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THE STORY OF ECLIPSES
SIMPLY TOLD FOR GENERAL READERS.
WITH ESPECIAL REFERENCE TO THE TOTAL ECLIPSE OF THE SUN OF MAY 28, 1900.
BY
GEORGE F. CHAMBERS, F.R.A.S.
Of the Inner Temple, Barrister-at-Law.
AUTHOR OF
"THE STORY OF THE SOLAR SYSTEM"; "THE STORY OF THE STARS"; "A HANDBOOK OF DESCRIPTIVE ASTRONOMY," ETC.
LONDON: GEORGE NEWNES, LTD. SOUTHAMPTON STREET, STRAND 1899.
The rights of translation and of reproduction are reserved.
PREFACE.
The present Volume is intended as a sequel to my two former volumes in the Newnes Series of "Useful Stories," entitled respectively the "Story of the Solar System," and the "Story of the Stars." It has been written not only as a necessary complement, so to speak, to those works, but because public attention is already being directed to the forthcoming total eclipse of the Sun on May 28, 1900. This eclipse, though only visible as a partial one in England, will be total no further off than Portugal and Spain. Considering also that the line of totality will pass across a large tract of country forming part of the United States, it may be inferred that there will be an enormous number of English-speaking spectators of the phenomenon. It is for these in general that this little book has been written. For the guidance of those who may be expected to visit Portugal or Spain, a temporary Appendix has been prepared, giving a large amount of information showing how those countries can be best reached, whether by sea or overland, from the shores of England.
If anyone is inclined to doubt whether an eclipse expedition is likely to provide non-astronomical tourists with incidents of travel, pleasant, profitable, and even amusing, perhaps the doubt will be removed by a perusal of the accounts of Sir F. Galton's trip to Spain in 1860 (Vacation Tourists in 1860, p. 422), or of Professor Tyndall's trip to Algeria in 1870 (Hours of Exercise in the Alps, p. 429), or of Professor Langley's Adventures on Pike's Peak in the Rocky Mountains, Colorado, U.S., in 1878 (Washington Observations, 1876, Appendix III. p. 203); or of some of the many Magazine and other narratives of the Norway eclipse of 1896 and the Indian eclipse of 1898.
Subject to these special points no further prefatory explanation seems needed, the general style of the contents being, mutatis mutandis, identical with the contents of the Volumes which have gone before.
I have to thank my friend, Dr. A. M. W. Downing, the Superintendent of the Nautical Almanac, for kindly verifying the calculations in chapters II. and III.
G. F. C. NORTHFIELD GRANGE, EASTBOURNE, 1899.
CONTENTS.
CHAP. PAGE
I. INTRODUCTION 9
II. GENERAL IDEAS 11
III. THE SAROS AND THE PERIODICITY OF ECLIPSES 18
IV. MISCELLANEOUS THEORETICAL MATTERS CONNECTED WITH ECLIPSES OF THE SUN (CHIEFLY) 34
V. WHAT IS OBSERVED DURING THE EARLIER STAGES OF AN ECLIPSE OF THE SUN 40 The Moon's Shadow and the Darkness it causes 41 Shadow Bands 46 The Approach of Totality 49 The Darkness of Totality 53 Meteorological and other effects 54
VI. WHAT IS OBSERVED DURING THE TOTAL PHASE OF AN ECLIPSE OF THE SUN 56 Baily's Beads 57 The Corona 62
VII. WHAT IS OBSERVED AFTER THE TOTAL PHASE OF AN ECLIPSE OF THE SUN IS AT AN END 73
VIII. ECLIPSES OF THE SUN MENTIONED IN HISTORY—CHINESE 75
IX. ARE ECLIPSES ALLUDED TO IN THE BIBLE 86
X. ECLIPSES MENTIONED IN HISTORY—CLASSICAL 107
XI. ECLIPSES MENTIONED IN HISTORY—THE CHRISTIAN ERA TO THE NORMAN CONQUEST 128
XII. ECLIPSES MENTIONED IN HISTORY—MEDIAEVAL AND MODERN 145
XIII. ECLIPSES MENTIONED IN HISTORY—NINETEENTH CENTURY 162
XIV. THE ELECTRIC TELEGRAPH AS APPLIED TO ECLIPSES OF THE SUN 179
XV. ECLIPSES OF THE MOON—GENERAL PRINCIPLES 186
XVI. ECLIPSES OF THE MOON MENTIONED IN HISTORY 197
XVII. CATALOGUES OF ECLIPSES: AND THEIR CALCULATION 218
XVIII. STRANGE ECLIPSE CUSTOMS 224
XIX. ECLIPSES IN SHAKESPEARE AND THE POETS 229
XX. BRIEF HINTS TO OBSERVERS OF ECLIPSES 233
XXI. TRANSITS AND OCCULTATIONS 235
APPENDIX—INFORMATION RESPECTING THE TOTAL ECLIPSE OF MAY 28, 1900, FOR TRAVELLERS VISITING PORTUGAL AND SPAIN 239
LIST OF ILLUSTRATIONS.
PAGE
FIG. 1. TOTAL ECLIPSE OF THE SUN, SEPTEMBER 7, 1858 Frontispiece
" 2. THEORY OF TOTAL ECLIPSE OF THE SUN 15
" 3. THEORY OF AN ANNULAR ECLIPSE OF THE SUN 16
" 4. ANNULAR ECLIPSE OF THE SUN 16
" 5. PARTIAL ECLIPSE OF THE SUN 17
" 6. SHADOW BANDS 47
" 7. RAYS OF LIGHT SEEN DURING TOTALITY 49
" 8. BRUSHES OF LIGHT 57
" 9. "BAILY'S BEADS," FOUR STAGES, AT BRIEF INTERVALS (MAY 15, 1836) 58
" 10. CORONA OF 1882. SUN-SPOT MAXIMUM 68
" 11. CORONA OF 1867. SUN-SPOT MINIMUM 70
" 12. ECLIPSE OF JAN. 11, 689 B.C. AT JERUSALEM 100
" 13. THEORY OF AN ECLIPSE OF THE MOON 187
" 14. CONDITIONS OF ECLIPSES OF THE MOON 189
" 15. OCCULTATION OF JUPITER, AUG. 7, 1889 (IMMERSION) 237
" 16. OCCULTATION OF JUPITER, AUG. 7, 1889 (IMMERSION) 237
" 17. OCCULTATION OF JUPITER, AUG. 7, 1889 (EMERSION) 238
" 18. OCCULTATION OF JUPITER, AUG. 7, 1889 (EMERSION) 238
" 19. PATH OF THE TOTAL ECLIPSE OF THE SUN OF MAY 28, 1900 at end of book.
