THE SCIENCE OF ASTRONOMY.
In a work of this kind, professedly devoted to a very brief and popular
view of the different scientific subjects, much cannot be said on any
special branch of science; it will be better, therefore, to take up one
subject in astronomy, and by discussing it in a simple manner, our young
friends may be stimulated to learn more of those glorious truths which
are to be found in the published works of many eminent astronomers, and
especially in that of Mr. Hind, called "The Illustrated London
Astronomy." One of the most interesting subjects is the phenomenon of
the eclipse of the sun; and as 1858 is likely to be long remembered for
its "annular eclipse," we shall devote some pages and illustrations to
this subject.
Eclipses of the sun are of three kinds--partial, annular, and total.
Many persons have probably seen large partial eclipses of the sun, and
may possibly suppose that a total eclipse is merely an intensified form
of a partial one; but astronomers assert that no degree of partial
eclipse, even when the very smallest portion of the sun remains visible,
gives the slightest idea of a total one, either in the solemnity and
overpowering influence of the spectacle, or the curious appearances
which accompany it.
The late Mr. Baily said of an eclipse (usually called that of Thales),
which caused the suspension of a battle between the Lydians and Medes,
that only a total eclipse could have produced the effect ascribed to it.
Even educated astronomers, when viewing with the naked eye the sun
nearly obscured by the moon in an annular eclipse, could not tell that
_any part_ of the sun was hidden, and this was remarkably verified in
the annular eclipse of the 15th March of this year.
During the continuance of a total eclipse of the sun, we are permitted a
hasty glance at some of those secrets of Nature which are not revealed
at any other time--glories that hold in tremulous amazement even veteran
explorers of the heavens and its starry worlds.
The general meaning of an eclipse may be shown very nicely by lighting a
common oil, or oxy-hydrogen lantern in a darkened room, and throwing the
rays which proceed from it on a three-feet globe. The lantern may be
called the sun, and, of course, it is understood that correct
comparative sizes are not attempted in this arrangement; if it were so,
the globe representing the earth would have to be a mere speck, for if
we make the model of the sun in proportion to a three-feet globe, no
ordinary lecture hall would contain it. This being premised, attention
is directed to the lantern, which, like the sun, is self-luminous, and
is giving out its own rays; these fall upon the globe we have designated
the earth, and illuminate one-half, whilst the other is shrouded in
darkness, reminding us of the opacity of the earth, and teaching, in a
familiar [Page 20] manner, the causes of day and night. Another globe,
say six inches in diameter, and supported by a string, may be compared
to the moon, and, like the earth, is now luminous, and shines only by
borrowed light: the moon is simply a reflector of light; like a sheet of
white cardboard, or a metallic mirror. When, therefore, the small globe
is passed between the lantern and the large globe, a shadow is cast on
the large globe: it is also seen that only the half of the small globe
turned towards the lantern is illuminated, while the other half,
opposite the large globe, is in shadow or darkness. And here we
understand why the moon appears to be black while passing before the
sun; so also by moving the small globe about in various curves, it is
shown why eclipses are only visible at certain parts of the earth's
surface; and as it would take (roughly speaking) fifty globes as large
as the moon to make one equal in size to our earth, the shadow it casts
must necessarily be small, and cannot obscure the whole hemisphere of
the earth turned towards it. An eclipse of the sun is, therefore, caused
by the opaque mass of moon passing between the sun and the earth. Whilst
an eclipse of the moon is caused by the earth moving directly between
the sun and the moon: the large shadow cast by the earth renders a total
eclipse of the moon visible to a greater number of spectators on that
half of the earth turned towards the moon. All these facts can be
clearly demonstrated with the arrangement already described, of which we
give the following pictorial illustration:--
[Illustration: Fig. 25.]
