WHAT IS OBSERVED DURING THE TOTAL PHASE OF AN ECLIPSE OF THE SUN.
The central feature of every total eclipse of the Sun is undoubtedly the
Corona[16] and the phenomena connected with it; but immediately before
the extinction of the Sun's light and incidental thereto there are some
minor features which must be briefly noticed.
[Illustration: FIG. 8.--BRUSHES OF LIGHT.]
The Corona first makes its appearance on the side of the dark Moon
opposite to the disappearing crescent, but brushes of light are
sometimes observed on the same side, along the convex limb of the
disappearing crescent. The appearance of the brushes will be
sufficiently realised by an inspection of the annexed engraving without
the necessity of any further verbal description. These brushes are
little, if at all, coloured, and must not be confused with the "Red
Flames" or "Prominences" hereafter to be described.
BAILY'S BEADS.
When the disc of the Moon has advanced so much over that of the Sun as
to have reduced the Sun almost to the narrowest possible crescent of
light, it is generally noticed that at a certain stage the crescent
suddenly breaks up into a succession of spots of light. These spots are
sometimes spoken of as "rounded" spots, but it is very doubtful whether
(certainly in view of their supposed cause) they could possibly be
deemed ever to possess an outline, which by any stretch, could be called
"rounded." Collating the recorded descriptions, some such phrase as
"shapeless beads" of light would seem to be the most suitable
designation. These are observed to form before the total phase, and
often also after the total phase has passed. Under the latter
circumstances, the beads of light eventually run one into another, like
so many small drops of water merging into one big one. The commonly
received explanation of "Baily's Beads" is that they are no more than
portions of the Sun's disc, seen through valleys between mountains of
the Moon, the said mountains being the cause why the bright patches are
discontinuous. It is exceedingly doubtful whether this is the true
explanation. The whole question is involved in great uncertainty, and
well deserves careful study during future eclipses; but this it is not
likely to get, in view of the current fashion of every sufficiently
skilled observer concentrating his attention on matters connected with
the solar Corona (observed spectroscopically or otherwise), to the
exclusion of what may be called older subjects of study. I will dismiss
Baily's Beads from our consideration with the remark that the first
photograph of them was obtained at Ottumwa, Illinois, U.S., during the
eclipse of 1869.
[Illustration: FIG. 9.--"BAILY'S BEADS," FOUR STAGES, AT BRIEF INTERVALS.
MAY 15, 1836.]
"Baily's Beads" received their name from Mr. Francis Baily, who, in
1836, for the first time exhaustively described them; but they were
probably seen and even mentioned long before his time. At the total
eclipse of the Sun, seen at Penobscot in North America, on October 27,
1780, they would seem to have been noticed, and perhaps even earlier
than that date.
Almost coincident with the appearance of Baily's Beads, that is, either
just before or just after, and also just before or just after the
absolute totality (there seems no certain rule of time) jets of red
flame are seen to dart out from behind the disc of the Moon. It is now
quite recognised as a certain fact that these "Red Flames" belong to the
Sun and are outbursts of hydrogen gas. Moreover, they are now commonly
called "Prominences," and with the improved methods of modern science
may be seen almost at any time when the Sun is suitably approached; and
they are not restricted in their appearance to the time when the Sun is
totally eclipsed as was long supposed.
I may have more to say about these Red Flames later on; but am at
present dealing only with the outward appearances of things.
Carrington's description has been considered very apt. One which he saw
in 1851 he likened to "a mighty flame bursting through the roof of a
house and blown by a strong wind."
Certain ambiguous phrases made use of in connection with eclipses of
ancient date may perhaps in reality have been allusions to the Red
Flames; otherwise the first account of them given with anything like
scientific precision seems to be due to a Captain Stannyan, who observed
them at Berne during the eclipse of 1706. His words are that the Sun at
"his getting out of his eclipse was preceded by a blood-red streak from
its left limb which continued not longer than six or seven seconds of
time; then part of the Sun's disc appeared all of a sudden."
Some subsequent observers spoke of the Red Flames as isolated jets of
red light appearing here and there; whilst others seem to have thought
they had seen an almost or quite continuous ring of red light around the
Sun. The last-named idea is now recognised as the more accurate
representation of the actual facts, the Red Flames being emanations
proceeding from a sort of shell enveloping the Sun, to which shell the
name of "Chromosphere" has now come to be applied.