THE STORY OF ECLIPSES.
CHAPTER I.
INTRODUCTION.
It may, I fear, be taken as a truism that "the man in the street" (collectively, the "general public") knows little and cares less for what is called physical science. Now and again when something remarkable happens, such as a great thunderstorm, or an earthquake, or a volcanic eruption, or a brilliant comet, or a total eclipse, something in fact which has become the talk of the town, our friend will condescend to give the matter the barest amount of attention, whilst he is filling his pipe or mixing a whisky and soda; but there is not in England that general attention given to the displays of nature and the philosophy of those displays, which certainly is a characteristic of the phlegmatic German. However, things are better than they used to be, and the forthcoming total eclipse of the Sun of May 28, 1900 (visible as it will be as a partial eclipse all over Great Britain and Ireland, and as a total eclipse in countries so near to Great Britain as Spain and Portugal, to say nothing of the United States), will probably not only attract a good deal of attention on the part of many millions of English-speaking people, but may also be expected to induce a numerically respectable remnant to give their minds and thoughts, with a certain amount of patient attention, to the Science and Philosophy of Eclipses.
There are other causes likely to co-operate in bringing this about. It is true that men's minds are more enlightened at the end of the 19th century than they were at the end of the 16th century, and that a trip to Spain will awaken vastly different thoughts in the year 1900 to those which would have been awakened, say in the year 1587; but for all that, a certain amount of superstition still lingers in the world, and total eclipses as well as comets still give rise to feelings of anxiety and alarm amongst ill-educated villagers even in so-called civilized countries. Some amusing illustrations of this will be presented in due course. For the moment let me content myself by stating the immediate aim of this little book, and the circumstances which have led to its being written. What those circumstances are will be understood generally from what has been said already. Its aim is the unambitious one of presenting in readable yet sound scientific language a popular account of eclipses of the Sun and Moon, and (very briefly) of certain kindred astronomical phenomena which depend upon causes in some degree similar to those which operate in connection with eclipses. These kindred phenomena are technically known as "Transits" and "Occultations." Putting these two matters entirely aside for the present, we will confine our attention in the first instance to eclipses; and as eclipses of the Sun do not stand quite on the same footing as eclipses of the Moon, we will, after stating the general circumstances of the case, put the eclipses of the Moon aside for a while.
CHAPTER II.
GENERAL IDEAS.
The primary meaning of the word "Eclipse" ([Greek: ekleipsis]) is a forsaking, quitting, or disappearance. Hence the covering over of something by something else, or the immersion of something in something; and these apparently crude definitions will be found on investigation to represent precisely the facts of the case.
Inasmuch as the Earth and the Moon are for our present purpose practically "solid bodies," each must cast a shadow into space as the result of being illuminated by the Sun, regarded as a source of light. What we shall eventually have to consider is: What results arise from the existence of these shadows according to the circumstances under which they are viewed? But before reaching this point, some other preliminary considerations must be dealt with.
The various bodies which together make up the Solar system, that is to say, in particular, those bodies called the "planets"—some of them "primary," others "secondary" (alias "Satellites" or "Moons")—are constantly in motion. Consequently, if we imagine a line to be drawn between any two at any given time, such a line will point in a different direction at another time, and so it may occasionally happen that three of these ever-moving bodies will come into one and the same straight line. Now the consequences of this state of things were admirably well pointed out nearly half a century ago by a popular writer, who in his day greatly aided the development of science amongst the masses. "When one of the extremes of the series of three bodies which thus assume a common direction is the Sun, the intermediate body deprives the other extreme body, either wholly or partially, of the illumination which it habitually receives. When one of the extremes is the Earth, the intermediate body intercepts, wholly or partially, the other extreme body from the view of the observers situate at places on the Earth which are in the common line of direction, and the intermediate body is seen to pass over the other extreme body as it enters upon or leaves the common line of direction. The phenomena resulting from such contingencies of position and direction are variously denominated Eclipses, Transits, and Occultations, according to the relative apparent magnitudes of the interposing and obscured bodies, and according to the circumstances which attend them."[1]
The Earth moves round the Sun once in every year; the Moon moves round the Earth once in every lunar month (27 days). I hope everybody understands those essential facts. Then we must note that the Earth moves round the Sun in a certain plane (it is nothing for our present purpose what that plane is). If the Moon as the Earth's companion moved round the Earth in the same plane, an eclipse of the Sun would happen regularly every month when the Moon was in "Conjunction" ("New Moon"), and also every month at the intermediate period there would be a total eclipse of the Moon on the occasion of every "Opposition" (or "Full Moon"). But inasmuch as the Moon's orbit does not lie in quite the same plane as the Earth's, but is inclined thereto at an angle which may be taken to average about 5-1/8 deg., the actual facts are different; that is to say, instead of there being in every year about 25 eclipses (solar and lunar in nearly equal numbers), which there would be if the orbits had identical planes, there are only a very few eclipses in the year, never, under the most favourable circumstances, more than 7, and sometimes as few as 2. Nor are the numbers equally apportioned. In years where there are 7 eclipses, 5 of them may be of the Sun and 2 of the Moon; where there are only 2 eclipses, both must be of the Sun. Under no circumstances can there be in any one year more than 3 eclipses of the Moon, and in some years there will be none. The reasons for these diversities are of a technical character, and a full elucidation of them would not be of interest to the general reader. It may here be added, parenthetically, that the occasions will be very rare of there being 5 solar eclipses in one year. This last happened in 1823,[2] and will only happen once again in the next two centuries, namely in 1935. If a total eclipse of the Sun happens early in January there may be another in December of the same year, as in 1889 (Jan. 1 and Dec. 22). This will not happen again till 2057, when there will be total eclipses on Jan. 5 and Dec. 26. There is one very curious fact which may be here conveniently stated as a bare fact, reserving the explanation of it for a future page, namely, that eclipses of the Sun and Moon are linked together in a certain chain or sequence which takes rather more than 18 years to run out when the sequence recurs and recurs ad infinitum. In this 18-year period, which bears the name of the "Saros," there usually happen 70 eclipses, of which 41 are of the Sun and 29 of the Moon. Accordingly, eclipses of the Sun are more numerous than those of the Moon in the proportion of about 3 to 2, yet at any given place on the Earth more lunar eclipses are visible than solar eclipses, because the former when they occur are visible over the whole hemisphere of the Earth which is turned towards the Moon whilst the area over which a total eclipse of the Sun is visible is but a belt of the Earth no more than about 150 to 170 miles wide. Partial eclipses of the Sun, however, are visible over a very much wider area on either side of the path traversed by the Moon's shadow.