In using this apparatus, it should be explained that if the moon were as
large as the sun, the shadow would be cylindrical like the figure 1, and
of an unlimited length. If she were of greater magnitude, it would
precisely resemble the shadow cast in the experiment already adduced
with the lantern and shown at No. 2. But being so very much smaller than
the sun, the moon projects a shadow which converges to a point as shown
in the third diagram.
[Page 21]
[Illustration: Fig. 26.]
[Illustration: Fig. 27.]
[Illustration: Fig. 28.]
In order to comprehend the difference between an annular and a total
eclipse of the sun, it is necessary to mention the apparent sizes of the
sun and moon: thus, the former is a very large body--viz., eight hundred
and eighty-seven thousand miles in diameter; but then, the sun is a very
long way off from the earth, and is ninety millions of miles distant
from us; therefore, he does not appear to be very large: indeed, the sun
seems to be about the same size as the moon; for, although the sun's
diameter is (roughly speaking) four hundred times greater than that of
the moon, he is four hundred times further away from us, and,
consequently, the sun and moon _appear_ to be the same size, and when
they come in a straight line with the eye, the nearer and smaller body,
the moon, covers the larger and more distant mass, the sun; and hence,
we have either an annular, or a total eclipse, showing how a small body
may come between the eye and a larger body, and either partially or
completely obscure it.
[Page 22]
With respect to an annular eclipse, it must be remembered, that the
paths of all bodies revolving round others are elliptical; _i.e._, they
take place in the form of an ellipse, which is a figure easily
demonstrated; and is, in fact, one of the conic sections.
If a slice be taken off a cone, parallel with the base, we have a circle
thus--
[Illustration: Fig. 29.]
If it be cut obliquely, or slanting, we see at once the figure spoken
of, and have the ellipse as shown in this picture.
[Illustration: Fig. 30.]
Now, the ellipse has two points within it, called "the foci," and these
are easily indicated by drawing an ellipse on a diagram-board, in which
two nails have been placed in a straight line, and about twelve inches
apart. Having tied a string so as to make a loop, or endless cord, a
circle may first be drawn by putting the cord round one of the nails,
and holding a piece of chalk in the loop of the string, it may be
extended to its full distance, and a circle described; here a figure is
produced round one point, and to show the difference between a circle
and an ellipse, the endless cord is now placed on the two nails, and the
chalk being carried round inside the string, no longer produces the
circle, but that familiar form called the oval. As a gardener would say,
an oval has been struck; and the two points round which it has been
described, [Page 23] are called the _foci_. This explanation enables us
to understand the next diagram, showing the motion of the earth round
the sun; the latter being placed in one of the foci of a very moderate
ellipse, and the various points of the earth's orbit designated by the
little round globes marked A, B, C, D, where it is evident that the
earth is nearer to the sun at B than at D. In this diagram the ellipse
is exaggerated, as it ought, in fact, to be very nearly a circle.
[Illustration: Fig. 31.]
[Illustration: Fig. 32.]
We are about three millions of miles nearer to the sun in the winter
than we are in the summer; but from the more oblique or slanting
direction of the rays of the sun during the winter season, we do not
derive any increased heat from the greater proximity. The sun,
therefore, apparently varies in size; but this seeming difference is so
trifling that it is of no importance in the discussion: and here we may
ask, why does [Page 24] the earth move round the sun? Because it is
impelled by _two forces_, one of which has already been fully explained,
and is called the _centrifugal_ power, and the other, although termed
the _centripetal_ force, is only another name for the "attraction of
gravitation."
[Illustration: Fig. 33.]
[Illustration: Fig. 34.]