As regards the Moon itself during the continuance of the total phase,
all that need be said is that our satellite usually exhibits a disc
which is simply black; but on occasions observers have called it purple
or purplish. Although during totality the Moon is illuminated by a full
allowance of Earth-shine (light reflected by the Earth into space), yet
from all accounts this is always insufficient to reveal any traces of
the irregularities of mountains and valleys, etc., which exist on the
Moon.
When during totality any of the brighter planets, such as Mercury,
Venus, Mars, Jupiter, or Saturn, happen to be in the vicinity of the Sun
they are generally recognised; but the stars seen are usually very few,
and they are only very bright ones of the 1st or 2nd magnitudes. Perhaps
an explanation of the paucity of stars noticed is to be found in the
fact that the minds of observers are usually too much concentrated on
the Sun and Moon for any thought to be given to other things or other
parts of the sky.
Perhaps this is a convenient place in which to recall the fact that
there has been much controversy in the astronomical world during the
last 50 years as to whether there exist any undiscovered planets
revolving round the Sun within the orbit of Mercury. Whilst there is
some evidence, though slight, that one or more such planets have been
seen, opponents of the idea base their scepticism on the fact that with
so many total eclipses as there have been since 1859 (when Lescarbault
claimed to have found a planet which has been called "Vulcan"), no
certain proof has been obtained of the existence of such a planet; and
what better occasion for finding one (if one exists of any size) than
the darkness of a total solar eclipse? At present it must be confessed
that the sceptics have the best of it.
THE CORONA.
We have now to consider what I have already called the central feature
of every total eclipse. It was long ago compared to the nimbus often
placed by painters around the heads of the Virgin Mary and other saints
of old; and as conveying a rough general idea the comparison may still
stand. It has been suggested that not a bad idea of it may be obtained
by looking at a Full Moon through a wire-gauze window-screen. The Corona
comes into view a short time (usually to be measured by seconds) before
the total extinction of the Sun's rays, lasts during totality and
endures for a brief interval of seconds (or it might be a minute) after
the Sun has reappeared. It was long a matter of discussion whether the
Corona belonged to the Sun or the Moon. In the early days of telescopic
astronomy there was something to be said perhaps on both sides, but it
is now a matter of absolute certainty that it belongs to the Sun, and
that the Moon contributes nothing to the spectacle of a total eclipse of
the Sun, except its own solid body, which blocks out the Sun's light,
and its shadow, which passes across the Earth.
Of the general appearance of the Corona some idea may be obtained from
Fig. 1 (see Frontispiece) which so far as it goes needs little or no
verbal description. Stress must however be laid on the word "general"
because every Corona may be said to differ from its immediate
predecessor and successor, although, as we shall see presently, there
is strong reason to believe that there is a periodicity in connection
with Coronas as with so many other things in the world of Astronomy. A
curious point may here be mentioned as apparently well established,
namely, that when long rays are noticed in the Corona they do not seem
to radiate from the Sun's centre as the short rays more or less seem to
do. Though the aggregate brilliancy of the Corona varies somewhat yet it
may be taken to be much about equal on the whole to the Moon at its
full. The Corona is quite unlike the Moon as regards heat for its
radiant heat has been found to be very well marked.
There is another thing connected with the Sun's Corona which needs to be
mentioned at the outset and which also furnishes a reason for treating
it in a somewhat special manner. The usual practice in writing about
science is to deal with it in the first instance descriptively, and then
if any historical information is to be given to exhibit that separately
and subsequently. But our knowledge of the Sun's Corona has developed so
entirely by steps from a small beginning that it is neither easy nor
advantageous to keep the history separate or in the background and I
shall therefore not attempt to do so.
Astronomers are not agreed as to what is the first record of the Corona.