Confining our attention in the first instance to eclipses of the Sun, the diagrams fig. 2 and fig. 3 will make clear, with very little verbal description, the essential features of the two principal kinds of eclipses of the Sun. In these figures S represents the Sun, M the Moon and E the Earth. They are not, of course, even approximately drawn to scale either as to the size of the bodies or their relative distances, but this is a matter of no moment as regards the principles involved. M being in sunshine receives light on, as it were, the left hand side, which faces S the Sun. The shadow of the Moon cast into space is, in the particular case, thrown as regards its tip on to the Earth and is intercepted by the Earth. Persons at the moment situated on the Earth within the limits of this shadow will not see any part of the Sun at all; they will see, in fact, nothing but the Moon as a black disc with only such light behind and around it as may be reflected back on to the sky by the illuminated (but to the Earth invisible) hemisphere of the Moon, or as may proceed from the Sun's Corona (to be described presently). The condition of things therefore is that known as a "total" eclipse of the Sun so far as regards the inhabitants of the narrow strip of Earth primarily affected.
Fig. 3 represents nearly but not quite the same condition of things. Here the Earth and the Moon are in those parts of their respective orbits which put the two bodies at or near the maximum distance possible from the Sun and from one another. The Moon casts its usual shadow, but the tip does not actually reach any part of the Earth's surface. Or, in other words, to an observer on the Earth the Moon is not big enough to conceal the whole body of the Sun. The result is this; at the instant of central coincidence the Moon covers up only the centre of the Sun, leaving the outer edge all round uncovered.
This outer edge shows as a bright ring of light, and the eclipse is of the sort known as an "annular" eclipse of the Sun.[3] As the greatest breadth of the annulus can never exceed 11/2 minutes of arc, an annular eclipse may sometimes, in some part of its track, become almost or quite total, and vice versa.
The idea will naturally suggest itself, what exactly does happen to the inhabitants living outside (on the one side or the other) of the strip of the Earth where the central line of shadow falls? This depends in every case on circumstances, but it may be stated generally that the inhabitants outside the central line but within 1000 to 2000 miles on either side, will see a larger or smaller part of the Sun concealed by the Moon's solid body, simultaneously with the total concealment of the Sun to the favoured individuals who live, or who for the moment are located, within the limits of the central zone.
Now we must advance one stage in our conceptions of the movements of the Earth and the Moon, so far as regards the bearing of those movements on the question of eclipses. The Earth moves in a plane which is called the "Plane of the Ecliptic," and correspondingly, the Sun has an apparent annual motion in the same plane. The Moon moving in a different plane, inclined to the first mentioned one to the extent of rather more than 5 deg., the Moon's orbit will evidently intersect the ecliptic in two places. These places of intersection are called "Nodes," and the line which may be imagined to join these Nodes is called the "Line of Nodes." When the Moon is crossing the ecliptic from the S. to the N. side thereof, the Moon is said to be passing through its "Ascending Node" ([Symbol: Ascending node]); the converse of this will be the Moon passing back again from the N. side of the ecliptic to the S. side, which is the "Descending Node" ([Symbol: Descending node]). Such changes of position, with the terms designating them, apply not only to the Moon in its movement round the Earth, but to all the planets and comets circulating round the Sun; and also to satellites circulating round certain of the planets, but with these matters we have no concern now.
FOOTNOTES:
[Footnote 1: D. Lardner, Handbook of Astronomy, 3rd ed., p. 288.]
[Footnote 2: But not one of them was visible at Greenwich.]
[Footnote 3: Latin Annulus, a ring.]
CHAPTER III.
THE "SAROS" AND THE PERIODICITY OF ECLIPSES.