To show their mutual relations, let us suppose that, at the creation of
the universe, the earth, marked A, was hurled from the hand of its
Maker; according to the law of inertia, it would continue in a straight
line, A C, for ever through space, provided it met with no resistance or
obstruction. Let us now suppose the earth to have arrived at the point
B, and to come within the sphere of the attraction of the sun S; [Page
25] here we have at once contending forces acting at right angles to
each other; either the earth must continue in its original direction, A
C, or fall gradually to the sun. But, mark the beauty and harmony of the
arrangement: like a billiard-ball, struck with equal force at two points
at right angles to each other, it takes the mean between the two, or
what is termed the diagonal of the parallelogram (as shown in our
drawing of a billiard-table), and passes in the direction of the curved
line, B D; having reached D, it is again ready to fly off at a tangent;
the centrifugal force would carry it to E, but again the gravitating
force controls the centripetal, and the earth pursues its elliptical
path, or orbit, till the Almighty Author who bade it move shall please
to reverse the command.
[Illustration: Fig. 35.]
The mutual relations of the centripetal and centrifugal forces may be
illustrated by suspending a tin cylindrical vessel by two strings, and
having filled it with water, the vessel may be swung round without
spilling a single drop; of course, the movement must be commenced
carefully, by making it oscillate like a pendulum.
[Illustration: Fig. 36.]
The cord which binds it to the finger may be compared to the centripetal
force, whilst the centrifugal power is illustrated by the water pressing
against the sides and remaining in the vessel. Upon the like principles
the moon revolves about the earth, but her orbit is more elliptical than
that of the earth around the sun; and it is evident from our diagram
that the moon is much further from the earth at A than at B. As a
natural consequence, the moon appears sometimes a little larger and
sometimes smaller than the sun; the apparent mean diameter of the latter
being thirty-two minutes, whilst the moon's apparent diameter varies
from twenty-nine and a half to thirty-three and a half minutes. Now, if
the moon passes exactly between us and the sun when she is apparently
largest, then a total eclipse takes place; whereas, if she glides
between the sun and ourselves when smallest--_i.e._, when furthest off
from the earth--then she is not sufficiently [Page 26] large to cover
the sun entirely, but a ring of sunlight remains visible around her, and
what is called an annular eclipse of the sun occurs. This fact may be
shown in an effective manner by placing the oxy-hydrogen lantern before
a sheet, or other white surface, and throwing a bright circle of light
upon it, which may be called the sun; then, if a round disc of wood be
passed between the lantern and the sheet, at a certain distance from the
nozzle of the lantern, all the light is cut off, the circle of light is
no longer apparent, and we have a resemblance to a total eclipse.
[Illustration: Fig. 37.]
By taking the round disc of wood further from the lantern, and repeating
the experiment, it will be found that the whole circle of light is not
obscured, but a ring of light appears around the dark centre,
corresponding with the phenomenon called the annular (ring-shaped)
eclipse.
If a bullet be placed very near to one eye whilst the other remains
closed, a large target may be wholly shut out from vision; but if the
bullet be adjusted at a greater distance from the eye, then the centre
only will be obscured, and the outer edge or ring of the target remains
visible.
When the advancing edge, or first _limb_, as it is termed, of the moon
approaches very near to the second limb of the sun, the two are joined
together for a time by alternations of black and white points, called
Baily's beads.
This phenomenon is supposed to be caused partly by the uneven and
mountainous edge of the moon, and partly by that inevitable fault of
telescopes, and of the nervous system of the eye, which tends to enlarge
the images of luminous objects, producing what is called irradiation. It
is exceedingly interesting to know that, although the clouds obscured
the annular eclipse of 1858, in many parts of England, we are yet [Page
27] left the recorded observations of one fortunate astronomer, Mr. John
Yeats, who states that--
"All the phenomena of an annular eclipse were clearly and beautifully
visible on the Fotheringay-Castle-mound, which is a locality easily
identified. Baily's beads were perfectly plain on the completion of the
_annulus_, which occurrence took place, according to my observation, at
about seventy seconds after 1 o'clock; it lasted about eighty seconds.
The 'beads,' like drops of water, appeared on the upper and under sides
of the moon, occupying fully three-fourths of her circumference.
