WHAT IS OBSERVED DURING THE EARLIER STAGES
OF AN ECLIPSE OF THE SUN.
The information to be given in this and the next following chapters will
almost exclusively concern total and annular eclipses of the Sun, for,
in real truth, there is practically only one thing to think about during
a partial eclipse of the Sun. This is, to watch when the Moon's black
body comes on to the Sun and goes off again, for there are no subsidiary
phenomena, either interesting or uninteresting, unless, indeed, the
eclipse should be nearly total. The progress of astronomical science in
regard to eclipses has been so extensive and remarkable of late years
that, unless the various points for consideration are kept together
under well-defined heads, it will be almost impossible either for a
writer or a reader to do full justice to the subject. Having regard to
the fact that the original conception of this volume was that it should
serve as a forerunner to the total solar eclipse of May 28, 1900 (and
through that to other total eclipses), from a popular rather than from a
technical standpoint, I think it will be best to mention one by one the
principal features which spectators should look out for, and to do so as
nearly as may be in the order which Nature itself will observe when the
time comes.
Of course the commencement of an eclipse, which is virtually the moment
when the encroachment on the circular outline of the Sun by the Moon
begins, or can be seen, though interesting as a proof that the
astronomer's prophecy is about to be fulfilled, is not a matter of any
special importance, even in a popular sense, much less in a scientific
sense. As a rule, the total phase does not become imminent, so to speak,
until a whole hour and more has elapsed since the first contact; and
that hour will be employed by the scientific observer, less in looking
at the Sun than in looking at his instruments and apparatus. He will do
this for the purpose of making quite sure that everything will be ready
for the full utilisation to the utmost extent of the precious seconds of
time into which all his delicate observations have to be squeezed during
the total phase.
With these preliminary observations I shall proceed now to break up the
remainder of what I have to say respecting total eclipses into what
suggest themselves as convenient sectional heads.
THE MOON'S SHADOW AND THE DARKNESS IT CAUSES.
In awaiting the darkness which is expected to manifest itself an
unthinking and inexperienced observer is apt to look out for the coming
obscurity, as he looks out for night-fall half an hour or more after
sunset and during the evening twilight. The darkness of an eclipse is
all this and something more. It is something more in two senses; for the
interval of time between the commencement of an eclipse and totality is
different in duration and different in quality, so to speak, from the
diminution of daylight on the Earth which ensues as the twilight of
evening runs its course. Speaking roughly, sunset may be described as an
almost instantaneous loss of full sunlight; and the gradual loss of
daylight is noticeable even at such short intervals as from one five
minutes to another. This is by no means the case previous to a total
eclipse of the Sun. When that is about to occur, the reduction of the
effective sunlight is far more gradual. For instance, half an hour after
an eclipse has commenced more than half the Sun's disc will still be
imparting light to the Earth: but half an hour after sunset the
deficiency of daylight will be very much more marked and, if no
artificial light is at hand, very much more inconvenient.
If there should be within easy reach of the observer's post a bushy
tree, such for instance as an elm, 30 ft. or 40 ft. high, and spreading
out sufficiently for him to place himself under it in a straight line
with the Sun, and with a nice smooth surface of ground for the sun's
rays to fall on, he will see a multitude of images of the Sun thrown
upon the ground.
Until the eclipse has commenced these images will be tiny circles
overlapping one another, and of course each of these circles means so
many images of the Sun. These images indeed can be seen on any fine day,
and the circles increase in size in proportion to the height of the
foliage above the ground, being something like 1 inch for every 10 feet.
It may be remarked, by the way, that the images are circles, because the
Sun is a source of light having a circular outline, and is not a point
of light like a star. If it were, the outline of the foliage would be
reproduced on the ground leaf for leaf. It follows naturally from all
this that when in consequence of there being an eclipse in progress the
shape of the Sun's contour gradually changes, so will the shape of the
Solar images on the ground change, becoming eventually so many
crescents. Moreover, the horns of the crescent-shaped images will be in
the reverse direction to the horns of the actual crescent of the Sun at
the moment, the rays of the Sun crossing as they pass through the
foliage, just as if each interstice were a lens.
Supposing there are some spots on the Sun at a time when an eclipse is
in progress the Moon's passage over these spots may as well be noticed.
In bygone years some amount of attention was devoted to this matter with
the view of ascertaining whether any alteration took place in the
appearance of the spots; distortion, for instance, such as might be
produced by the intervention of a lunar atmosphere. No such distortion
was ever noticed, and observations with this idea in view may be said to
possess now only an academic interest, for it may be regarded as a
well-established fact that the Moon has no atmosphere.