To bring about an eclipse of the Sun, two things must combine: (1) the Moon must be at or near one of its Nodes; and (2), this must be at a time when the Moon is also in "Conjunction" with the Sun. Now the Moon is in Conjunction with the Sun (= "New Moon") 12 or 13 times in a year, but the Sun only passes through the Nodes of the Moon's orbit twice a year. Hence an eclipse of the Sun does not and cannot occur at every New Moon, but only occasionally. An exact coincidence of Earth, Moon, and Sun, in a straight line at a Node is not necessary to ensure an eclipse of the Sun. So long as the Moon is within about 181/2 deg. of its Node, with a latitude of not more than 1 deg. 34', an eclipse may take place. If, however, the distance is less than 151/4 deg. and the latitude less than 1 deg. 23' an eclipse must take place, though between these limits[4] the occurrence of an eclipse is uncertain and depends on what are called the "horizontal parallaxes" and the "apparent semi-diameters" of the two bodies at the moment of conjunction, in other words, on the nearness or "far-offness" of the bodies in question. Another complication is introduced into these matters by reason of the fact that the Nodes of the Moon's orbit do not occupy a fixed position, but have an annual retrograde motion of about 191/4 deg., in virtue of which a complete revolution of the Nodes round the ecliptic is accomplished in 18 years 218-7/8 days (= 18.5997 years).
The backward movement of the Moon's Nodes combined with the apparent motion of the Sun in the ecliptic causes the Moon in its monthly course round the Earth to complete a revolution with respect to its Nodes in a less time (27.2 days) than it takes to get back to Conjunction with the Sun (29.5 days); and a curious consequence, as we shall see directly, flows from these facts and from one other fact. The other fact is to the Sun starting coincident with one of the Moon's Nodes, returns on the Ecliptic to the same Node in 346.6 days. The first named period of 27.2 days is called the "Nodical Revolution of the Moon" or "Draconic Month," the other period of 29.5 days is called the "Synodical Revolution of the Moon." Now the curious consequence of these figures being what they are is that 242 Draconic Months, 223 Lunations, and 19 Returns of the Sun to one and the same Node of the Moon's orbit, are all accomplished in the same time within 11 hours. Thus (ignoring refinements of decimals):—
Days Days. Years. Days. Hours.
242 times 27.2 = 6585.36 = 18 10 81/2 223 times 29.5 = 6585.32 = 18 10 73/4 19 times 346.6 = 6585.78 = 18 10 183/4
The interpretation to be put upon these coincidences is this: that supposing Sun and Moon to start together from a Node they would, after the lapse of 6585 days and a fraction, be found again together very near the same Node. During the interval there would have been 223 New and Full Moons. The exact time required for 223 Lunations is such that in the case supposed the 223rd conjunction of the two bodies would happen a little before they reached the Node; their distance therefrom would be 28' of arc. And the final fact is that eclipses recur in almost, though not quite, the same regular order every 6585-1/3 days, or more exactly, 18 years, 10 days, 7 hours, 42 minutes.[5] This is the celebrated Chaldean "SAROS," and was used by the ancients (and can still be used by the moderns in the way of a pastime) for the prediction of eclipses alike of the Sun and of the Moon.
* * * * *
At the end of a Saros period, starting from any date that may have been chosen, the Moon will be in the same position with respect to the Sun, nearly in the same part of the heavens, nearly in the same part of its orbit, and very nearly indeed at the same distance from its Node as at the date chosen for the terminus a quo of the Saros. But there are trifling discrepancies in the case (the difference of about 11 hours between 223 lunations and 19 returns of the Sun to the Moon's Node is one) and these have an appreciable effect in disturbing not so much the sequence of the eclipses in the next following Saros as their magnitude and visibility at given places on the Earth's surface. Hence, a more accurate succession will be obtained by combining 3 Saros periods, making 54 years, 31 days; while, best of all, to secure an almost perfect repetition of a series of eclipses will be a combination of 48 Saroses, making 865 years for the Moon; and of about 70 Saroses, or more than 1200 years for the Sun.
These considerations are leading us rather too far afield. Let us return to a more simple condition of things. The practical use of the Saros in its most elementary conception is somewhat on this wise. Given 18 or 19 old Almanacs ranging, say, from 1880 to 1898, how can we turn to account the information they afford us in order to obtain from them information respecting the eclipses which will happen between 1899 and 1917? Nothing easier. Add 18^y 10^d 7^h 42^m to the middle time of every eclipse which took place between 1880 and 1898 beginning, say, with the last of 1879 or the first of 1880, and we shall find what eclipses will happen in 1898 and 17 following years, as witness by way of example the following table:—
Error of Saros by d. h. m. Exact Calculation. MOON. 1879 Dec. 28 4 26 p.m. (Mag. 0.17) 18 10 7 42 ——————————————————————- (Mag. 0.16) 1898 Jan. 8 12 8 a.m. (civil time) +3 m.
d. h. m. SUN. 1880 Jan. 11 10 48 p.m. (Total) 18 10 7 42 ——————————————————————- (Total) 1898 Jan. 22 6 30 a.m. (civil time) -1 h. 7 m.
d. h. m. MOON. 1880 June 22 1 50 p.m. (Mag. Total) 18 11 7 42 ——————————————————————- (Mag. 0.93) 1898 July 3 9 32 p.m. +35 m.
d. h. m. SUN. 1880 July 7 1 35 p.m. (Mag. Annular) 18 11 7 42 ——————————————————————- (Mag. Annular) 1898 July 18 9 17 p.m. +1 h. 10 m.
d. h. m. SUN. 1880 Dec. 2 3 11 a.m. (civil time). (Mag. 0.04) 18 11 7 42 ——————————————————————- (Mag. 0.02) 1898 Dec. 13 10 53 a.m. -1 h. 5 m.
d. h. m. MOON. 1880 Dec. 16 3 39 p.m. (Mag. Total) 18 11 7 42 ——————————————————————- (Mag. Total) 1898 Dec. 27 11 21 p.m. -13 m.
d. h. m. SUN. 1880 Dec. 31 1 45 p.m. (Mag. 0.71) 18 11 7 42 ——————————————————————- (Mag. 0.72) 1899 Jan. 11 9 27 p.m. -1 h. 11 m.
There having been 5 recurrences of Feb. 29 between Dec. 1879 and Jan. 1899, 5 leap years having intervened, we have to add an extra day to the Saros period in the later part of the above Table.[6]
Let us make another start and see what we can learn from the eclipses, say, of 1883.
