is essential that the interior stores should be
brought rapidly to the surface; and this is accomplished, not, as in
solids, by conduction, but by actual transport, or “convection.” Only
the enormous elasticity of highly compressed gases could render this
process swift enough to sustain the incessant outpourings of heat from
the photosphere. It may be accompanied by an actual rise in temperature.
If the sun be truly gaseous throughout, it _must_ be so accompanied. The
reason of this seeming anomaly is that a sphere of radiating and
contracting gas develops by shrinkage more heat than it can dispose of
by radiation. Whether or no the sun comes within the scope of this
principle, known as “Lane’s Law,” cannot at present be decided. It is,
in other words, an open question whether the sun is growing hotter or
colder. Help towards answering it might have been expected from the
study of geological climates; but their variations have evidently been
due to a complexity of causes. At any rate, the sun’s decline, if the
inevitable turning-point has already been reached, is going on with
extreme slowness.
The visible structure of the photosphere, or lustrous envelope of the
solar globe, is, in itself, suggestive of the vertical circulation by
which the indispensable communications between its interior and exterior
are kept up. It is composed of brilliant granules and dusky interstices,
the former representing, it is supposed, the vividly incandescent
summits of uprushing currents, the latter the cooled, descending
return-flows. It may be safely described as the limiting surface of
thermal interchange, and is often spoken of as a cloud-sphere, or level
of condensation, where the ascending vapours, like mounting volumes of
water-gas in our atmosphere, are chilled into liquid droplets. To the
brilliant luminosity of these incandescent droplets, the blaze of the
solar emissions is ascribed. Or the droplets might equally well be solid
particles on the model of the ice-spicules collected to form the
delicate fields of cirrus in our upper air. The cloud theory of the
photosphere is, however, hampered by the difficulty of finding a
substance capable of liquefying or solidifying at a temperature of
8,000° C. Carbon has generally been selected as the material of the
solar “granules,” but carbon evaporates at about 4,000°, and although
its boiling point might be raised by enormous pressure, there are no
signs that the requisite conditions exist in the sun. Hence, some
speculators turn towards electricity as the exciting agent of the
photospheric radiance; but it would be waste of time to attempt, at
present, to discuss the vague possibilities connected with an hypothesis
which offers no holding ground for distinct reasoning.
[Illustration:
FIG. 1.—_Photograph of a Sun-spot._ (From _Knowledge_, February,
1890.)
]
The photospheric texture is often rent and perforated. This ragged
condition (well exemplified in Fig. 1 from a photograph taken by Dr.
Janssen at Meudon) is accompanied or caused by a violent disturbance of
the sun’s bodily circulation. A typical sun-spot consists of a dark
opening, or “umbra,” within which a still darker “nucleus” can often be
discerned. The umbra is garnished all round with a semi-luminous
“penumbra,” composed of elongated shining bodies placed side by side,
and all, when undisturbed, pointing radially inwards towards the centre
of the spot. The effect has been compared to that of “straw-thatching,”
although the solar “straws” are, at times, thrown somewhat wildly about.
Where they hang over the _eaves_ of the spot they are always brightest,
because set most closely together. The penumbra may be called a modified
extension of the ordinary mottled surface of the photosphere, the
lustrous grains being drawn out into filaments, the “pores” into obscure
interspaces.
Spots commonly occur in groups (as in our Figure) belonging to a single
area of disturbance marked by the brightening, and probably by an
elevation of the photosphere. The members of such families show curious
and unexplained mutual relations. The size of these extraordinary
formations is on the gigantic scale of all solar phenomena. They are
often visible, individually or collectively, to the naked eye, and
attracted notice accordingly in pre-telescopic times. In 1858, a spot
opened to the extent of 144,000 miles, so that sixteen earths, side by
side, might have been engulfed in it. A still more remarkable outbreak
took place in February, 1892. Three thousand three hundred and sixty
million square miles of the photosphere were riddled as if by some
tremendous bombardment, the extreme dimensions of the affected district
being 150,000 by 75,000 miles. This spot, the largest ever photographed
at Greenwich, attained its acme on February 13th, when a magnetic storm
and widely diffused auroral display attested the sympathy of the earth
with commotions in the sun. Five times brought back to view by the sun’s
rotation, its history was followed from November until March; but this
duration is not an extreme case, a spot having been known to survive
throughout eighteen rotations. Although the group of February, 1892,
covered ¹⁄₇₀₀th of the sun’s entire surface, its proportions were
outdone by those of a spot and its immediate attendants, without
counting outliers, measured by Sir John Herschel at the Cape, March
29th, 1837.
Spots are always associated with faculæ. The two are correlated
phenomena. There is no certainty as to their order of precedence, if any
fixed order there be, but faculæ both survive spots and develop apart
from them. Not infrequently the faculæ garlanding a spot throw a
“bridge” right across it (see Fig. 1). In stereoscopic views these
brilliant projections show as veritable _suspension bridges_. They float
almost palpably at a high altitude above the black gulf they span.
The distribution of spots is easily perceived to depend immediately upon
the sun’s rotation. Two zones of its surface, parallel to the solar
equator, are alone infested by them. These may be defined as lying
between 6° and 35° of north and south latitude; but the prohibition of
spot-development is much more absolute in the polar than in the
equatorial direction. One solitary macula has been observed in 50° north
latitude.
The periodicity of sun-spots was first recognised by Schwabe at Dessau
in 1851. Since abundantly confirmed, it constitutes one of the
fundamental data of solar physics. Once in about eleven years a
“maximum” is attained; for months together the photosphere is never calm
and unbroken; its agitated condition betrays the turmoil of the
interior. The superabundance of spots is succeeded, after some years, by
a scarcity, or “minimum,” when the perturbing agencies appear to have
sunk into repose, preparatory to another outburst of activity. In this
highly irregular, although well-marked, cycle, the ascent is almost
always much more rapid than the descent; the upspringing of the
disturbance occupies, as a rule, not much more than half the time
allotted to its quieting down. Nor is its intensity by any means
uniform. High and low maxima alternate with, or succeed each other, with
no obvious regularity. Sometimes we have a divided or double maximum, as
in 1882–4, followed by an unusually swift ebb of agitation. The minimum
of 1889 was premature and brief; for spots were again numerous in 1891,
and developed prodigiously throughout the years 1892 and 1893. Only in
January, 1894, a slight falling off became apparent, and the
tranquillity which set in with 1895 may very probably reign with only
temporary interruption for some time. The cause of these vicissitudes is
completely unknown; but they so closely resemble, in character, the
changes of variable stars, that it seems impossible to exclude the sun
from that category, spot-maxima corresponding with stellar light-maxima
and _vice versâ_.
[Illustration:
FIG. 2.—_Sun-spots and Magnetic Variations._ (From Langley’s “New
Astronomy.”)
