THE NEW ASTRONOMY
THE NEW ASTRONOMY
BY
SAMUEL PIERPONT LANGLEY, PH.D., LL.D.
DIRECTOR OF THE ALLEGHENY OBSERVATORY, MEMBER NATIONAL ACADEMY,
FELLOW ROYAL ASTRONOMICAL SOCIETY, ETC., ETC.
Illustrated
[Illustration]
BOSTON.
TICKNOR AND COMPANY
211 Tremont Street
1888
COPYRIGHT, 1884, 1885, 1886, AND 1887, BY THE CENTURY CO.;
AND 1887, BY S. P. LANGLEY.
_All rights reserved._
University Press:
JOHN WILSON AND SON, CAMBRIDGE.
PREFACE.
I have written these pages, not for the professional reader, but with
the hope of reaching a part of that educated public on whose support
he is so often dependent for the means of extending the boundaries of
knowledge.
It is not generally understood that among us not only the support
of the Government, but with scarcely an exception every new private
benefaction, is devoted to “the Old” Astronomy, which is relatively
munificently endowed already; while that which I have here called “the
New,” so fruitful in results of interest and importance, struggles
almost unaided.
We are all glad to know that Urania, who was in the beginning but a
poor Chaldean shepherdess, has long since become well-to-do, and dwells
now in state. It is far less known than it should be that she has a
younger sister now among us, bearing every mark of her celestial birth,
but all unendowed and portionless. It is for the reader’s interest in
the latter that this book is a plea.
CONTENTS.
CHAPTER PAGE
I. SPOTS ON THE SUN 1
II. THE SUN’S SURROUNDINGS 35
III. THE SUN’S ENERGY 70
IV. THE SUN’S ENERGY (_Continued_) 91
V. THE PLANETS AND THE MOON 117
VI. METEORS 175
VII. COMETS 199
VIII. THE STARS 221
INDEX 253
LIST OF ILLUSTRATIONS.
FIGURE PAGE
1. THE SUN’S SURROUNDINGS 4
2. VIEW OF THE SUN ON SEPT. 20, 1870 6
3. THE SUN ON SEPT. 22, 1870 6
4. THE SUN ON SEPT. 26, 1870 7
5. THE SUN ON SEPT. 19, 1870 8
6. THE SUN ON SEPT. 20, 1870 8
7. THE SUN ON SEPT. 21, 1870 9
8. THE SUN ON SEPT. 22, 1870 9
9. THE SUN ON SEPT. 23, 1870 10
10. THE SUN ON SEPT. 26, 1870 10
11. NASMYTH’S WILLOW LEAVES 11
12. THE CACTUS TYPE 12
13. EQUATORIAL TELESCOPE AND PROJECTION 13
14. POLARIZING EYE-PIECE 14
15. SPOT OF SEPT. 21, 1870 15
16. SPOT OF MARCH 5, 1873 15
17. SUN ON MARCH 5, 1873 18
18. “THE PLUME” SPOT OF MARCH 5 AND 6, 1873 19
19. TYPICAL SUN-SPOT OF DECEMBER, 1873 21
20. FROST CRYSTAL 23
21. CYCLONE SPOT 24
22. SPOT OF MARCH 31, 1875 25
23. CIRROUS CLOUD 27
24. SPOT OF MARCH 31, 1875 28
25. TYPICAL ILLUSTRATION OF FAYE’S THEORY 29
26. SPOT OF OCT. 13, 1876 30
27. PHOTOGRAPH OF EDGE OF SUN 31
28. FACULA 33
29. LUNAR CONE SHADOW 36
30. TRACK OF LUNAR SHADOW 39
31. INNER CORONA ECLIPSE OF 1869 40
32. SKETCH OF OUTER CORONA, 1869 41
33. TACCHINI’S DRAWING OF CORONA OF 1870 43
34. WATSON’S NAKED-EYE DRAWING OF CORONA OF 1870 44
35. PHOTOGRAPH SHOWING COMMENCEMENT OF OUTER CORONA 45