Error of Saros by Exact Calculation. h. m. MOON 1883 April 22 11 39 a.m. (Mag. 0.8) 18 11 7 42 —————————————————————— (Mag. Penumbral) 1901 May 3 7 21 p.m. +51 m.
h. m. SUN 1883 May 6 9 45 p.m. Visible, Philippines. (Mag. Total) 18 11 7 42 —————————————————————— (Mag. Total) 1901 May 18 5 27 a.m. (civil time). -2 m.
h. m. MOON 1883 Oct. 16 6 54 a.m. Visible, California. (Mag. 0.28) 18 11 7 42 —————————————————————— (Mag. 0.23) 1901 Oct. 27 2 36 p.m. -39 m.
h. m. SUN 1883 Oct. 30 11 37 p.m. Visible, N. Japan. (Mag. Annular) 18 11 7 42 —————————————————————— (Mag. Annular) 1901 Nov. 11 7 19 a.m. (civil time) +1 m.
The foregoing does not by any means exhaust all that can be said respecting the Saros even on the popular side.
If the Saros comprised an exact number of days, each eclipse of a second Saros series would be visible in the same regions of the Earth as the corresponding eclipse in the previous series. But since there is a surplus fraction of nearly one-third of a day, each subsequent eclipse will be visible in another region of the Earth, which will be roughly a third of the Earth's circumference in longitude backwards (i.e. about 120 deg. to the W.), because the Earth itself will be turned on its axis one-third forwards.
After what has been said as to the Saros and its use it might be supposed that a correct list of eclipses for 18.03 years would suffice for all ordinary purposes of eclipse prediction, and that the sequence of eclipses at any future time might be ascertained by adding to some one eclipse which had already happened so many Saros periods as might embrace the years future whose eclipses it was desired to study. This would be true in a sense, but would not be literally and effectively true, because corresponding eclipses do not recur exactly under the same conditions, for there are small residual discrepancies in the times and circumstances affecting the real movements of the Earth and Moon and the apparent movement of the Sun which, in the lapse of years and centuries, accumulate sufficiently to dislocate what otherwise would be exact coincidences. Thus an eclipse of the Moon which in A.D. 565 was of 6 digits[7] was in 583 of 7 digits, and in 601 nearly 8. In 908 the eclipse became total, and remained so for about twelve periods, or until 1088. This eclipse continued to diminish until the beginning of the 15th century, when it disappeared in 1413. Let us take now the life of an eclipse of the Sun. One appeared at the North Pole in June A.D. 1295, and showed itself more and more towards the S. at each subsequent period. On August 27, 1367, it made its first appearance in the North of Europe; in 1439 it was visible all over Europe; in 1601, being its 19th appearance, it was central and annular in England; on May 5, 1818, it was visible in London, and again on May 15, 1836. Its three next appearances were on May 26, 1854, June 6, 1872, and June 17, 1890. At its 39th appearance, on August 10, 1980, the Moon's shadow will have passed the equator, and as the eclipse will take place nearly at midnight (Greenwich M.T.), the phenomenon will be invisible in Europe, Africa, and Asia. At every succeeding period the central line of the eclipse will lie more and more to the S., until finally, on September 30, 2665, which will be its 78th appearance, it will vanish at the South Pole.[8]
The operation of the Saros effects which have been specified above, may be noticed in some of the groups of eclipses which have been much in evidence (as will appear from a subsequent chapter), during the second half of the 19th century. The following are two noteworthy Saros groups of Solar eclipses:—
1842 July 8. 1850 Aug. 7. 1860 " 18. 1868 " 17. 1878 " 29. 1886 " 29. 1896 Aug. 9. 1904 Sept. 9.
If the curious reader will trace, by means of the Nautical Almanac (or some other Almanac which deals with eclipses in adequate detail), the geographical distribution of the foregoing eclipses on the Earth's surface, he will see that they fulfil the statement made on p. 24 (ante), that a Saros eclipse when it reappears, does so in regions of the Earth averaging 120 deg. of longitude to the W. of those in which it had, on the last preceding occasion, been seen; and also that it gradually works northwards or southwards.
But a given Saros eclipse in its successive reappearances undergoes other transformations besides that of Terrestrial longitude. These are well set forth by Professor Newcomb[9]:—
"Since every successive recurrence of such an eclipse throws the conjunction 28' further toward the W. of the node, the conjunction must, in process of time, take place so far back from the node that no eclipse will occur, and the series will end. For the same reason there must be a commencement to the series, the first eclipse being E. of the node. A new eclipse thus entering will at first be a very small one, but will be larger at every recurrence in each Saros. If it is an eclipse of the Moon, it will be total from its 13th until its 36th recurrence. There will be then about 13 partial eclipses, each of which will be smaller than the last, when they will fail entirely, the conjunction taking place so far from the node that the Moon does not touch the Earth's shadow. The whole interval of time over which a series of lunar eclipses thus extend will be about 48 periods, or 865 years. When a series of solar eclipses begins, the penumbra of the first will just graze the earth not far from one of the poles. There will then be, on the average, 11 or 12 partial eclipses of the Sun, each larger than the preceding one, occurring at regular intervals of one Saros. Then the central line, whether it be that of a total or annular eclipse, will begin to touch the Earth, and we shall have a series of 40 or 50 central eclipses. The central line will strike near one pole in the first part of the series; in the equatorial regions about the middle of the series, and will leave the Earth by the other pole at the end. Ten or twelve partial eclipses will follow, and this particular series will cease."
These facts deserve to be expanded a little.