"Prior to this, the upper edge of the moon seemed dark and rough, and
there were no other changes of colour. At 12.43, the cusps, for a few
moments, bore a very black aspect.
"There was nothing like intense darkness during the eclipse, and less
gloom than during a thunderstorm. Bystanders prognosticated rain; but it
was the shadow of a rapidly-declining day. At 12 o'clock, a lady living
on the farm suddenly exclaimed, 'The cows are coming home to be milked!'
and they came, all but one; that followed, however, within the hour.
Cocks crowed, birds flew low or fluttered about uneasily, but every
object far and near was well defined to the eye.
"A singular broadway of light stretched north and south for upwards of a
quarter of an hour; from about 12.54 to 1.10 P.M."
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
If the annular eclipse of the sun be a matter for wonderment, the total
eclipse of the same is much more surprising; no other expression than
that of _awfully grand_, can give an idea of the effects of totality,
and of the suddenness with which it obscures the light of heaven. The
darkness, it is said, comes dropping down like a mantle, and as the
moment of full obscuration approaches, people's countenances become
livid, the horizon is indistinct and sometimes invisible, and there is a
general appearance of horror on all sides. These are not simply the
inventions [Page 28] of active human imaginations, for they produce
equal, if not greater effects, upon the brute creation. M. Arago quotes
an instance of a half-starved dog, who was voraciously devouring some
food, but dropped it the instant the darkness came on. A swarm of ants,
busily engaged, stopped when the darkness commenced, and remained
motionless till the light reappeared. A herd of oxen collected
themselves into a circle and stood still, with their horns outward, as
if to resist a common enemy; certain plants, such as the convolvulus and
silk-tree acacia, closed their leaves. The latter statement was
corroborated during the annular eclipse of the 15th of March, 1858, by
Mr. E. S. Lane, who states, that crocuses at the Observatory, Beeston,
had their blossoms expanded before the eclipse; they commenced closing,
and were quite shut at about one minute previous to the greatest
darkness; and the flowers opened partially about twenty minutes
afterwards. A "_total eclipse_" of the sun has always impressed the
human mind with terror and wonder in every age: it was always supposed
to be the forerunner of evil; and not only is the mind powerfully
impressed, as darkness gradually shuts out the face of the sun, but at
the moment of totality, a magnificent corona, or glory of light, is
visible, and prominences, or flames, as they are often termed, make
their appearance at different points round the circle of the dark mass.
This glory does not flash suddenly on the eye; but commencing at the
first limb of the sun, passes quickly from one limb to the other. Our
illustration shows "the corona" and the "rose-coloured prominences,"
whose nature we shall next endeavour to explain. Professor Airy
describes the change from the last narrow crescent of light to the
entire dark moon, surrounded by a ring of faint light, as most curious,
striking, and magical in effect. The progress of the formation of the
corona was seen distinctly. [Page 29] It commenced on the side of the moon
opposite to that at which the sun disappeared, and in the general decay
and disease which seemed to oppress all nature, the moon and the corona
appeared almost like a local sore in that part of the sky, and in some
places were seen double. Its texture appeared as if fibrous, or composed
of entangled threads; in other places brushes, or feathers of light
proceeded from it, and one estimate calculated the light at about
one-seventh part of a full moon light. The question, whether the corona
is concentric with the sun and moon, was specially mooted by M. Arago,
and Professor Baden Powell has produced such excellent imitations of the
"corona" by making opaque bodies occult, or conceal, very bright points,
that it cannot be considered as material or real, although it ought to
be remembered that the best theory of the zodiacal light represents it
to be a nebulous mass, increasing in density towards the sun, and yet no
portion of this nebulous mass was seen during the totality. But by far
the most remarkable of all the appearances connected with a "total
eclipse" are the rose-coloured prominences, mountains, or flames,
projecting from the circumference of the moon to the inner ring of the
corona; and, although they had been observed by Vaserius (a Swedish
astronomer) in 1733, they took the modern astronomers entirely by
surprise in 1842, and they were not prepared with instruments to
ascertain the nature of these strange and almost portentous forms. In
1851, however, great preparations were made to throw further light on
the subject. Professor Airy went to make his observations, and he says,
"That the suddenness of the darkness in 1851 appeared much more striking
than in 1842, and the forms of the rose-coloured mountains were most
curious. One reminded him of a boomerang (that curious weapon thrown so
skilfully by the aborigines of Australia); this same figure has been
spoken of by others as resembling a Turkish scimitar, strongly coloured
with rose-red at the borders, but paler in the centre. Another form was
a pale-white semicircle based on the moon's limbs; a third figure was a
red detached cloud, or balloon, of nearly circular form, separated from
the moon by nearly its own breadth; a fourth appeared like a small
triangle, or conical red mountain, perhaps a little white in the
interior;" and the Professor proceeds to say, "I employed myself in an
attempt to draw roughly the figures, and it was impossible, after
witnessing the increase in height of some, and the disappearance of
another, and the arrival of new forms, not to feel convinced that the
phenomena belonged to the sun, and _not_ to the moon."