]
Solar disturbances, however originating, are a sort of universal
pulse-beat, with which the earth, and doubtless every other member of
the solar cortège, throb in unison. The accompanying diagram (Fig. 2)
shows how closely the magnetic needle sympathises with the variations in
the state of the sun. The amplitude of its daily oscillations is
represented by the dotted curve, while the smooth curve is constructed
from the relative numbers of spots. The striking conformity in point of
time-development, between two effects so disparate in their nature,
extends to minute details. Violent commotions on the sun seldom fail to
be reflected in magnetic storms and auroral manifestations on the earth;
and exact correspondences have sometimes been observed; yet it does not
seem possible to trace these simultaneous effects to the immediate
magnetic action of the sun.
No meteorological cycle corresponding with the spot-cycle has yet been
satisfactorily made out. The direct diminution of heat and light through
the obscuration of a small part of the sun’s photosphere amounts, at the
utmost, to ¹⁄₁₀₀₀th of the whole. The spots are far from being totally
dark or cool. Their blackest nuclei are really no less brilliant than
limelight; while about half as much heat is derived from them as from
the surrounding disc when they are centrally situated, and 80 per cent.
when they are near the limb.[9] Their dimming and cooling effects then
are insignificant; they are probably more than compensated by the
quickening of the sun’s circulatory processes, and consequent increase
of emission, through the disturbance of internal equilibrium of which
outbreaks of spots are among the consequences.
The spot-zones do not always occupy the same positions. They shift with
the progress of the eleven-year cycle. This curious circumstance,
discovered by R. C. Carrington in 1856, illustrates, in his words, “the
regular irregularity, and irregular regularity,” distinguishing solar
periodicity. At maxima, the mean latitude of the zones in question is
about 16°; but they close down towards the equator as each wave of
agitation dies out, its few latest products appearing in quite low
latitudes. Then, when minimum is passed, a fresh start is made with the
opening of a few small spots in 30° or 35° north or south latitude; and
this newly-organised disturbance begins to descend as before, gaining
strength as it proceeds. Thus, each impulse acts independently of the
succeeding one.
The most cursory observation of sun-spots suffices to show that the
shining body marked by them rotates on an axis from west to east, in the
same direction as the planetary revolutions. True, they emerge to sight
on its eastern, and vanish at its western limb; but this is because we
are located at its _backside_, and see their courses inverted. Attempts,
however, to fix the sun’s period of rotation were long baffled; for the
spots, instead of being carried round as if attached to a rigid surface,
gave signs of possessing “proper motions” of uncertain and inconstant
amount. The subject was first thoroughly investigated by Carrington; and
he reached the unexpected conclusion that the sun has no uniform period,
but gyrates in a composite fashion, quickest at the equator, and
gradually slower towards the poles. From less than twenty-five days, he
found the time of circuit to lengthen steadily to twenty-seven and a
half in 50° of latitude. The axis round which this remarkably
conditioned movement is performed makes an angle of 7° 15′ with the pole
of the ecliptic; it inclines towards the earth’s northern hemisphere
from June to December, when the spots describe, in crossing the disc,
paths curved downwards (to the eye of a northern observer); but the
conditions being reversed between December and June, their paths are
then curved upwards; while on June 3rd and December 5th, they pursue
straight tracks, the earth being on those two days in the line of
intersection between the sun’s equatorial plane and that of the
ecliptic.
Only a rough approximation, however, to the laws of solar rotation can
be derived from spots. For they do not simply drift with the
photospheric currents, but are subject to accelerations and retardations
connected with their internal economy, as well as to mutual attractions
and repulsions depending, it is supposed, upon their electrical
condition. Fortunately, however, a method has been perfected by which
these complications are abolished. Something has already been said as to
spectroscopic determinations of motion in the line of sight. They are
evidently applicable to the sun’s axial movement. For, through its
effect, his eastern limb is always advancing uniformly towards us, while
the western limb is retreating at the same rate. Thus, the whole
Fraunhofer spectrum is shifted slightly upward, or towards the blue, at
the left-hand edge of the solar disc, and as much towards the red at the
right-hand edge. The same lines of solar absorption, in fact, taken from
opposite sides of the solar equator, and placed end to end, appear
evidently notched, and can be distinguished at a glance from terrestrial
absorption lines, which, having nothing to do with the sun’s rotation,
show no break at the junction of their sections. They in this way
“virtually map” themselves, as Professor Langley proved experimentally
in 1877.
In 1887–9, M. Dunér, of Upsala, succeeded in extending these delicate
measurements to within fifteen degrees of the sun’s poles, where the
movement is so slow that it can only, by incredible refinements, be
dealt with successfully. The upshot was to emphasise the law of
slackening _angular_ speed detected by Carrington and confirmed by
Spoerer. From 25½ days at the Equator, the sun’s period of rotation was
found to become protracted to 38½ days at the seventy-fifth parallel of
latitude. Its investigation from photographs of faculæ has been lately
carried out by M. Stratonoff at Taschkent in Russia. The results of the
three methods are collected in the following little table.[10]
THE SUN’S ROTATION.
┌────────────────┬────────────────┬────────────────┬────────────────┐
│ │ Period from │ Period from │ Period from │
│ Mean Solar │ Faculæ. │ Spots. │ Spectroscopic │
│ Latitude. │ (Stratonoff.) │ (Spoerer.) │ Measures. │
│ │ │ │ (Dunér.) │
├────────────────┼────────────────┼────────────────┼────────────────┤
│ 0° │ 24^d·66 │ 25^d·09 │ 25^d·46 │
│ 15° │ 25 ·26 │ 25 ·44 │ 26 ·35 │
│ 30° │ 25 ·48 │ 26 ·53 │ 27 ·57 │
└────────────────┴────────────────┴────────────────┴────────────────┘
These facts, although so various, are not necessarily discordant. They
apply to different parts of the great solar machine, each one of which
may rotate with a certain independence. The spots drift, more or less
passively, _with_ the photosphere. The faculæ are elevated above it, and
appear to be everywhere accelerated relatively to its systematic
currents. The strata originating the Fraunhofer lines, to which alone
the spectroscope is applied, display, on the contrary, effects of
retardation. “This peculiar law of the sun’s rotation,” Professor Holden
remarks, “shows conclusively that it is not a rigid body, in which case,
every one of its layers in every latitude must necessarily rotate in the
same time. It is more like a vast whirlpool where the velocities of
rotation depend on the situation of the rotating masses, not only as to
latitude, but also as to depth beneath the exterior surface.”
Solar chemistry progresses by successive interpretations; and the
characters to be read are so multitudinous and so similar as to require
very delicate discrimination. The work, carried on simultaneously in the
sun and laboratory, becomes more arduous as it advances, and is still
far from complete. Indeed, the difficulties attending detailed
comparisons between the Fraunhofer lines and the innumerable components
of terrestrial spectra, would be insuperable but for the aid of
photography, here, as elsewhere, the versatile handmaiden of physical
astronomy.
Here is a list of 36 solar elements published by Professor Rowland of
Baltimore in 1891, and arranged according to the number of their
representative lines in the solar spectrum.