36. ECLIPSE OF 1857, DRAWING BY LIAIS 48
37. ENLARGEMENT OF PART OF FIG. 38 49
38. FAC-SIMILE OF PHOTOGRAPH OF CORONA OF 1871 51
39. “SPECTRES” 54
40. OUTER CORONA OF 1878 57
41. SPECTROSCOPE SLIT AND SOLAR IMAGE 59
42. SLIT AND PROMINENCES 59
43. TACCHINI’S CHROMOSPHERIC CLOUDS 62
44. TACCHINI’S CHROMOSPHERIC CLOUDS 62
45. VOGEL’S CHROMOSPHERIC FORMS 64
46. TACCHINI’S CHROMOSPHERIC FORMS 66
47. ERUPTIVE PROMINENCES 67
48. SUN-SPOTS AND PRICE OF GRAIN 77
49. SUN-SPOT OF NOV. 16, 1882, AND EARTH 80
50. GREENWICH RECORD OF DISTURBANCE OF MAGNETIC NEEDLE,
NOV. 16 AND 17, 1882 81
51. SUN-SPOTS AND MAGNETIC VARIATIONS 87
52. GREENWICH MAGNETIC OBSERVATIONS, AUG. 3 AND 5, 1872 89
53. ONE CUBIC CENTIMETRE 93
54. POUILLET’S PYRHELIOMETER 93
55. BERNIÈRES’S GREAT BURNING-GLASS 103
56. A “POUR” FROM THE BESSEMER CONVERTER 105
57. PHOTOMETER-BOX 108
58. MOUCHOT’S SOLAR ENGINE 109
59. ERICSSON’S NEW SOLAR ENGINE, NOW IN PRACTICAL USE IN NEW
YORK 113
60. SATURN 119
61. THE EQUATORIAL TELESCOPE AT WASHINGTON 122
62. JUPITER, MOON, AND SHADOW 125
63. THREE VIEWS OF MARS 129
64. MAP OF MARS 129
65. THE MOON 137
66. THE FULL MOON 141
67. GLASS GLOBE, CRACKED 145
68. PLATO AND THE LUNAR ALPS 149
69. THE LUNAR APENNINES: ARCHIMEDES 153
70. VESUVIUS AND NEIGHBORHOOD OF NAPLES 157
71. PTOLEMY AND ARZACHEL 161
72. MERCATOR AND CAMPANUS 165
73. WITHERED HAND 168
74. IDEAL LUNAR LANDSCAPE AND EARTH-SHINE 169
75. WITHERED APPLE 171
76. GASSENDI. NOV. 7, 1867 173
77. THE CAMP AT MOUNT WHITNEY 177
78. VESUVIUS DURING AN ERUPTION 183
79. METEORS OBSERVED NOV. 13 AND 14, 1868, BETWEEN MIDNIGHT
AND FIVE O’CLOCK, A. M. 189
80. COMET OF DONATI, SEPT. 16, 1858 201
81. “A PART OF A COMET” 203
82. COMET OF DONATI, SEPT. 24, 1858 205
83. COMET OF DONATI, OCT. 3, 1858 209
84. COMET OF DONATI, OCT. 9, 1858 213
85. COMET OF DONATI, OCT. 5, 1858 217
86. TYPES OF STELLAR SPECTRA 222
87. THE MILKY WAY 225
88. SPECTRA OF STARS IN PLEIADES 231
89. SPECTRUM OF ALDEBARAN 235
90. SPECTRUM OF VEGA 235
91. GREAT NEBULA IN ORION 239
92. A FALLING MAN 243
93. A FLASH OF LIGHTNING 245
THE NEW ASTRONOMY.
I.
SPOTS ON THE SUN.
The visitor to Salisbury Plain sees around him a lonely waste,
utterly barren except for a few recently planted trees, and otherwise
as desolate as it could have been when Hengist and Horsa landed in
Britain; for its monotony is still unbroken except by the funeral
mounds of ancient chiefs, which dot it to its horizon, and contrast
strangely with the crowded life and fertile soil which everywhere
surround its borders. In the midst of this loneliness rise the rude,
enormous monoliths of Stonehenge,--circles of gray stones, which seem
as old as time, and were there, as we now are told, the temple of a
people which had already passed away, and whose worship was forgotten,
when our Saxon forefathers first saw the place.