Still the question remains unanswered, what are these "rose-coloured
prominences?" If they belong to the sun, and are mountains in that
luminary, they must be some thirty or forty thousand miles in height.
M. Faye has formally propounded the theory, that they are caused by
refraction, or a kind of mirage, or the distortion of objects caused by
heated air. This phenomenon is not peculiar to any country, though most
frequently observed near the margin of lakes and rivers, and on hot
sandy plains. M. Monge, who accompanied Buonaparte in his [Page 30]
expedition to Egypt, witnessed a remarkable example between Alexandria
and Cairo, where, in all directions, green islands appeared surrounded
by extensive lakes of pure, transparent water. M. Monge states that
"Nothing could be conceived more lovely or picturesque than the
landscape. In the tranquil surface of the lake, the trees and houses
with which the islands are covered were strongly reflected with vivid
and varied hues, and the party hastened forward to enjoy the refreshment
apparently proffered them; but when they arrived, the lake, on whose
bosom the images had floated--the trees, amongst whose foliage they
arose, and the people who stood on the shore, as if inviting their
approach, had all vanished, and nothing remained but the uniform and
irksome desert of sand and sky, with a few naked and ragged Arabs."
If M. Monge and his party had not been undeceived, by actually going to
the spot, they would, one and all, have been firmly convinced that these
visionary trees, lakes, and buildings had a real existence. This kind of
mirage is known in Persia and Arabia by the name of "serab" or
miraculous water, and in the western districts of India by that of
"scheram." This illusion is the effect of unusual refraction, and M.
Faye attempts to account for the rose-coloured mountains by something of
a similar nature.
It is right, however, to mention, that learned astronomers do not
consider this theory of any value.
Lieutenant Patterson, one of the observers of the eclipse of 1851, says,
that "It is very remarkable that the flames or prominences correspond
exactly (at least as far as he could judge) with the spots on the sun's
surface." Taking this statement with that of M. Faye, it may be assumed,
as a new idea, and nothing more, that these prominences are, after all,
mere aerial pictures of these openings in the sun's atmosphere, or what
are called "sun spots." In the "Edinburgh Philosophical Journal," it is
said, that although it has lately been shown in the Edinburgh
Observatory that it is possible to produce, by certain optical
experiments, red flames on the sun's limb of precisely the rose-coloured
tint described, yet, on weighing the whole of the evidence, there does
seem a great preponderance in favour of the eclipse flames being real
appendages of the sun, and in that case they must be masses of such vast
size as to play no unimportant part in the economy of that stupendous
orb.