Iron (2000 +)
Nickel
Titanium
Manganese
Chromium
Cobalt
Carbon (200 +)
Vanadium
Zirconium
Cerium
Calcium (75 +)
Scandium
Neodymium
Lanthanum
Yttrium
Niobium
Molybdenum
Palladium
Magnesium (20 + )
Sodium (11 + )
Silicon
Hydrogen
Strontium
Barium
Aluminium (4)
Cadmium
Rhodium
Erbium
Zinc
Copper (2)
Silver (2)
Glucinium (2)
Germanium
Tin
Lead (1)
Potassium (1)
Only two of these substances, carbon and silicon, are non-metallic,
hydrogen ranking as a gaseous metal. Neither oxygen, nitrogen, nor
argon, have yet spoken their “Adsum,” but it is not impossible that they
may do so in the future. Negative evidence, at any rate, is, in
spectroscopic inquiries, absolutely inconclusive.
The spectra of sun-spots are, as might have been expected, characterised
by a great increase of absorption. There is a general darkening which
extends far up in the ultra-violet, and is modified, in the green and
blue, into remarkable dusky gratings made up of closely-set fine rays;
and some of the ordinary Fraunhofer lines are besides thickened and
blackened. The formation in spots of oxides is thought by Dr. Scheiner
to be possibly indicated by these symptoms; “if so,” he adds, “the
presence of oxygen in the sun would thus be indirectly suggested.”[11]
Bright lines, too, flash out in the immediate neighbourhood of
sun-spots, especially the “great twin brethren,” “H” and “K,” due to
calcium, which stand in imposing breadth and strength at the violet end
of the Fraunhofer spectrum, and are of corresponding importance as
indexes to solar phenomena. With this pair, brilliant hydrogen rays are
often associated, besides other “reversals,” by which, upon the
customary dark lines, flaming rays of identical wave-lengths are
superposed. But these signs of incandescence evidently belong to the
facular stratum high up above the spot-umbra.
So long ago as 1769, the observations of Dr. Wilson of Glasgow were
believed to have established, once for all, that spots are funnel-shaped
depressions in the photosphere. But the perspective effects from which
he argued are certainly not always, perhaps not very often, present. Mr.
Howlett, after thirty-five years—1859 to 1895—devoted to testing the
truth of the traditional conviction, has at last succeeded in shaking,
if not in overthrowing, it. Most solar observers now admit that spots
are of extremely various and extremely variable construction, so that
the obscure umbra, at times a sort of pit or crater, in which vapours,
cooled by expansion, well up from below, may, at another stage in the
life-history even of the same spot, represent an actual accumulation of
absorbent material above the brilliant solar cloud envelope. In any
case, a spotted area appears to be an area of elevation. This might be
due to a wide-spreading relief of pressure, or an accession of internal
heat. The fact emerged clearly from a series of measurements of the
sun’s diameter executed by M. Sykora at Charkow, Russia, in 1895.[12]
The intensity of the agitations connected with sun-spots can be most
fully appreciated from spectroscopic observations. Lines torn,
displaced, and _branching_, testify to velocities in the line of sight
of the matter surrounding or overlaying them up to three or four hundred
miles a second! These tumultuous uprushes and downrushes are not of a
systematic nature; they afford no insight, consequently, into the
formative laws of spots. Of these we are indeed far more ignorant than
Sir William Herschel supposed himself to be. Recent work on the sun has
provided a grand store of facts ascertained with surprising skill and
ingenuity. But they want _colligating_. No framework has yet been
constructed that will hold them, each in its proper place. It has been
truly said: “Considering the rapid progress which has been made in the
observational or practical side of solar physics, it must be confessed
that the theoretical side has been very imperfectly developed. Almost
every student of solar physics has his own theory, and usually he
himself is the only one who believes in it.”
Since Sir John Herschel propounded his “cyclonic theory” of sun-spots in
1847, there has been a marked tendency to assimilate solar to
terrestrial phenomena. But the circumstances of the two bodies are so
utterly unlike that such attempts can only prove misleading. The earth
is a solid globe warmed from without, hence, with hot tropical and
frigid polar regions. This disparity is the prime motor in the
circulation of its atmosphere and oceans; a circulation, essentially in
latitude, directed towards the equalisation of temperature. The sun, on
the contrary, is heated from within; there is no appreciable difference
of temperature between its poles and equator; and its circulation is of
the bodily kind belonging to fluid masses, and is carried on by vertical
currents effecting exchanges of heat between the surface and the
profundities beneath. Were these to stop, or even notably to slacken,
the sun would promptly cease to shine, and lapse into the condition of a
“dark star.” It is not then surprising that the drifting movements of
the photosphere are _along_, not _across_, parallels of latitude. Solar
meteorology, in a word, has almost nothing in common with terrestrial
meteorology; and explanatory schemes, based upon an analogy which does
not exist, must sooner or later be consigned to the limbo of vanities.
CHAPTER III.
THE SUN’S SURROUNDINGS.
“What we ordinarily call the sun,” wrote the late Mr. Ranyard, “is only
the bright spherical nucleus of a nebulous body.”[13] But it is only
when the interposing moon cuts off the dazzling rays of the nucleus that
we see directly anything of its nebular surroundings. Partial or annular
eclipses are of little or no use for this purpose; the revelation
belongs exclusively to the sombre, yet splendid moments of totality. No
sooner has the last glint of sunshine vanished than the corona starts
into view, encompassing the black lunar globe with a sort of “glory” of
silvery streamers. Its radiated shape suggests vacillation of form and a
flickering radiance; yet its immobility is absolute. The awe and wonder
of the sight tend, for the moment, to supersede scientific curiosity,
and they are enhanced by the perception, at the base of the corona, of
the serrated scarlet “chromosphere” fringing the moon’s circumference,
while the towering “prominences” that are usually seen to spring from it
produce the startling effect of a conflagration.
These marvellous appendages received no adequate notice until their
disclosure during the total eclipse of July 8, 1842. Even the
uninstructed crowds in the streets of Milan and Pavia shouted with
amazement at what they saw; while by solar students the recurrence of
similar opportunities has ever since been eagerly anticipated and
diligently turned to account. The question that first pressed for
solution related to the local habitation of prominences; for some
unwisely persisted in attaching them to the moon. A decisive answer was
given by photography at its first _effective_ application to eclipses on
July 18, 1860. From a comparison of negatives exposed at the beginning
and end of totality, it became clearly apparent that the moon had, in
the interval, moved _over_ the prominences, uncovering, to a small
extent, those on the west side and concealing those on the east.
Their solar connexion having thus been established by the camera, the
spectroscope was called upon to determine their physical and chemical
nature. An admirable opportunity for taking this further step was
presented by the Indian eclipse of August 18, 1868. The result was
decisive. The light of a huge spire of flame, 89,000 miles high, had no
sooner passed through a prism than its gaseous origin declared itself.
The spectrum consisted of several hydrogen lines, and one unknown line
in the yellow, slightly more refrangible than the sodium-pair D_{1}, and
D_{2}, and hence called D_{3}. “Je verrai ces lignes-là en dehors des
éclipses!” M. Janssen exclaimed, as they caught his eye; and on the
following morning, at Guntoor in the Neilgherries, he actually started
daylight spectroscopic work at the edge of the sun. He owed his success
to a perfectly simple principle. The ordinary invisibility of
prominences is due to the drowning of their light in reflected sunshine.