During the passage of the Moon over Sun-spots an opportunity is afforded
of comparing the blackness, or perhaps we should rather say, the
intensity of the shade of a Sun-spot with the blackness of the Moon's
disc. Testimony herein is unanimous that the blackness of the Moon
during the stages of partial eclipse is intense compared with the
darkest parts of a Sun-spot; and this, be it remembered, in spite of the
fact that during the partial phase the atmosphere between the observer
and the Sun is brilliantly illuminated, whilst the Moon itself, being
exposed to Earth-shine, is by no means absolutely devoid of all
illumination.
When the Moon is passing across the Sun there have often been noticed
along the limb of the Moon fringes of colour, and dark and bright bands.
This might not necessarily be a real appearance for it is conceivable
that such traces of colour might be due to the telescopes employed not
having been truly achromatic, that is, not sufficiently corrected for
colour; but making every allowance for this possible source of mistake
there yet remains proof that the colour which has often been seen has
been real.
As to whether the Moon's limb can be seen during a partial eclipse, or
during the partial phase of what is to be a total eclipse, the evidence
is somewhat conflicting. There is no doubt that when the totality is
close at hand the Moon's limb can be seen projected on the Corona
(presently to be described); but the question is, whether the far-off
limb of the Moon can be detected in the open sky whilst something like
full daylight still prevails on the Earth. Undoubtedly the preponderance
of evidence is against the visibility of the Moon as a whole, under such
circumstances; but there is nevertheless some testimony to the contrary.
A French observer, E. Liais, said that three photographic plates of the
eclipse of 1858 seen in S. America all showed the outer limb of the
Moon with more or less distinctness. This testimony, be it noted, is
photographic and not visual; and on the whole it seems safest to say
that there is very small probability of the Moon as a whole ever being
seen under the circumstances in question.
What has just been said concerns the visibility of the Moon during quite
the early, or on the other hand during quite the late, stages of a total
eclipse. Immediately before or after totality the visibility of the
whole contour of the Moon is a certain fact; and the only point upon
which there is a difference of opinion is as to what are the time-limits
beyond which the Moon must not be expected to be seen. The various
records are exceedingly contradictory: perhaps the utmost that can be
said is that the whole Moon must not be expected to be visible till
about 20 minutes before totality, or for more than 5 minutes after
totality--but it must be admitted that these figures are very uncertain
in regard to any particular eclipse.
It has been sometimes noticed when the crescent of the Sun had become
comparatively small, say that the Sun was about 7/8ths covered, that the
uncovered portion exhibited evident colour which has been variously
described as "violet," "brick-red," "reddish," "pink," "orange,"
"yellowish." The observations on this point are not very numerous and,
as will appear from the statement just made, are not very consistent;
still it seems safe to assume that a hue, more or less reddish, does
often pervade the uncovered portion of a partially-eclipsed Sun.
The remark just made as regards the Sun seems to have some application
to the Moon. There are a certain number of instances on record that what
is commonly spoken of as the black body of the Moon does, under certain
circumstances, display traces of red which has been variously spoken of
as "crimson," "dull coppery," "reddish-brownish" and "dull glowing
coal."
SHADOW BANDS.
Let us suppose that we have a chance of observing a total eclipse of the
Sun; have completed all our preliminary preparations; have taken note of
everything which needs to be noted or suggests itself for that purpose
up till nearly the grand climax; and that the clock tells us that we are
within, say, five minutes of totality. Somewhere about this time perhaps
we shall be able to detect, dancing across the landscape, singular wavy
lines of light and shade. These are the "Shadow Bands," as they are
called. The phrase is curiously inexplicit, but seemingly cannot be
improved upon at present because the philosophy of these
appearances--their origin and the laws which regulate their
visibility--are unknown, perhaps because amid the multitude of other
things to think about sufficient attention has hitherto not been paid to
the study of them. These shadow bands are most striking if a high
plastered wall, such as the front of a stone or stuccoed house, is in
their track as a screen to receive them. The shadow bands seem to vary
both in breadth and distance apart at different eclipses, and also in
the speed with which they pass along. Though, as already stated, little
is known of their origin yet they may be conceived to be due to
irregularities in the atmospheric refraction of the slender beam of
light coming from the waning or the waxing crescent of the Sun, for be
it understood they may be visible after totality as well as before it.
It is to be remarked that they have never been photographed.
[Illustration: FIG. 6.--SHADOW BANDS.]
In addition to the shadow bands there are instances on record of the
limbs of the Sun's crescent appearing to undulate violently on the
approach of totality. These undulations were noticed by Airy in 1842
about 6 minutes before totality. Blake, in America in 1869, observed the
same phenomenon 8 minutes before totality. In other cases the interval
would seem to have been very much shorter--a mere matter of seconds. A
very singular observation was made in 1858 by Mr. J. D. Smith at Laycock
Abbey, Wiltshire, on the occasion of the annular eclipse of that year.