During the last eclipse great disappointment was felt that the darkness
was so insignificant, although, when we consider the enormous
light-giving power of the sun, and know that it was not wholly obscured,
we could hardly have expected any other result. There can be no doubt
that a decided change in the amount of light is only to be observed
during a total eclipse of the sun, one of which occurred on the 7th of
September, 1858; but, unfortunately, it was only visible in South
America; we must therefore content ourselves with the descriptions of
those astronomers who can be fully relied on. From the graphic account
given by Professor Piazzi Smyth, the astronomer-royal for Scotland,
[Page 31] of a total eclipse as seen by him on the western coast of
Norway, we may form some notion of the imposing appearance of the
surrounding country when obscured during the occurrence of this rare
astronomical phenomenon.
The Professor remarks, "To understand the scene more fully, the reader
must fancy himself on a small, rocky island on a mountainous coast, the
weather calm, and the sky at the beginning of the eclipse seven-tenths
covered with thin and bright cirro-strati clouds. As the eclipse
approaches, the clouds gradually darken, the rays of the sun are no
longer able to penetrate them through and through, and drench them with
living light as before, but they become darker than the sky against
which they are seen. The air becomes sensibly colder, the clouds still
darker, and the whole atmosphere murkier.
"From moment to moment as the totality approaches, the cold and darkness
advance apace; and there is something peculiarly and terribly convincing
in the two different senses, so entirely coinciding in their indications
of an unprecedented fact being in course of accomplishment. Suddenly,
and apparently without any warning (so immensely greater were its
effects than those of anything else which had occurred), the totality
supervenes, and darkness _comes down_. Then came into view lurid lights
and forms, as on the extinction of candles. This was the most striking
point of the whole phenomenon, and made the Norse peasants about us flee
with precipitation, and hide themselves for their lives.
"Darkness reigned everywhere in heaven and earth, except where, along
the north-eastern horizon, a narrow strip of unclouded sky presented a
low burning tone of colour, and where some distant snow-covered
mountains, beyond the range of the moon's shadow, reflected the faint
mono-chromatic light of the partially eclipsed sun, and exhibited all
the detail of their structure, all the light, and shade, and markings of
their precipitous sides with an apparently supernatural distinctness.
After a little time, the eyes seemed to get accustomed to the darkness,
and the looming forms of objects close by could be discerned, all of
them exhibiting a dull-green hue; seeming to have exhaled their natural
colour, and to have taken this particular one, merely by force of the
red colour in the north.
"Life and animation seemed, indeed, to have now departed from everything
around, and we could hardly but fear, against our reason, that if such a
state of things was to last much longer, some dreadful calamity must
happen to us all; while the lurid horizon, northward, appeared so like
the gleams of departing light in some of the grandest paintings by Danby
and Martin, that we could not but believe, in spite of the alleged
extravagances of these artists, that Nature had opened up to the
constant contemplation of their mind's-eye some of those magnificent
revelations of power and glory which others can only get a glimpse of on
occasions such as these."
It can be easily imagined, that under such peculiar and awful
circumstances, the careful observation of these effects must be somewhat
difficult, [Page 32] and the only wonder is that the astronomical
observations are conducted with any certainty at all.
In the eclipse of 1842, it was not only the vivacious Frenchman who was
carried away in the impulse of the moment, and had afterwards to plead
that "_he was no more than a man_" as an excuse for his unfulfilled part
in the observations, but the same was the case with the grave Englishman
and the more stolid German. In 1851, much the same failure in the
observations occurred; and on some person asking a worthy American, who
had come with his instruments from the other side of the world expressly
to observe the eclipse, what he had succeeded in doing? he merely
answered, with much quiet impressiveness, "_That if it was to be
observed over again, he hoped he would be able to do something, but
that, as it was, he had done nothing: it had been too much for him._"
This is not quite so bad as the fashionable lady who had been invited to
look at an eclipse of the sun through a grand telescope, but arriving
too late, inquired whether "it could not be shown _over again_."
With this brief glance at the science of astronomy, we once more return
to the term "gravity," which will introduce to us some new and
interesting facts, under the head of what is called "centre of gravity."