But sunshine, because it is continuous—that is, made up of beams of all
refrangibilities—can be weakened to almost any extent by dispersion,
while the detached prominence-rays lose nothing by being separated.
Hence, the result of passing the mixed light from near the solar limb
through a train of prisms is that the tell-tale bright lines stand out
distinctly from an _emaciated_ prismatic background. The method was
independently discovered by Mr. Norman Lockyer in England, and his and
Janssen’s communications on the subject were laid before the French
Academy of Sciences on the same day of October, 1868. It has proved of
inestimable value, and was further improved in 1869 by Dr. Huggins’s
device for viewing these objects in their proper shapes through an open
slit, instead of building them up in narrow sections by successive
observations through a narrow one. This was made possible by the
intensity of their light. They can be observed in variously coloured
images corresponding to the different rays they emit; but the least
refrangible of the hydrogen series—the blood-red C (alias Hκ)—is
generally chosen as being the most brilliant and best defined.
The unrecognised substance giving the yellow prominence-line was named
by Dr. Frankland “helium.” It evidently existed near the sun in enormous
quantities, and in close companionship with hydrogen. Yet no dark line
corresponding to its absorption was to be found in the Fraunhofer
spectrum, although it now and then emerged in spot-spectra. Conjectures
were rife as to its nature and relations. It was generally believed to
be specifically lighter than hydrogen, and some held it a product of its
dissociation, and so of a different elemental standing. Everything about
it, however, remained doubtful until, in March, 1895, Professor Ramsay
produced a sample for inspection close at hand, extracted by heat from
the rare mineral “clevite.” The recognition-mark was its emission, when
electrically excited, of the solar D_{3}, with which were associated
several other chromospheric rays previously registered as of unknown
origin, but now linked together as vibrations of the same molecules. A
sudden and entirely unlooked-for advance was thus made in the chemistry
of the sun’s surroundings.
Helium is a colourless gas of about twice the density of hydrogen. Its
peculiar qualities are shared only by argon, the new constituent of the
earth’s atmosphere. Both have unusual thermal relations; both are
chemically inert. They refuse to combine with any other element, and
thus stand apart from the round of multiform change involving the whole
material world. Helium is nevertheless distributed freely throughout the
universe. Hydrogen itself is scarcely more ubiquitous.
A considerable mass of information regarding the solar prominences was
rapidly collected by means of the Janssen-Lockyer invention. They were
at once divided into two classes. The “quiescent” kind occur in all
solar latitudes; they change their shapes very gradually; they have no
immediate relationship with spots. In form they resemble _pillared
clouds_ resting in banks like heavy cumuli, or floating, like expanses
of thin cirrus, high above the chromosphere with which they are
ordinarily connected by slender supports or conduit-pipes. But these are
at times invisible or non-existent. Father Secchi occasionally watched
isolated cloudlets form and grow spontaneously as if by condensation
from saturated air; and on October 13, 1880, Professor Young made a
confirmatory observation. About 11 A.M. he noticed a detached fiery mass
at an elevation of 67,500 miles above the limb. “It grew rapidly,
without any sensible rising or falling, and in an hour developed into a
large stratiform cloud, irregular on the upper surface, but nearly flat
beneath. From this lower surface pendent filaments grew out, and by the
middle of the afternoon the object had become one of the ordinary
stemmed prominences.”[14] The size of these formations is enormous. They
vary in height from about 10,000 to 100,000 miles; and ranges of them
450,000 miles in extent have been photographed during total eclipses.
[Illustration:
FIG. 3.—_Curves showing the development of Sun-spots and Prominences
during the period 1880 to 1891._ (Sidgreaves.)
]
The second class of prominences, known as “eruptive,” are obviously
manifestations of intense energy. In some of their forms they suggest
geyser-like spoutings of incandescent vapours. They represent swords and
scimetars, palms with twisted trunks composed of mounting flames,
igneous vegetation of sundry types. Their chemistry is much more complex
than that of the quiescent sort. Not only hydrogen and helium, but iron,
magnesium, sodium, and a number of other metals enter into their
composition. Belonging to the same order of disturbance with spots, they
are closely conjoined with them, both in time and space. They conform to
the sun-spot cycle, as well as to the “law of zones,” showing that
photospheric and chromospheric disturbances spring from a common cause.
Fig. 3 (from the _Observatory_ for March, 1893) embodies a comparison
between the “spotted area” as determined at Greenwich 1880–1891, and the
“profile area” of prominences (without distinction of kind) observed
spectroscopically at Stonyhurst during the years 1880–1892. The
agreement between the two curves is very striking; but the minimum of
solar activity in 1889 is decidedly better represented by the
prominence-tracing. Father Sidgreaves, director of the Stonyhurst
Observatory, adds the important remark that wide-spreading elevations of
the chromosphere attend spot-maxima, while depressions of equal extent
occur at minima.
The chromosphere is a solar envelope, but not a solar atmosphere. It
completely surrounds the sun to the depth of about 4,000 miles with a
close tissue of scarlet flames, their filamentous or tufted summits
swaying and intercrossing as if under the gusty sweep of fiery winds.
Any of these summits which attain an unwonted height become
“prominences,” but it is a mere matter of convention when the change of
nomenclature should take place. The chemical composition of the
chromosphere does not differ essentially from that of prominences. Its
permanent constituents were found by Professor Young to be hydrogen,
helium, “coronium,” and calcium, the last represented _only_ by “H” and
“K.” But disturbances never failed to be indicated by the blaze of
metallic lines, of which 273 in all have been determined by the same
authority. Their appearance signified, without doubt, the injection from
below of the corresponding vapours, chiefly those of iron, titanium,
sodium, magnesium, strontium, barium, and manganese. At moments the
reinforcement of the spectrum with bright rays was so extensive that it
seemed as if the entire “reversing layer” had been uplifted bodily into
the chromosphere.
The reversing layer lies quite close to the photosphere. It is scarcely
more than 300 miles deep, and is hence invisible except during about a
second at the beginning and end of total eclipses. Young was the first
to be favoured with a sight of it, on December 22, 1870. No sooner was
the direct solar spectrum intercepted by the moon, than “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.”[15] It was
afterwards frequently observed, and at last satisfactorily photographed
by Mr. Shackleton, a member of Sir George Baden-Powell’s expedition to
Novaya Zemlya, for the purpose of observing the total solar eclipse of
August 9, 1896. The permanent record then secured was of peculiar
importance as affording the means of confronting in detail the
components of the vario-tinted flash at the eclipsed sun’s limb with the
dusky legion of the Fraunhofer lines. The correspondence is striking,
and leaves no doubt that Young’s stratum is the actual locality where
the characteristic solar spectrum is produced. It may be described as an
universal solar ocean of glowing metallic vapours, the rays emanating
from which, although vivid when seen _off_ the sun, are thrown out in
dark relief by projection upon the white-hot photosphere. The existence
of just such a heterogeneous absorbing layer had been predicted, on
theoretical grounds, some years before it came into view.