In the centre of the inner circle is a stone which is believed once
to have been the altar; while beyond the outmost ring, quite away to
the northeast upon the open plain, still stands a solitary stone,
set up there evidently with some special object by the same unknown
builders. Seen under ordinary circumstances, it is difficult to divine
its connection with the others; but we are told that once in each
year, upon the morning of the longest day, the level shadow of this
distant, isolated stone is projected at sunrise to the very centre of
the ancient sanctuary, and falls just upon the altar. The primitive
man who devised this was both astronomer and priest, for he not only
adored the risen god whose first beams brought him light and warmth,
but he could mark its place, and though utterly ignorant of its nature,
had evidently learned enough of its motions to embody his simple
astronomical knowledge in a record so exact and so enduring that though
his very memory has gone, common men are still interested in it; for,
as I learned when viewing the scene, people are accustomed to come from
all the surrounding country, and pass in this desolate spot the short
night preceding the longest day of the year, to see the shadow touch
the altar at the moment of sunrise.
Most great national observatories, like Greenwich or Washington,
are the perfected development of that kind of astronomy of which
the builders of Stonehenge represent the infancy. Those primitive
men could know where the sun would rise on a certain day, and make
their observation of its place, as we see, very well, without knowing
anything of its physical nature. At Greenwich the moon has been
observed with scarcely an intermission for one hundred and fifty
years, but we should mistake greatly did we suppose that it was for
the purpose of seeing what it was made of, or of making discoveries in
it. This immense mass of Greenwich observations is for quite another
purpose,--for the very practical purpose of forming the lunar tables,
which, by means of the moon’s place among the stars, will tell the
navigator in distant oceans where he is, and conduct the fleets of
England safely home.
In the observatory at Washington one may see a wonderfully exact
instrument, in which circles of brass have replaced circles of stone,
all so bolted between massive piers that the sun can be observed by
it but once daily, as it crosses the meridian. This instrument is the
completed attainment along that long line of progress in one direction,
of which the solitary stone at Stonehenge marks the initial step,--the
attainment, that is, purely of precision of measurement; for the
astronomer of to-day can still use his circles for the special purpose
of fixing the sun’s place in the heavens, without any more knowledge of
that body’s chemical constitution than had the man who built Stonehenge.
Yet the object of both is, in fact, the same. It is true that the
functions of astronomer and priest have become divided in the advance
of our modern civilization, which has committed the special cultivation
of the religious aspect of these problems to a distinct profession;
while the modern observer has possibly exchanged the emotions of awe
and wonder for a more exact knowledge of the equinox than was possessed
by his primitive brother, who both observed and adored. Still, both
aim at the common end, not of learning what the sun is made of, but
of where it will be at a certain moment; for the prime object of
astronomy, until very lately indeed, has still been to say _where_
any heavenly body is, and not _what_ it is. It is this precision of
measurement, then, which has always--and justly--been a paramount
object of this oldest of the sciences, not only as a good in itself,
but as leading to great ends; and it is this which the poet of Urania
has chosen rightly to note as its characteristic, when he says,--
“That little Vernier, on whose slender lines
The midnight taper trembles as it shines,
Tells through the mist where dazzled Mercury burns,
And marks the point where Uranus returns.”
But within a comparatively few years a new branch of astronomy has
arisen, which studies sun, moon, and stars for what they are in
themselves, and in relation to ourselves. Its study of the sun,
beginning with its external features (and full of novelty and interest,
even, as regards those), led to the further inquiry as to what it was
made of, and then to finding the unexpected relations which it bore to
the earth and our own daily lives on it, the conclusion being that, in
a physical sense, it made us and re-creates us, as it were, daily, and
that the knowledge of the intimate ties which unite man with it brings
results of the most practical and important kind, which a generation
ago were unguessed at.