We have seen that all eclipses may be grouped in a series, and that 18 years or thereabouts is the duration of each series, or Saros cycle. But these cycles are themselves subject to cycles, so that the Saros itself passes through a cycle of about 64 Saroses before the conditions under which any given start was made, come quite round again. Sixty-four times 18 make 1152, so that the duration of a Solar eclipse Great Cycle may be taken at about 1150 years. The progression of such a series across the face of the Earth is thus described by Mrs. Todd, who gives a very clear account of the matter:—
"The advent of a slight partial eclipse near either pole of the Earth will herald the beginning of the new series. At each succeeding return conformably to the Saros, the partial eclipse will move a little further towards the opposite pole, its magnitude gradually increasing for about 200 years, but during all this time only the lunar penumbra will impinge upon the Earth. But when the true shadow begins to touch, the obscuration will have become annular or total near the pole where it first appeared. The eclipse has now acquired a track, which will cross the Earth slightly farther from that pole every time it returns, for about 750 years. At the conclusion of this interval, the shadow path will have reached the opposite pole; the eclipse will then become partial again, and continue to grow smaller and smaller for about 200 years additional. The series then ceases to exist, its entire duration having been about 1150 years. The series of "great eclipses" of which two occurred in 1865 and 1883, while others will happen in 1901, 1919, 1937, 1955, and 1973, affords an excellent instance of the northward progression of eclipse tracks; and another series, with totality nearly as great (1850, 1868, 1886, 1904, and 1922), is progressing slowly southwards."
The word "Digit," formerly used in connection with eclipses, requires some explanation. The origin of the word is obvious enough, coming as it does from the Latin word Digitus, a finger. But as human beings have only eight fingers and two thumbs it is by no means clear how the word came to be used for twelfths of the disc of the Sun or Moon instead of tenths. However, such was the case; and when a 16th-century astronomer spoke of an eclipse of six digits, he meant that one-half of the luminary in question, be it Sun or Moon, was covered. The earliest use of the word "Digit" in this connection was to refer to the twelfth part of the visible surface of the Sun or Moon; but before the word went out of use, it came to be applied to twelfths of the visible diameter of the disc of the Sun or Moon, which was much more convenient. However, the word is now almost obsolete in both senses, and partial eclipses, alike of the Sun and of the Moon, are defined in decimal parts of the diameter of the luminary—tenths or hundredths according to the amount of precision which is aimed at. Where an eclipse of the Moon is described as being of more than 12 Digits or more than 1.0 (= 1 diameter) it is to be understood that the eclipse will be (or was) not only total, but that the Moon will be (or was) immersed in the Earth's shadow with a more or less considerable extent of shadow encompassing it.
There are some further matters which require to be mentioned connected with the periodicity of eclipses. To use a phrase which is often employed, there is such a thing as an "Eclipse Season," and what this is can only be adequately comprehended by looking through a catalogue of eclipses for a number of years arranged in a tabular form, and by collating the months or groups of months in which batches of eclipses occur. This is not an obvious matter to the casual purchaser of an almanac, who, feeling just a slight interest in the eclipses of a coming new year, dips into his new purchase to see what those eclipses will be. A haphazard glance at the almanacs of even two or three successive years will probably fail to bring home to him the idea that each year has its own eclipse season in which eclipses may occur, and that eclipses are not to be looked for save at two special epochs, which last about a month each, and which are separated from one another and from the eclipse seasons of the previous and of the following years by intervals of about six months, within a few days more or less. Such, however, is the case. A little thought will soon make it clear why such should be the case. We have already seen that the Moon's orbit, like that of every other planetary member of the Solar System, has two crossing places with respect to the ecliptic which are called "Nodes." We know also that the apparent motion of the Sun causes that body to traverse the whole of the ecliptic in the course of the year. The conjoint result of all this is that the Moon passes through a Node twice in every lunar month of 27 days, and the Sun passes (apparently) through a Node twice in every year. The first ultimate result of these facts is that eclipses can only take place at or near the nodal passages of the Moon and the Sun, and that as the Sun's nodal passages are separated by six months in every case the average interval between each set of eclipses, if there is more than one, must in all cases be six months, more or less by a few days, dependent upon the latitude and longitude of the Moon at or about the time of its Conjunction or Opposition under the circumstances already detailed. If the logic of this commends itself to the reader's mind, he will see at once why eclipses or groups of eclipses must be separated by intervals of about half an ordinary year. Hence it comes about that, taking one year with another, it may be said that we shall always have a couple of principal eclipses with an interval of half a year (say 183 days) between each; and that on either side of these dominant eclipses there will, or may be, a fortnight before or a fortnight after, two other pairs of eclipses with, in occasional years, one extra thrown in. It is in this way that we obtain what it has already been said dogmatically that we do obtain; namely, always in one year two eclipses, which must be both of the Sun, or any number of eclipses up to seven, which number will be unequally allotted to the Sun or to the Moon according to circumstances.
Though it is roughly correct to say that the two eclipse seasons of every year run to about a month each, in length, yet it may be desirable to be a little more precise, and to say that the limits of time for solar eclipses cover 36 days (namely 18 days before and 18 days after the Sun's nodal passages); whilst in the case of the Moon, the limits are the lesser interval of 23 days, being 111/2 on either side of the Moon's nodal passages.
We have already seen[10] that the Moon's nodes are perpetually undergoing a change of place. Were it not so, eclipses of the Sun and Moon would always happen year after year in the same pair of months for us on the Earth. But the operative effect of the shifting of the nodes is to displace backwards the eclipse seasons by about 20 days. For instance in 1899 the eclipse seasons fall in June and December. The middle of the eclipse seasons for the next succeeding 20 or 30 years will be found by taking the dates of June 8 and December 2, 1899, and working the months backwards by the amount of 19-2/3 days for each succeeding year. Thus the eclipse seasons in 1900 will fall in the months of May and November; accordingly amongst the eclipses of that year we shall find eclipses on May 28, June 13, and November 22.
Perhaps it would tend to the more complete elucidation of the facts stated in the last half dozen pages, if I were to set out in a tabular form all the eclipses of a succession, say of half a Saros or 9 years, and thus exhibit by an appeal to the eye directly the grouping of eclipse seasons the principles of which I have been endeavouring to define and explain in words.