The movements taking place in eruptive prominences are often of
portentous speed. They are betrayed, so far as they coincide with the
visual ray, by spectroscopic line-displacements; so far as they are
directed _across_ the visual ray, by immediate observation of the
spectroscopic images. Thus, the up-and-downrushes of flaming hydrogen
above spots on the disc reach velocities of 320 miles a second; and
solar tornadoes (detected by Mr. Lockyer more than a quarter of a
century ago) are often observed to whirl at rates which would be
incredible were they less well authenticated. Vertical explosions at the
limb, on the other hand, of still more unruly violence are rendered
manifest by displacements, not of the emitted lines, but of the
radiating substances themselves.
On September 19th and 20th, 1893, Father Fényi, director of the Kalocsa
Observatory in Hungary, witnessed the development and dissolution of a
pair of objects perhaps the most extraordinary in the astonishing record
of solar phenomena.[16] They broke out within nineteen hours of each
other, showed a close similarity of shape and structure, underwent
analogous changes, and, strangest of all, were situated at almost
diametrically opposite points of the solar limb. The first was already,
when first viewed at 2 P.M., 168,000 miles high; within half an hour, it
had sprung up to 224,000 miles (8′ 18″), and again subsided into a
commonplace flame of the modest dimension of 13,650 miles (30″). The
rate of ascent, directly measured (always necessarily through the medium
of the spectroscope), was 132 miles a second. This vast, though
transient construction, seemed to be formed of a multitude of distinct
fiery tongues, each leaping and flaring independently. As a whole, it
was also tongue-shaped, and “stood erect nearly in the direction of the
sun’s radius,” travelling, meanwhile, towards the earth at an average
rate of 186 miles a second.
The companion-prominence began to show at nine next morning, and, rising
with a velocity of 300 miles per second, attained in twelve minutes to a
height of 220,000 miles. This tremendous apparition was of the same
“ragged” texture as its predecessor, and shone, even in its loftiest
fragments, with the same intense glow. As might have been expected from
its opposite position, its radial movement was _from_ the earth. A
prominence measured by the same observer, July 15, 1895, was diminishing
its distance from the earth with the extraordinary velocity of 533 miles
a second; and on September 30 of the same year, a colossal object
resembling the bent and riven trunk of a great tree, was in the course
of half an hour flung upwards to a minimum altitude of 313,000 miles,
and had again faded out of sight. “The appearance,” Father Fényi wrote,
“of all the numerous great eruptions which I have observed has been such
as would be produced by a kind of explosion over a spotted region,
which, seizing upon a prominence already developed, hurls it upward from
the surface, tears it to pieces, and brings it to a speedy end.” The
matter thus acted upon is of enormous volume, but negligeable mass.
Photographs of prominence-spectra, obtained by Dr. Schuster during the
eclipse of May 17, 1882, brought out the remarkable predominance in
their light of the “H” and “K” emissions of calcium. It was
re-discovered by means of spectrographs of those objects, taken in 1891
without an eclipse, by Professor Hale at Chicago, and by M. Deslandres
in Paris. Both investigators promptly seized upon the advantage it
offered for their chemical delineation in full daylight. The lines in
question are dark and abnormally wide in the sun itself, bright and
sharp in prominences. Thus, at these particular parts of the spectrum,
the obliterating effects of scattered sunlight are non-existent. Just
here, too, photographic sensitiveness is at its maximum. Hence, by
working with either of these lines (K is preferable) nothing could be
easier than to get impressions of the brilliant forms of prominences
relieved against the background of solar absorption. (See Figures 4 and
5.) The thin, bright line is _sheltered_ from daylight glare by the
dusky, broad one. By the use of a “double slit,” the method was
completed. This, again, was simultaneously invented by Hale and
Deslandres, although they had, without suspecting it, been anticipated
by Janssen in 1869. The second slit is adjusted so as to exclude all but
a single ray of the spectrum formed by dispersing the light admitted
through the first. An unlimited power of selection is in this way
afforded as to the quality of light to be employed; but for general
purposes, K is not likely to be superseded.
In the Chicago spectroheliograph, two moveable slits, together with a
powerful diffraction spectroscope, are attached to a twelve-inch
refractor. With this instrument, monochromatic impressions of the sun
with its spots, faculæ, and flame-garland are obtained without
difficulty. To begin with, the solar disc is covered with a metal
diaphragm, then the first slit is caused to traverse the artificially
eclipsed image, the second following at such a rate that the K line
alone always falls upon the sensitive plate. The result is a complete
photographic record of the chromosphere and prominences. The diaphragm
having been then removed, the return journey of the slits is very
quickly made, so as to guard against the formidable actinic strength of
even that small element of direct sunlight contained in the K line. The
object of the second transit is to _insert_ an autographic print of the
sun itself into the space previously left blank to receive it. The
entire operation occupies less than one minute. Portrayed thus in
calcium light, the solar disc has a strange effect. It is entirely
overspread with a reticulation of irregular bright markings, greatly
emphasized over the spot-zones, and corresponding in general with the
positions of faculæ. According to Professor Hale, these masses and
wreathings of calcium vapour _are_ faculæ. M. Deslandres regards them
rather as gaseous formations connected with faculæ. Their extension and
intensity are at times so great that M. Deslandres has actually
succeeded, through the prevalence of their light, in photographing the
sun as a “bright-line star.” The double-slit method also affords the
means of studying the distribution of each element of the reversing
layer in the leisure of ordinary daylight, as M. Deslandres has shown by
some preliminary experiments.[17]
To this extent astronomers have made themselves independent of eclipses.
These momentous occurrences are, fortunately, not needed for researches
concerned with distinct coloured rays separable by dispersion from
diffuse sunshine. But with the corona it is different. For here we have
a white glory to deal with. Coronal light is derived from three sources:
from the original incandescence of solid or liquid particles, from
sunshine reflected by them, and from gaseous emissions. The most
conspicuous of these is a green ray of unknown chemical meaning. It
proceeds from every part of the corona, even from the dark rifts
separating its brilliant streamers, and the inconceivably tenuous
substance to which it owes its origin has, accordingly, received the
name of “coronium.” The coronal spectrum includes many other bright
lines, especially in the ultra-violet, photographed during eclipses; but
the hydrogen, helium, and calcium lines which accompany them probably
represent scattered chromospheric light.
[Illustration:
FIG. 4.—_Eruptive Prominence photographed by Professor Hale at the
Kenwood Observatory, March 24, 1895, at 22h. 40m. Chicago mean
time._ (_The photosphere is covered with a metallic disc._)
]
[Illustration:
FIG. 5.—_The same, 18m. later._
(From the _Astrophysical Journal_, May, 1896.)
]
The green coronal ray is much too faint to be isolated with the
spectroscope; but the continuous coronal spectrum has maxima of
intensity compared with ordinary daylight, which suggested to Dr.
Huggins, in 1882, a differential method of photographing the entire
structure apart from eclipses. It has however, as yet come to nothing,
and Hale and Deslandres have been equally unsuccessful with their
“double slit” apparatus. Hence, it is only by favour of the moon that
this wonderful appendage can be investigated, and the available moments
have not been allowed to pass in vain.
[Illustration:
FIG. 6.—_The Eclipsed Sun, photographed at Sohag in Egypt, May 17,
1882. A Comet is almost involved in the Corona._ (From
“Philosophical Transactions,” vol. clxxv.)