He says[13]:--"Both my brother and myself were distinctly impressed with
the conviction that the withdrawal of light was not continuous, but by
pulsations, or, as it were, waves of obscuration, the darkness
increasing by strokes which sensibly smote the eye, and were repeated
distinctly some five or seven times after we had remarked the phenomenon
and before the time of greatest obscuration. This did not occur on the
return of light, which came back continuously and without shock or
break." Rümker mentions that though this phenomenon was very apparent to
the naked eye it was not visible in the telescope.
Faint rays or brushes of light sometimes seem to spring from the
diminishing crescent of the Sun. These rays generally are very transient
and not very conspicuous, and perhaps must be distinguished as regards
both their appearance and their origin from the more striking rays
which are usually seen a few minutes before or after totality, and which
are generally associated with, or even deemed to belong to, the Corona.
Fig. 7 represents these rays as seen in Spain on July 18, 1860, some
minutes after totality. They are described as having been well defined,
but at some moments more marked than at others, and though well-defined
yet constantly varying. Radiations of light more or less of the
character just described may probably be regarded as a standing feature
of every total eclipse.
[Illustration: FIG. 7.--RAYS OF LIGHT SEEN NEAR THE TIME OF TOTALITY.]
THE APPROACH OF TOTALITY.
The next thing to think about and to look out for is the approach of the
Moon's shadow. I have mentioned this already,[14] and also the
appalling velocity with which it seems to approach. By this time the
coming darkness, which characterises every total phase, will have
reached an advanced stage of development. The darkness begins to be
felt. The events which manifest themselves at this juncture on the Earth
(rather than in the sky around the Sun) are so graphically described by
the American writer whom I have already quoted, and who writes,
moreover, from personal experience, that I cannot do better than
transfer her striking account to my pages.[15] "Then, with frightful
velocity, the actual shadow of the Moon is often seen approaching, a
tangible darkness advancing almost like a wall, swift as imagination,
silent as doom. The immensity of nature never comes quite so near as
then, and strong must be the nerves not to quiver as this blue-black
shadow rushes upon the spectator with incredible speed. A vast, palpable
presence seems overwhelming the world. The blue sky changes to gray or
dull purple, speedily becoming more dusky, and a death-like trance
seizes upon everything earthly. Birds, with terrified cries, fly
bewildered for a moment, and then silently seek their night-quarters.
Bats emerge stealthily. Sensitive flowers, the scarlet pimpernel, the
African mimosa, close their delicate petals, and a sense of hushed
expectancy deepens with the darkness. An assembled crowd is awed into
absolute silence almost invariably. Trivial chatter and senseless joking
cease. Sometimes the shadow engulfs the observer smoothly, sometimes
apparently with jerks; but all the world might well be dead and cold and
turned to ashes. Often the very air seems to hold its breath for
sympathy; at other times a lull suddenly awakens into a strange wind,
blowing with unnatural effect. Then out upon the darkness, gruesome but
sublime, flashes the glory of the incomparable corona, a silvery, soft,
unearthly light, with radiant streamers, stretching at times millions of
uncomprehended miles into space, while the rosy, flaming protuberances
skirt the black rim of the Moon in ethereal splendour. It becomes
curiously cold, dew frequently forms, and the chill is perhaps mental as
well as physical. Suddenly, instantaneous as a lightning flash, an arrow
of actual sunlight strikes the landscape, and Earth comes to life again,
while corona and protuberances melt into the returning brilliance, and
occasionally the receding lunar shadow is glimpsed as it flies away with
the tremendous speed of its approach."
In connection with the approach of the Moon's shadow, it is to be noted
that at totality the heavens appear in a certain sense to descend upon
the Earth. If an observer is looking upwards towards the zenith over his
head, he will see the clouds appear to drop towards the Earth, and the
surrounding gloom seems also to have the effect of vitiating one's
estimate of distances. To an observer upon a high hill, a plain below
him appears to become more distant. Although what has been called the
descent of the clouds (that is to say their appearance of growing
proximity) is most manifest immediately before the totality, yet a sense
of growing nearness may sometimes be noticed a very considerable time
before the total phase is reached.
Whilst on the subject of clouds, it may be mentioned that although there
is in the vault of heaven generally during the total phase an
appreciable sensation of black darkness, more or less absolute, that is
to say, either blackish or greyish, yet in certain regions of the sky,
(generally in the direction of the horizon) the clouds, when there are
any, often exhibit colours in strata, orange hue below and red above,
with indigo or grey or black higher up still, right away to the Sun's
place. The cause of these differences is to be found in the fact that
the lower part of the atmosphere within the area of the Moon's shadow
is, under the circumstances in question, illuminated by light which
having passed through many miles of atmosphere near to the Earth's
surface, has lost much from the violet end of its spectrum, leaving an
undue proportion of the red end.