Approximate Mid-interval.
1894. March 21. [Symbol: Moon] } March 29. * April 6. [Symbol: Sun] }
Sept. 15. [Symbol: Moon] } Sept. 22. ** Sept. 29. [Symbol: Sun] }
1895. March 11. [Symbol: Moon] } March 18. * March 26. [Symbol: Sun] }
Aug. 20. [Symbol: Sun] } Sept. 4. [Symbol: Moon] } Sept. 4. ** Sept. 18. [Symbol: Sun] }
1896. Feb. 13. [Symbol: Sun] } Feb. 20. * Feb. 28. [Symbol: Moon] }
Aug. 9. [Symbol: Sun] } Aug. 16. ** Aug. 23. [Symbol: Moon] }
1897. Feb. 1. [Symbol: Sun] Feb. 1. *
July 29. [Symbol: Sun] July 29. **
1898. Jan. 7. [Symbol: Moon] } Jan. 14. * Jan. 22. [Symbol: Sun] }
July 3. [Symbol: Moon] } July. 10. ** July 18. [Symbol: Sun] }
Dec. 13. [Symbol: Sun] } Dec. 27. [Symbol: Moon] } Dec. 27. * 1899. Jan. 11. [Symbol: Sun] }
June 8. [Symbol: Sun] } June 15. ** June 23. [Symbol: Moon] }
Dec. 2. [Symbol: Sun] } Dec. 9. * Dec. 16. [Symbol: Moon] }
1900. May 28. [Symbol: Sun] } June 5. ** June 13. [Symbol: Moon] }
Nov. 22. [Symbol: Sun] Nov. 22. *
1901. May 3. [Symbol: Moon] } May 10. ** May 18. [Symbol: Sun] }
Oct. 27. [Symbol: Moon] } Nov. 3. * Nov. 11. [Symbol: Sun] }
1902. April 8. [Symbol: Sun] } April 22. [Symbol: Moon] } April 22. ** May 7. [Symbol: Sun] }
Oct. 17. [Symbol: Moon] } Oct. 24. * Oct. 31. [Symbol: Sun] }
The Epochs in the last column which are marked with stars (*) or (**) as the case may be, represent corresponding nodes so that from any one single-star date to the next nearest single-star date means an interval of one year less (on an average) the 19-2/3 days spoken of on p. 32 (ante). A glance at each successive pair of dates will quickly disclose the periodical retrogradation of the eclipse epochs.
FOOTNOTES:
[Footnote 4: These limits are slightly different in the case of eclipses of the Moon. (See p. 190, post.)]
[Footnote 5: This assumes that 5 of these years are leap years.]
[Footnote 6: If there are 5 leap years in the 18, the odd days will be 10; if 4 they will be 11; if only 3 leap years (as from 1797 to 1815 and 1897 to 1915), the odd days to be added will be 12.]
[Footnote 7: See p. 28 (post) for an explanation of this word.]
[Footnote 8: In Mrs. D. P. Todd's interesting little book, Total Eclipses of the Sun (Boston, U.S., 1894), which will be several times referred to in this work, two maps will be found, which will help to illustrate the successive northerly or southerly progress of a series of Solar eclipses, during centuries.]
[Footnote 9: In his and Professor Holden's Astronomy for Schools and Colleges, p. 184.]
[Footnote 10: See p. 19 (ante).]
CHAPTER IV.
MISCELLANEOUS THEORETICAL MATTERS CONNECTED WITH ECLIPSES OF THE SUN (CHIEFLY).
One or two miscellaneous matters respecting eclipses of the Sun (chiefly) will be dealt with in this chapter. It is not easy to explain or define in words the circumstances which control the duration of a Solar eclipse, whereas in the case of a lunar eclipse the obscuration is the same in degree at all parts of the Earth where the Moon is visible. In the case of a Solar eclipse it may be total, perhaps, in Africa, may be of six digits only in Spain, and of two only in England. Under the most favourable circumstances the breadth of the track of totality across the Earth cannot be more than 170 miles, and it may be anything less than that down to zero where the eclipse will cease to be total at all, and will become annular. The question whether a given eclipse shall exhibit itself on its central line as a total or an annular one depends, as has been already explained, on the varying distances of the Earth and the Moon from the Sun in different parts of their respective orbits. Hence it follows that not only may an eclipse show itself for several Saros appearances as total and afterwards become annular, and vice versa, but on rare occasions one and the same eclipse may be annular in one part of its track across the Earth and total in another part, a short time earlier or later. This last-named condition might arise because the Moon's distance from the Earth or the Sun had varied sufficiently during the progress of the eclipse to bring about such a result; or because the shadow just reaching the Earth and no more the eclipse would be total only for the moment when a view perpendicular upwards could be had of it, and would be annular for the minutes preceding and the minutes following the perpendicular glimpse obtained by observers actually on the central line. The eclipse of December 12, 1890, was an instance of this.
If the paths of several central eclipses of the Sun are compared by placing side by side a series of charts, such as those given in the Nautical Almanac or in Oppolzer's Canon, it will be noticed that the direction of the central line varies with the season of the year. In the month of March the line runs from S.W. to N.E., and in September from N.W. to S.E. In June the line is a curve, going first to the N.E. and then to the S.E. In December the state of things is reversed, the curve going first to the S.E. and then to the N.E. At all places within about 2000 miles of the central line the eclipse will be visible, and the nearer a place is to the central line, so much the larger will be the portion of the Sun's disc concealed from observers there by the Moon. If the central line runs but a little to the N. of the Equator in Winter or of 25 deg. of N. latitude in Summer, the eclipse will be visible all over the Northern Hemisphere, and the converse will apply to the Southern Hemisphere. It is something like a general rule in the case of total and annular eclipses, though subject to many modifications, that places within 200-250 miles of the central line will have partial eclipse of 11 digits; from thence to 500 miles of 10 digits, and so on, diminishing something like 1 digit for every 250 miles, so that at 2000 miles, or rather more, the Sun will be only to a very slight extent eclipsed, or will escape eclipse altogether.