]
One result fully ascertained is that it changes in form concurrently
with the progress of the sun-spot period. The maximum coronal type is
entirely different from the minimum type, and reappears in unmistakable
connexion with vehement solar disturbance. This cyclical relation was
first pointed out by Mr. Ranyard. On July 29, 1878, a totality of 165
seconds was observed, under splendid conditions of weather, in the
Western States of North America. No prominences worthy of note were
visible, but the corona wore a most surprising aspect. A pair of
enormous equatorial streamers stretched east and west of the sun to a
distance of at least ten millions of miles. Indeed, they came to no
definite end. They were best seen with the naked eye, and made no show
on sensitive plates, but the application of low telescopic powers
disclosed, near the base of the effusions, a mass of delicate and
complex detail. The solar poles were as distinctively, although not so
strikingly, garnished as the solar equator. Each was the centre from
which diverged a dense brush of straight, electrical-looking rays. The
sun was at the time in a state of profound tranquillity; and it was
recalled that, at the previous minimum, in 1867, Grosch had delineated,
at Santiago, just the same equatorial extensions, and just the same
polar brushes. The connexion was emphasised during the maximum of
1882–4, by the substitution, when the moon covered the sun on May 17,
1882, and May 6, 1883, of a dazzling stellate formation for the winged
corona of 1878. In Fig. 6 is reproduced a photograph by Dr. Schuster of
the Sohag, or Egyptian corona, with the added embellishment of a comet
hurrying up to perihelion, conspicuous to the eye at the time, but never
seen again.
In 1889 the minimum type of corona reasserted itself. A drawing made by
Miss M. L. Todd during the eclipse of January 1, gave the characteristic
equatorial “fish-tails,” reaching out on the west to four solar
diameters.[18] And although the camera, owing to special difficulties,
has not yet been able to pursue them so far, Professor Barnard’s
exquisite picture (Fig. 7), taken at Bartlett’s Springs, California,
with an exposure of 4½ seconds, portrays the type to perfection, with
its suggested indefinite expansions, “the soft feathery details of the
inner corona, and the delicate fan-structures at the poles.” Two minute
notches mark the points where a couple of prominences have, by the
intensity of their actinic power, _eaten into_ the black circumference
of the lunar image.
[Illustration:
FIG. 7.—_The Corona of January 1, 1889, photographed by Professor E.
E. Barnard._
]
Nine negatives were secured by the artist, but at a considerable
personal sacrifice. “So impressive,” he wrote, “was the magnificent
spectacle upon the crowd that had gathered just outside our enclosure,
that not a murmur was heard. The frightened, half-whining bark of a dog,
and the click-click of the driving clock, alone were audible. When the
sun suddenly burst forth, an almost instantaneous and highly-surprised
cackling of the chickens, that had hastily sought their roosts at the
beginning of totality, would have been amusing could one have shaken off
the dazed feeling at the unexpectedly rapid termination of the
semi-darkness. My own feelings were those of excessive disappointment
and depression. So intent was I in watching the cameras and making the
exposures, that I did not look up to the sun during totality, and
therefore saw nothing of the corona.”
On April 16, 1893, at the height of the last sun-spot maximum, a
shadow-track crossed South America and Central Africa. Once more the
coronal type had changed. Not a trace remained of the equatorial
“wings”; not a trace of the polar “fans.” Instead, the “compass-card”
aureole of 1882 and 1883, shaped regardless of heliographic latitude,
reemerged from beneath the veil of daylight. That the sun’s filmy
“crown” follows, after its own inexplicable fashion, the general round
of solar vicissitudes, no longer admitted of a doubt. The fact is thus
stated by M. Deslandres, who observed the eclipse at Fundium, in the
Senegal district.
“The form of the corona,” he says, “undergoes periodical variations,
which follow the simultaneous periodical variations already ascertained
for spots, faculæ, prominences, auroræ, and terrestrial magnetism. This
important relation, indicated by preceding eclipses, is strongly
confirmed by the eclipse of 1893.”[19]
Professor Schaeberle’s photographs, taken on the same occasion at Mina
Bronces in Chili, marked a decided advance in coronal portraiture. The
sun’s disc measured four inches on his plates, exposed with a
photoheliograph forty feet in length; and the details of inner coronal
construction came out accordingly with unprecedented perfection. The
corona of August 9, 1896, reproduced the most striking features of the
corona observed August 29, 1886; and both corresponded to an
intermediate epoch of the spot-cycle. The polar brushes were present
without the equatorial extensions, while in both a protruding ray made
an angle of some thirty or forty degrees with the solar axis. This
distinctive trait imprinted itself with surprising emphasis on some of
Sir George Baden-Powell’s Novaya Zemlya photographs.
Researches, prosecuted under cover of eighteen eclipses, have greatly
strengthened the visible analogy between coronal streamers, auroral
coruscations, and comets’ tails. The persuasion that electrical
discharges in high vacua are concerned in all these phenomena is not
easily resisted. Repulsive forces such as are at work in Crookes’ tubes
perhaps come into play, on the vast solar scale, to produce the strange
and beautiful luminous forms revealed during eclipses. Their tenuity is
certainly extreme. They probably contain very much less matter, volume
for volume, than the incredibly exhausted tubes of modern physicists.
The unresisted passage of comets through the corona demands this
supposition, which is in complete accord with the fineness of the
Fraunhofer lines. The corona shows no increase of density downwards, and
the chromosphere very little. Hence neither can be a true solar
atmosphere, weighing freely upon the sun’s surface. For, under the
immense power of solar gravity, the accumulated pressure of the
superincumbent layers, even if there were only one hundred miles’
thickness of them, could not be intelligibly conveyed in figures; how
much less when the piling-up of the aerial strata is reckoned by
thousands of miles!
To recapitulate. Starting from the photosphere, we meet first an
envelope producing the _general_ absorption, by which sunlight is
enfeebled and reddened as if by the interposition of a slightly rufous
shade. Next comes the reversing layer composed of mixed incandescent
vapours, giving rise, by their _selective_ absorption, to the Fraunhofer
lines. No alterations in correspondence with the spot-cycle have yet
been determined in either of these couches, which, close as they lie to
the photosphere, remain, nevertheless, apparently indifferent to its
agitations. They are overspread by the chromosphere and prominences;
while above and beyond shines the mysterious corona; both chromosphere
and corona strictly conforming, by manifest changes, to the sun’s
periodicity. One other solar appendage remains to be noticed.