It is commonly associated with a total eclipse which occurred in the 1st
century A.D. and possibly in the year 96 A.D. Some details of the
discussion will be found in a later chapter,[17] and I will make no
further allusion to the matter here. Passing over the eclipses of 968
A.D. and 1030 A.D. the records of both of which possibly imply that the
Corona was noticed, we may find ourselves on thoroughly firm ground in
considering the eclipse of April 9, 1567. Clavius, a well-known writer
on chronology, undoubtedly saw then the Corona in the modern acceptation
of the word but thought it merely the uncovered rim of the Sun. In reply
to this Kepler showed by some computations of his own, based on the
relative apparent sizes of the Sun and Moon, that Clavius's theory was
untenable. Kepler, however, put forth a theory of his own which was no
better, namely, that the Corona was due to the existence of an
atmosphere round the Moon and proved its existence. From this time
forwards we have statements, by various observers, applying to various
eclipses, of the Corona seeming to be endued with a rotatory motion. The
Spanish observer, Don A. Ulloa, in 1778, wrote thus respecting the
Corona seen in that year:--"After the immersion we began to observe round
the Moon a very brilliant circle of light which seemed to have a rapid
circular motion something similar to that of a rocket turning about its
centre." Modern observations furnish no counterpart of these ideas of
motion in the Corona. Passing over many intervening eclipses we must
note that of 1836 (which gave us "Baily's Beads") as the first which set
men thinking that total eclipses of the Sun exhibited subsidiary
phenomena deserving of careful and patient attention. Such attention
was given on the occasion of the eclipses of 1842 and 1851, still
however without the Corona attracting that interest which it has gained
for itself more recently. It was noticed indeed that the Corona always
first showed itself on the side of the Moon farthest from the vanishing
crescent but the full significance of this fact was not at first
realised. Mrs. Todd well remarks:--"In the early observations of the
Corona it was regarded as a halo merely and so drawn. Its real structure
was neither known, depicted, nor investigated. The earliest pictures all
show this. Preconceived ideas prejudiced the observers, and their
sketches were mostly structureless.... It should not be forgotten that
the Coronal rays project outward into space from a spherical Sun and do
not lie in a plane as they appear to the eye in photographs and
drawings." After remarking on the value of photographs of the Corona up
to a certain point because of their automatic accuracy Mrs. Todd very
sensibly says, "but pencil drawings, while ordinarily less trustworthy
because involving the uncertain element of personal equation are more
valuable in delineating the finest and faintest detail of which the
sensitive plate rarely takes note; the vast array of both, however,
shows marked differences in the structure and form of the Corona from
one eclipse to another though it has not yet revealed rapid changes
during any one observation. This last interesting feature can be studied
only by comparison of photographs near the beginning of an eclipse track
and its end, two or three hours of absolute time apart." Concerted
efforts to accomplish this were made in 1871, 1887, and 1889, but they
broke down because the weather failed at one or other end of the chain
of observing stations and a succession of photographs not simultaneous
but separated by sufficient intervals of time could not be had. The
eclipse of 1893, however, yielded successful though negative results.
Photographs in South America compared with photographs in Africa two
hours later in time disclosed no appreciable difference in the structure
of the Corona and its streamers. The eclipse of May 28, 1900, will
furnish the next favourable opportunity for a repetition of this
experiment by reason of the fact that the line of totality begins in
North America, crosses Portugal and Spain and ceases in Africa. In other
words, traverses countries eminently calculated to facilitate the
establishment of photographic observing stations where observations can
be made not simultaneously but at successive intervals spread over
several hours.
Although of course the Corona had been observed long before the year
1851, as indeed we have already seen, yet the eclipse of 1851 is the
farthest back which we can safely take as a starting-point for gathering
up thoroughly precise details, because it was the first at which
photography was brought into use. Starting, therefore, with that eclipse
I want to lay before the reader some of the very interesting and
remarkable generalisations which (thanks especially to Mr. W. H.
Wesley's skilful review of many of the photographic results) are now
gradually unfolding themselves to astronomers. To put the matter in the
fewest possible words there seems little or no doubt that according as
spots on the Sun are abundant or scarce so the Corona when visible
during an eclipse varies in appearance from one period of _eleven_ years
to another like period. Or, to put it in another way, given the date of
a coming total eclipse we can predict to a certain extent the probable
shape and character of the Corona if we know how the forthcoming date
stands as regards a Sun-spot maximum or minimum.
The most recent important eclipses up to date which have been observed,
namely those of April 16, 1893, Aug. 9, 1896, and Jan. 21, 1898, do not
add much to our useful records of the outward appearances presented by
the Corona. The 1896 Corona is described as intermediate between the two
Types respectively associated with years of maximum and minimum
Sun-spots, and this is as it should have been, albeit there was one
extension which reached to about two diameters of the Sun. The 1898
Corona yielded four long Coronal streamers reaching much farther from
the Sun than any previously seen, the two longest reaching to 4½ and 6
diameters of the Sun respectively. These dimensions are quite
unprecedented.
[Illustration: FIG. 10.--CORONA OF 1882. (SUN-SPOT MAXIMUM.)]