On certain occasions iridescent or rainbow-tinted clouds may be seen in
the vicinity of the Sun, either before, or during, or after totality,
depending on circumstances unknown. Such clouds have been observed at
all these three stages of a total eclipse. The effects of course are
atmospheric, and have no physical connection with either Sun or Moon.
THE DARKNESS OF TOTALITY.
With respect to the general darkness which prevails during totality,
great discrepancies appear in the accounts, not only as between
different eclipses, but in respect of the same eclipse observed by
different people at different places. Perhaps the commonest test applied
by most observers is that of the facility or difficulty of reading the
faces of chronometers or watches. Sometimes this is done readily, at
other times with difficulty. In India in 1868, one observer stated that
it was impossible to recognise a person's face three yards off, and
lamplight was needed for reading his chronometer. On the other hand in
Spain in 1860, it was noted that a thermometer, as well as the finest
hand-writing, could be read easily. The foregoing remarks apply to the
state of things in the open air. In 1860, it was stated that inside a
house in Spain the darkness was so great that people moving about had to
take great care lest they should run violently against the household
furniture.
Perhaps on the whole it may be said that the darkness of an ordinary
totality is decidedly greater than that of a full Moon night.
Many observers have noted during totality that even when there has not
been any very extreme amount of absolute darkness, yet the ruddy light
already mentioned as prevailing towards the horizon often gives rise to
weird unearthly effects, so that the faces of bystanders assume a
sickly livid hue not unlike that which results from the light of burning
salt.
METEOROLOGICAL AND OTHER EFFECTS.
It is very generally noticed that great changes take place in the
meteorological conditions of the atmosphere as an eclipse of the Sun
runs its course from partial phase to totality, and back again to
partial phase. It goes without saying that the obstruction of the solar
rays by the oncoming Moon would necessarily lead to a steady and
considerable diminution in the general temperature of the air. This has
often been made the matter of exact thermometric record, but it is not
equally obvious why marked changes in the wind should take place. As the
partial phase proceeds it is very usual for the wind to rise or blow in
gusts and to die away during totality, though there are many exceptions
to this, and it can hardly be called a rule.
The depression of temperature varies very much indeed according to the
locality where the eclipse is being observed and the local thermometric
conditions which usually prevail. The actual depression will often
amount to 10° or 20° and the deposit of dew is occasionally noticed.
In addition to the general effects of a total solar eclipse on men,
animals, and plants as summarised in the extract already made from Mrs.
Todd's book a few additional particulars may be given culled from many
recorded observations. Flowers and leaves which ordinarily close at
night begin long before totality to show signs of closing up. Thus we
are told that in 1836 "the crocus, gentian and anemone partially closed
their flowers and reopened them as the phenomenon passed off: and a
delicate South African mimosa which we had reared from a seed entirely
folded its pinnate leaves until the Sun was uncovered." In 1851 "the
night violet, which shortly before the beginning of the eclipse had
little of its agreeable scent about it, smelt strongly during the
totality."
In the insect world ants have been noticed to go on working during
totality, whilst grasshoppers are stilled by the darkness, and
earth-worms come to the surface. Birds of all kinds seem always upset in
their habits, almost invariably going to roost as the darkness becomes
intensified before totality. In 1868 "a small cock which had beforehand
been actively employed in grubbing about in the sand went to sleep with
his head under his wing and slept for about 10 minutes, and on waking
uttered an expression of surprise, but did not crow." In 1869 mention is
made of an unruly cow "accustomed to jump into a corn-field at night"
being found to have trespassed into the said corn-field during the total
phase.
The thrilling descriptions of the effects of the oncoming darkness of
totality, derived from the records of past total eclipses, are not
likely to be improved upon in the future, for we shall receive them more
and more from amateurs and less and less from astronomical experts.
Every additional total eclipse which happens testifies to the fact that
the time and thoughts of these latter classes of people will be to an
increasing degree dedicated to instrumental work rather than to simple
naked eye or even telescopic observation. The spectroscope and the
camera are steadily ousting the simple telescope of every sort and
unassisted eye observations from solar eclipse work.
Mrs. Todd has the following apt remarks by way of summary of the results
to an individual of observing a total eclipse of the Sun:--"I doubt if
the effect of witnessing a total eclipse ever quite passes away. The
impression is singularly vivid and quieting for days, and can never be
wholly lost. A startling nearness to the gigantic forces of Nature and
their inconceivable operation seems to have been established.
Personalities and towns and cities, and hates and jealousies, and even
mundane hopes, grow very small and very far away."
FOOTNOTES:
[Footnote 13: _Month. Not._, R.A.S., vol. xviii. p. 251.]
[Footnote 14: See p. 36 (_ante_).]
[Footnote 15: Mrs. D. P. Todd, _Total Eclipses of the Sun_, p. 21.]