After sunset in spring, and before sunrise in autumn, a mass of soft
luminosity, often brighter than the Milky Way, may be seen tapering
upward from the horizon along an axis approximating to the line of the
ecliptic. Its more conspicuous visibility at those times just reverses
the case of the harvest moon. As a rule, the apex of the cone barely
reaches the Pleiades; but it does not really end here. Thrice during the
present century, by Brorsen, Backhouse, and Barnard, the zodiacal
“counterglow” has been independently discovered and studied. This is a
hazy, luminous patch, ten to fifteen degrees across, and exactly 180°
from the sun. It represents the _opposition aspect_ of the Zodiacal
Light, hence proved to be a formation in planetary space, extending
considerably beyond the earth’s orbit. Two plausible hypotheses as to
its nature have been proposed. Professor Searle[20] holds it to
represent the reflection of sunlight from “an infinite number of small
asteroids.” Professor Bigelow[21] considers it as an amassment in the
plane of the sun’s equator—“a place of zero potential”—of the particles
electrically expelled from the poles. The Light is then, if this view be
correct, an extension of the corona—a sort of “pocket or receptacle,
wherein the coronal matter is accumulated and retained as a solar
accompaniment.” A continuous spectrum is derived from it; no element of
original emission can be detected; so that the spectroscope “holds the
balance even” between the two theories. If, however, the latter were
true, the Zodiacal Light should spread out from the sun’s equator; if
the former, then its medial plane should deviate very slightly from that
of the ecliptic, to which the fundamental, or “invariable” plane of the
solar system is inclined only one and a half degrees. M. Marchand’s
observations from the Pic du Midi[22] appear to be decisive on the
point. During three years, he mapped down the limits assigned by his
observations night after night, to an emanation which, in that pure air,
was seen to compass the entire sphere. The eventual comparison of his
collected data showed its axis to be a great circle sensibly coincident
with the sun’s equator. All reasonable doubt as to the nature of the
Zodiacal Light has thus been removed. It is a reservoir for the sun’s
waste matter—the sink, into which are daily flung the particles rejected
through the agency of the aigrettes and streamers composing the
wonderful eclipse-vision of the corona.
CHAPTER IV.
THE INTERIOR PLANETS.
The Interior Planets are those which revolve within the earth’s orbit.
They are two in number—Mercury and Venus. Mercury, with a diameter of
three thousand miles, is the smallest of the eight principal planets. It
pursues a track, too, more eccentric and more highly inclined to the
ecliptic than any other planetary orbit. The zodiac had of old to be
made 16° wide in order to afford room for its excursions. These
irregularities are, however, quite innocuous as regards the stability of
the system, for the reason that they belong to a body of insignificant
mass. The successive approaches to it of Encke’s comet have afforded a
means of ascertaining its gravitative power; and, according to the
latest report from this filmy messenger, it is even less than had been
supposed. Mercury, it appears, weighs little more than one-ten-millionth
of the sun, or one-thirtieth of the earth. And since its volume is about
one-nineteenth the terrestrial, the matter of which it is composed must
be less dense in the proportion of 30 to 19. So that the planet would
turn the balance against one equal globe of granite, or three and a half
of water. We can hence easily calculate that gravity, at Mercury’s
surface, possesses less than one-fourth its power at the earth’s
surface. A man of sixteen stone transported thither, would find himself
relieved of fully three-quarters of his habitual burthen.
The plane of Mercury’s orbit makes an angle of 7° with the ecliptic, and
he traverses it with a speed varying from 23 to 35 miles a second. The
corresponding distances from the sun are 43½ and 28½ million miles,
while the mean distance, or semi-major axis of the ellipse, measures
just 36 millions. Independently then of what we call seasons, Mercury is
subject, in the course of its year of 88 days, to considerable
vicissitudes of temperature. At perihelion it receives nine times, at
aphelion only four times, more heat than is imparted by the sun to an
equal area of the earth.
The crucial point as regards the physical condition of a planet is the
presence or absence of an atmosphere. And there is decisive evidence
that Mercury is in this respect poorly provided. Certain luminous
phenomena, often observed during its transits across the sun, appear to
be of purely optical production, since they are less conspicuous with
good than with indifferent telescopes; while, on the other hand, genuine
refractive effects are absent. A corresponding indication is afforded by
the low “albedo,” that is, the slight reflective power of this planet.
Of the light flooding its surface only 17 per cent.[23] is returned; 83
per cent. is absorbed. Now the albedo of clouds is about 72; a
cloud-wrapt globe is little less brilliant than if it were covered with
fresh-fallen snow. Hence a high albedo accompanies a dense, vapour-laden
atmosphere; a low albedo indicates a transparent one. And since Mercury,
which sends back only about as much light as if it were made of grey
granite, has the lowest albedo of any of the principal planets, it may
be safely concluded to possess the thinnest aerial covering. Yet it is
not, apparently, a totally airless globe. Spots upon its surface have
been seen to become effaced as if by atmospheric veilings; and the
spectroscope hints (although doubtfully) at aqueous absorption.
Mercury is “new” when nearest to the earth, and “full” when most remote
from it. At both these periods, moreover, its position with regard to
the sun renders it ordinarily invisible; so that it is usually seen as
either gibbous or crescent shaped. The study of its phases has brought
out a noteworthy circumstance. It is easy to understand that geometrical
light changes will not proceed by the same gradations upon a smooth and
upon a rugged globe, where they are complicated by irregular shadows and
illuminations. The laws of variation are quite different in each case,
and their respective prevalence can be distinguished by steady
observation. There seems no reason to doubt that the latter are obeyed
by Mercury. After several years’ watching of its phases, Professor G.
Müller[24] of Potsdam concludes them to be such as characterise a broken
and uneven surface.
[Illustration:
FIG. 8.—_Map of Mercury, by Schiaparelli._ (From _Astronomische
Nachrichten_, No. 2944.)
]
Little or nothing was known about the rotation of Mercury when
Schiaparelli of Milan undertook its determination in 1882. His
observations were made in full daylight, in order to reduce atmospheric
disturbances to a minimum; and he executed, in the course of a few
months, a series of 150 Mercurian delineations upon which is founded the
planisphere exhibited in Fig. 8. The surface of the planet, coloured
light rose with a coppery tinge, was seen to be diversified by
brownish-red markings which became effaced towards the limb as if
through atmospheric absorption. Although evidently of a permanent
nature, their outlines escaped precise definition. The most remarkable
circumstance about them was that they showed no effects of rotation.
During several consecutive hours of watching, they remained sensibly
fixed in their places. The conclusion was finally arrived at that
Mercury rotates on a nearly upright axis in the same time that it
revolves round the sun. Its day, no less than its year, is equal to 88
of our days. Consequently it turns at all times substantially the same
face towards the sun; and the “terminator,” that is, the dividing-line
between darkness and light, only “librates,” without travelling right
round the globe. The librations of Mercury are, however, extensive in
proportion to the eccentricity of its orbit; hence, five-eighths of its
surface come in for some share of illumination during the Mercurian
year. Over the remaining three-eighths darkness reigns supreme.
“There is no light in earth or heaven,
But the cold light of stars.”
Satisfactory confirmation of this curious result was obtained by Mr.
Percival Lowell at the Flagstaff Observatory in Arizona during the
autumn of 1896.[25] In Schiaparelli’s map, the axis of rotation lies in
the plane of the paper, and the centre of the projected sphere thus
represents the point on Mercury’s surface where the sun is vertical at
perihelion and aphelion; A and B, 23° 41′ to the east and west of it
respectively, marking the places where the sun is vertical at the
libration-limits. That formidable luminary oscillates from the zenith of
A to the zenith of B and back in 88 days, occupying, in consequence of
the planet’s unequal motion, 51 in describing the arc from east to west
(left to right), but only 37 in retracing it from west to east.[26]
The effects of these arrangements upon climate must be exceedingly
peculiar. They cannot readily be traced in detail; but, thin as the
Mercurian atmosphere is, it must be to some extent operative in
modifying the contrast in temperature between the two hemispheres.