The application of the spectroscope to observations of eclipses of the
Sun demands a few words of notice in this place, but it would not be
consistent with the plan of this work to go into details. Though the
spectroscope has been applied under many different circumstances to
different parts of the Sun's surroundings in connection with total
eclipses yet it is in regard to the Corona that most has been done and
most has been discovered. The substance of the discoveries made is that
the Corona shines with an intrinsic light of its own, that is to say,
that it is composed of constituents whose temperature is sufficiently
elevated to be self-luminous. These constituents are chiefly hydrogen;
the body which corresponds to the line D3 (of Fraunhofer's scale), and
which has been named "Helium"; and the body which corresponds to the
bright green line 1474 of Kirchoff's scale and which, since its
existence was first suspected and then assured, has been named
"Coronium."
The reader will not be surprised to learn, from what has gone before,
that an immense mass of records have accumulated respecting the
appearance of the Corona. Correspondingly numerous and divergent are the
theories which have been launched to explain the observations made. One
thing is in the highest degree probable, namely, that electricity is
largely concerned.
Going back to the question of Sun-spots regarded in their possible or
probable association with the Corona, the present position of matters
appears to be this: that there is a real connection between the general
form of the Corona and disturbances on the Sun, taking Sun-spots as an
indication of solar activity. When Sun-spots are at or near their
maximum, the Corona has generally been somewhat symmetrical, with
synclinal groups of rays making angles of 45° with its general axis. On
the other hand, at the epochs of minimum Sun-spots, the Corona shows
polar rifts much more widely open, with synclinal zones making larger
angles with the axis, and being, therefore, more depressed towards the
equatorial regions, in which, moreover, there is usually a very marked
extension of Coronal matter in the form of elongated streamers reaching
to several diameters of the Sun.
[Illustration: FIG. 11.--CORONA OF 1867. (SUN-SPOT MINIMUM.)]
This generalisation is well borne out by the maximum-epoch Coronas of
1870 and 1871, and the minimum-epoch Coronas of 1867, 1874, 1875, 1878,
and perhaps 1887, and certainly 1889. On the other hand, the eclipses of
1883, 1885 and 1886 do not strikingly confirm this theory. The eclipse
of 1883 was at a time of rapidly decreasing solar activity, yet the
Corona had the features of a Sun-spot maximum. The same, though in a
somewhat less degree, may be said of the eclipses of 1885 and 1886. At
the times of both of these eclipses the solar activity was decreasing.
The forthcoming eclipse of 1900 will nearly coincide with a Sun-spot
minimum, and if the above conclusions are well founded the Corona in
1900 should resemble that of 1889, and be characterised by, amongst
other things, some very elongated groups of rays extending in nearly
opposite directions.
We are still a long way off from being able to state with perfect
confidence what the Corona is. It is certainly a complex phenomenon, and
the various streamers which we see are not, as was at one time imagined,
a simple manifestation of one radiant light. Mrs. Todd thus conveniently
summarises the present state of our knowledge:--"The true corona appears
to be a triple phenomenon. First, there are the polar rays, nearly
straight throughout their visible extent. Gradually, as these rays start
out from points on the solar disc farther and farther removed from the
poles, they acquire increasing curvature, and very probably extend into
the equatorial regions, but are with great difficulty traceable there,
because projected upon and confused with the filaments having their
origin remote from the poles. Then there is the inner equatorial corona,
apparently connected intimately with truly solar phenomena, quite like
the polar rays; while the third element in the composite is the outer
equatorial corona, made up of the long ecliptic streamers, for the most
part visible only to the naked eye, also existing as a solar appendage,
and possibly merging into the zodiacal light. The total eclipses of a
half century have cleared up a few obscurities, and added many
perplexities. There is little or no doubt about the substantial, if not
entire, reality of the corona as a truly solar phenomenon. The Moon, if
it has anything at all to do with the corona, aside from the fact of its
coming in conveniently between Sun and Earth, so as to allow a brief
glimpse of something startlingly beautiful which otherwise could never
have been known, is probably responsible for only a very narrow ring of
the inner radiance of pretty even breadth all round. This diffraction
effect is accepted; but the problem still remains how wide this annulus
may be, and whether it may vary in width from one eclipse to another.
These questions once settled, the spurious structure may then be
excerpted from the true. Indeed the coronal streamers, delicately
curving and interlacing, may tell the whole story of the Sun's radiant
energy."
FOOTNOTES:
[Footnote 16: There seems sufficient evidence to show that the
Corona may be seen even on occasions when the Sun is not totally
eclipsed, provided that the visible crescent of the Sun is
exceedingly narrow.]
[Footnote 17: See p. 130 (_post_).]