The diameter of the Sun being 866,000 miles and the Moon being only 2160 miles or 1/400th how comes it to be possible that such a tiny object should be capable of concealing a globe 400 times bigger than itself? The answer is—Distance. The increased distance does it. The Moon at its normal distance from the Earth of 237,000 miles could only conceal by eclipse a body of its own size or smaller, but the Sun being 93,000,000 miles away, or 392 times the distance of the Moon, the fraction 1/392 representing the main distance of the Moon, more than wipes out the fraction 1/400 which represents our satellite's smaller size.
During a total eclipse of the Sun, the Moon's shadow travels across the Earth at a prodigious pace—1830 miles an hour; 301/2 miles a minute; or rather more than a 1/2 mile a second. This great velocity is at once a clue to the fact that the total phase during an eclipse of the Sun lasts for so brief a time as a few minutes; and also to the fact that the shadow comes and goes almost without being seen unless a very sharp watch is kept for it. Indeed, it is only observers posted on high ground with some miles of open low ground spread out under their eyes who have much chance of detecting the shadow come up, go over them, and pass forwards.
Places at or near the Earth's equator enjoy the best opportunities for seeing total eclipses of the Sun, because whilst the Moon's shadow travels eastwards along the Earth's surface at something like 2000 miles an hour, an observer at the equator is carried in the same direction by virtue of the Earth's axial rotation at the rate of 1040 miles an hour. But the speed imparted to an observer as the result of the Earth's axial rotation diminishes from the equator towards the poles where it is nil, so that the nearer he is to a pole the slower he goes, and therefore the sooner will the Moon's shadow overtake and pass him, and the less the time at his disposal for seeing the Sun hidden by the Moon.
It was calculated by Du Sejour that the greatest possible duration of the total phase of a Solar eclipse at the equator would be 7^m 58^s, and for the latitude of Paris 6^m 10^s. In the case of an annular eclipse the figures would be 12^m 24^s for the equator, and 9^m 56^s for the latitude of Paris. These figures contemplate a combination of all the most favourable circumstances possible; as a matter of fact, I believe that the greatest length of total phase which has been actually known was 61/2^m and that was in the case of the eclipse of August 29, 1886. It was in the open Atlantic that this duration occurred, but it was not observed. The maximum observed obscuration during this eclipse was no more than 4^m.
Though total eclipses of the Sun happen with tolerable frequency so far as regards the Earth as a whole, yet they are exceedingly rare at any given place. Take London, for instance. From the calculations of Hind, confirmed by Maguire,[11] it may be considered as an established fact that there was no total eclipse visible at London between A.D. 878 and 1715, an interval of 837 years. The next one visible at London, though uncertain, is also a very long way off. There will be a total eclipse on August 11, 1999, which will come as near to London as the Isle of Wight, but Hind, writing in 1871, said that he doubted whether there would be any other total eclipse "visible in England for 250 years[12] from the present time." Maguire states that the Sun has been eclipsed, besides twice at London, also twice at Dublin, and no fewer than five times at Edinburgh during the 846 years examined by him. In fact that every part of the British Isles has seen a total eclipse at some time or other between A.D. 878 and 1724 except a small tract of country at Dingle, on the West coast of Ireland. The longest totality was on June 15, 885, namely, 4^m 55^s, and the shortest in July 16, 1330, namely, 0^m 56^s.
Contrast with this the obscure island of Blanquilla, off the northern coast of Venezuela. The inhabitants of that island not long ago had the choice of two total eclipses within three and a half years, namely, August 29, 1886, and December 22, 1889; whilst Yellowstone, U.S., had two in twelve years (July 29, 1878, and January 1, 1889).
Counting from first to last, Du Sejour found that at the equator an eclipse of the Sun might last 4^h 29^m, and at the latitude of Paris 3^h 26^m. These intervals, of course, cover all the subordinate phases. The total phase which alone (with perhaps a couple of minutes added) is productive of spectacular effects, and interesting scientific results is a mere matter of minutes which may be as few as one (or less), or only as many as 6 or 8.
As a rule, a summer eclipse will last longer than a winter one, because in summer the Earth (and the Moon with it), being at its maximum distance from the Sun, the Sun will be at its minimum apparent size, and therefore the Moon will be able to conceal it the longer.
FOOTNOTES:
[Footnote 11: Month. Not., R.A.S., vol. xlv., p. 400. June 1885.]
[Footnote 12: Johnson makes the eclipse of June 14, 2151, to be "nearly, if not quite, total at London." Possibly it was this eclipse which Hind had in his thoughts when he wrote in the Times (July 28, 1871) the passage quoted above.]
CHAPTER V.
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.
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." Ruemker 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.
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 deg. or 20 deg. 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.]
CHAPTER VI.
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.
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.
"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 41/2 and 6 diameters of the Sun respectively. These dimensions are quite unprecedented.
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 deg. 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.
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).]
CHAPTER VII.
WHAT IS OBSERVED AFTER THE TOTAL PHASE OF AN ECLIPSE OF THE SUN IS AT AN END.
In a certain sense, a description of the incidents which precede the total disappearance of the Sun in connection with a total Eclipse will apply more or less to the second half of the phenomenon; only, of course, in the reverse order and on the opposite side of the compass. The Corona having appeared first of all on the W. side of the Sun, then having shown itself complete as surrounding the Sun, will begin to disappear on the W. side, and will be last seen on the E. side. Baily's Beads may or may not come into view; the Sun will reappear first as a very thin crescent, gradually widening; the quasi-nocturnal darkness visible on the Earth will cease, and eventually the Moon will completely pass away from off the Sun, and the Sun once again will exhibit a perfect circle of light. |
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