This new branch of inquiry is sometimes called Celestial Physics,
sometimes Solar Physics, and is sometimes more rarely referred to as
the New Astronomy. I will call it here by this title, and try to tell
the reader something about it which may interest him, beginning with
the sun.
[Illustration: FIG. 1.--THE SUN’S SURROUNDINGS.]
The whole of what we have to say about the sun and stars presupposes a
knowledge of their size and distance, and we may take it for granted
that the reader has at some time or another heard such statements as
that the moon’s distance is two hundred and forty thousand miles, and
the sun’s ninety-three million (and very probably has forgotten them
again as of no practical concern). He will not be offered here the
kind of statistics which he would expect in a college text-book; but
we must linger a moment on the threshold of our subject--the nature of
these bodies--to insist on the real meaning of such figures as those
just quoted. We are accustomed to look on the sun and moon as far off
together in the sky; and though we know the sun is greater, we are apt
to think of them vaguely as things of a common order of largeness,
away among the stars. It would be safe to say that though nine out of
ten readers have learned that the sun is larger than the moon, and, in
fact, larger than the earth itself, most of them do not at all realize
that the difference is so enormous that if we could hollow out the
sun’s globe and place the earth in the centre, there would still be so
much room that the moon might go on moving in her present orbit at two
hundred and forty thousand miles from the earth,--_all within the globe
of the sun itself_,--and have plenty of room to spare.
As to the distance of ninety-three million miles, a cannon-ball would
travel it in about fifteen years. It may help us to remember that at
the speed attained by the Limited Express on our railroads a train
which had left the sun for the earth when the “Mayflower” sailed from
Delftshaven with the Pilgrim Fathers, and which ran at that rate day
and night, would in 1887 still be a journey of some years away from
its terrestrial station. The fare at the customary rates, it may be
remarked, would be rather over two million five hundred thousand
dollars, so that it is clear that we should need both money and leisure
for the journey.
Perhaps the most striking illustration of the sun’s distance is
given by expressing it in terms of what the physiologists would call
velocity of nerve transmission. It has been found that sensation is not
absolutely instantaneous, but that it occupies a very minute time in
travelling along the nerves; so that if a child puts its finger into
the candle, there is a certain almost inconceivably small space of
time, say the one-hundredth of a second, before he feels the heat. In
case, then, a child’s arm were long enough to touch the sun, it can be
calculated from this known rate of transmission that the infant would
have to live to be a man of over a hundred before it knew that its
fingers were burned.
Trying with the help of these still inadequate images, we may get some
idea of the real size and distance of the sun. I could wish not to have
to dwell upon such figures, that seem, however, indispensable; but we
are now done with these, and are ready to turn to the telescope and see
what the sun itself looks like.
[Illustration: FIG. 2.--VIEW OF THE SUN ON SEPT. 20, 1870.]
[Illustration: FIG. 3.--THE SUN ON SEPT. 22, 1870.
(FROM A PHOTOGRAPH)]
The sun, as we shall learn later, is a star, and not a particularly
large star. It is, as has been said, “only a private in the host of
heaven,” but it is one of that host; it is one of those glittering
points to which we have been brought near. Let us keep in mind, then,
from the first, what we shall see confirmed later, that there is an
essentially similar constitution in them all, and not forget that when
we study the sun, as we now begin to do, we are studying the stars also.
If we were called on to give a description of the earth and all that
is on it, it would be easily understood that the task was impossibly
great, and that even an account of its most striking general features
might fill volumes. So it is with the sun; and we shall find that
in the description of the general character of its immediate surface
alone, there is a great deal to be told. First, let us look at a little
conventional representation (Fig. 1), as at a kind of outline of the
unknown regions we are about to explore. The circle represents the
Photosphere, which is simply what the word implies, that “sphere” of
“light” which we have daily before our eyes, or which we can study
with the telescope. Outside this there is a thin envelope, which rises
here and there into irregular prominences, some orange-scarlet, some
rose-pink. This is the Chromosphere, a thin shell, mainly of crimson
and scarlet tints, invisible even to the telescope except at the time
of a total eclipse, when alone its true colors are discernible, but
seen as to its form at all times by the spectroscope. It is always
there, not hidden in any way, and yet not seen, only because it is
overpowered by the intenser brilliancy of the Photosphere, as a
glow-worm’s shine would be if it were put beside an electric light.