Except in a few favoured localities, the existence of liquid water must
be impossible in either. Mercurian oceans, could they ever have been
formed, should long ago have been boiled off from the hot side, and
condensed in “thick-ribbed ice” on the cold side.
Mercury is then, according to our ideas, totally unfitted to be the
abode of organic life. Nor can it at any time have been more favourably
circumstanced than at present. We need not hesitate to assert that its
rotation was reduced to its actual minimum rate by the power of tidal
friction. The brake was, moreover, applied by the sun. The attainment of
rapid gyration was prevented by the resistance of solar tides raised on
a plastic mass. Disruption was accordingly rendered impossible. The
planet was, by anticipation, deprived of satellites, and remained
undivided and solitary.
Venus, the earths nearest planetary neighbour, might be called its twin.
Its diameter being 7,700 miles, it is of nearly the same size; it is not
greatly inferior in mean density; gravity at its surface is of more than
four-fifths its terrestrial strength, and it is supplied with an
extensive atmosphere. Its movements are placid and well-regulated. In a
period of 225 days it revolves at the rate of 22 miles per second in an
almost circular track, deviating but slightly from the plane of the
ecliptic. Its distance from the sun is 67,200,000 miles; hence it
receives just twice as much heat and light as the earth. Moreover, it
reflects at least 65 per cent. of the light incident upon it. Viewed in
the same telescopic field with Mercury during a close conjunction in
1878, it shone, James Nasmyth reported, like burnished silver, while
Mercury appeared as dull as zinc or lead. Yet Mercury is illuminated, on
an average, three and a half times more intensely than its neighbour.
Atmospheric effects are conspicuous on Venus. At the beginning and end
of transits, the part of the little black disc off the sun, has
constantly been seen silver-edged through refraction; and when the
planet, at inferior conjunction, passes above or below the sun, its
whole circumference is not unfrequently bordered with a halo of solar
rays, bent inwards as if by the action of a lens. Just in the same way,
the _geometrical_ rising of the heavenly bodies is _visually_
anticipated, and their setting delayed on the earth, by the curvature of
the beams refracted in passing through its atmosphere—or rather, through
half of it; while we, as spectators of Venus from the outside, perceive
the entire effect. Made on equal terms, the comparison is greatly to the
disadvantage of the earth. Refraction, as directly measured on Venus,
considerably exceeds its terrestrial amount; and the measurable
refraction is only that produced in the higher part of the air
surmounting the shell of clouds which constitutes the planet’s visible
surface. Thus, at the cloud-level a barometer would, by the lowest
estimate, stand at 35 inches, while at the same altitude of, say, two
miles, the column of mercury would, on the earth, drop to 21 inches. It
is, indeed, very likely that the aerial envelope of Venus weighs twice
as much as our own.
The occasional visibility of the dark side of Venus is still
unexplained. The appearance is indistinguishable except in scale from
that of the “old moon in the new moon’s arms”; but illumination by
earthshine, which is fully competent to produce the lunar effect,
practically vanishes at the distance of Venus. The “ashen light,” as it
is called, ordinarily shows only when the planet figures as a narrow
crescent; but M. Brenner of the Manora Observatory, who has a knack of
being unprecedented, saw it in June, 1895,[27] during the gibbous phase.
The appearances of this pale gleam follow no traceable law. They occur
unsought; and are recalcitrant to vigilant expectation. Their closest
analogy is with our auroræ. The “phosphorescence” of the dark side of
Venus may quite reasonably be set down as of an electrical nature. But
it does not seem, like terrestrial auroræ, to follow the lines of a
magnetic system.
Distinct spectroscopic indications of aqueous absorption in the
atmosphere of Venus were perceived, during the transits of 1874 and
1882, by Tacchini, Riccò, and Young. They accord well with the
“snow-caps,” which are one of the many puzzling Cytherean features.
Since these can be resolved into groups of brilliant points, they
represent, in the opinion of the late M. Trouvelot, mountainous
formations penetrating the reflective stratum, and shining, lustrous
with snow, in the clear upper air. They might almost equally well be
cloud-like condensations of a permanent kind, called into existence by
topographical peculiarities, and hence, after a fashion, _rooted in the
soil_. On the other hand, Mr. Lowell questions their reality in any
form; and his drawings represent extraordinarily sure seeing.
[Illustration:
FIG. 9.—_Venus, from a drawing by Mascari._ (_Nature_, February 20,
1896.)
]
The only point regarding the planet’s rotation upon which astronomers
are agreed is that its axis is nearly perpendicular to the place of its
orbit. As to its period, the divergence is enormous. It reaches all the
way from 24 hours to 225 days. Bad as is the telescopic holding-ground
on Mercury, that afforded by Venus is worse still. The disc falls off
rapidly in brightness from the limb towards the terminator, and is
sometimes diversified by filmy and indefinite markings, obviously of
atmospheric origin (in Fig. 9 the shadings are much too pronounced).
Attempts to use them as fiducial points are foredoomed to failure. The
period, accordingly, of 23^h 21^m arrived at by forcing into artificial
agreement the observations of Cassini at Bologna, of Bianchini and De
Vico in Rome, obtained small credit. The subject lay, as it were,
dormant until Schiaparelli made, in 1890, the provisional announcement
that Venus rotates on the same plan as Mercury. A clamour of
contradiction was immediately raised, and a large amount of evidence on
both sides of the question has since been collected. It is curious to
notice that, setting aside the opposite conclusions of Terby and
Brenner, the Alps mark a dividing-line between the pros and the cons.
Schiaparelli’s period of 224·7 days (ratified by himself in 1895) is
supported by Perrotin’s observations both at Nice and Mont Mounier; by
Tacchini’s at Rome, Cerulli’s at Teramo, and Mascari’s at the
complementary establishments of Catania and Mount Etna; while Niesten,
Trouvelot, Villiger, Stanley Williams, and Flammarion, all under some
disadvantage as regards climate, aver that the debated gyration is
performed in “about” 24 hours. Now, in the first place, a period of 24
hours is in itself open to suspicion, since all delicate observations
are liable to be affected by diurnal atmospheric variations; in the
second, it is mainly, if not entirely, based upon supposed changes in
almost evanescent shadings, while the long period of 224·7 days has been
derived fundamentally, from the immobility relative to the terminator,
of definite and permanent topographical features. The perfect roundness
of the disc of Venus affords independent proof of extremely slow
rotation.
Spectroscopic evidence may before long become available. The quantity to
be measured by the exquisite method of line-displacements is, indeed, at
the most extremely small. The equatorial velocity of Venus would, with
the 24-hour period, but slightly exceed a quarter of a mile a second;
but this effect being doubled by reflexion from the planet, and doubled
again by juxtaposition of light from its east and west limbs, could
probably be made distinctly perceptible. In the negative case, the value
of the support lent to the long-period hypothesis can only be appraised
by the degree of refinement attained in the research.