Outside all is the strange shape, which represents the mysterious
Corona, seen by the naked eye in a total eclipse, but at all other
times invisible even to telescope and spectroscope, and of whose true
nature we are nearly ignorant from lack of opportunity to study it.
[Illustration: FIG. 4.--THE SUN ON SEPT. 26, 1870.]
Disregarding other details, let us carry in our minds the three main
divisions,--the Photosphere, or daily visible surface of the sun, which
contains nearly all its mass or substance; the Chromosphere; and the
unsubstantial Corona, which is nevertheless larger than all the rest.
We begin our examination with the Photosphere.
There are records of spots having been seen with the naked eye before
the invention of the telescope, but they were supposed to be planets
passing between us and the surface; and the idea that the sun was pure
fire, necessarily immaculate, was taught by the professors of the
Aristotelian philosophy in mediæval schools, and regarded almost as
an article of religious faith. We can hardly conceive, now, the shock
of the first announcement that spots were to be found on the sun, but
the notion partook in contemporary minds at once of the absurd and the
impious; and we notice here, what we shall have occasion to notice
again, that these physical discoveries from the first affect men’s
thoughts in unexpected ways, and modify their scheme of the moral
universe as well as of the physical one.
[Illustration: FIG. 5.--SEPT. 19, 1870.]
[Illustration: FIG. 6.--SEPT. 20, 1870.
(ENGRAVED FROM A PHOTOGRAPH BY RUTHERFURD.)]
Very little indeed was added to the early observations of Fabricius and
Galileo until a time within the remembrance of many of us; for it is
since the advent of the generation now on the stage that nine-tenths of
the knowledge of the subject has been reached.
Let us first take a general view of the sun, and afterward study it
in detail. What we see with a good telescope in this general view is
something like this. Opposite are three successive views (Figs. 2, 3,
4) taken on three successive days,--quite authentic portraits, since
the sun himself made them; they being, in fact, projected telescopic
images which have been fixed for us by photography, and then exactly
reproduced by the engraver. The first was taken (by Mr. Rutherfurd, of
New York) on the 20th of September, 1870, when a remarkably large spot
had come into view. It is seen here not far from the eastern edge (the
left hand in the engraving), and numerous other spots are also visible.
The reader should notice the position of these, and then on turning to
the next view (Fig. 3, taken on September 22d) he will see that they
have all shifted their places, by a common motion toward the west. The
great spot on the left has now got well into view, and we can see its
separate parts; the group which was on the left of the centre has got
a little to the right of it, and so on. From the common motion of them
all, we might suspect that the sun was turning round on an axis like
the earth, carrying the spots with it; and as we continue to observe,
this suspicion becomes certainty. In the third view (Fig. 4), taken on
September 26th, the spot we first saw on the left has travelled more
than half across the disk, while others we saw on September 20th have
approached to the right-hand edge or passed wholly out of sight behind
it. The sun does rotate, then, but in twenty-five or twenty-six of
our days,--I say twenty-five _or_ twenty-six, because (what is very
extraordinary) it does not turn all-of-a-piece like the earth, but some
parts revolve faster than others,--not only faster in feet and inches,
but in the number of turns,--just as though the rim of a carriage wheel
were to make more revolutions in a mile than the spokes, and the spokes
more than the hub. Of course no solid wheel could so turn without
wrenching itself in pieces, but that the great solar wheel does, is
incontestable; and this alone is a convincing proof that the sun’s
surface is not solid, but liquid or gaseous.
[Illustration: FIG. 7.--SEPT. 21, 1870.]
[Illustration: FIG. 8.--SEPT. 22, 1870.]
But let us return to the great spot which we saw coming round the
eastern edge. Possibly the word “great” may seem misapplied to what was
but the size of a pin-head in the first engraving, but we must remember
that the disk of the sun there shown is in reality over 800,000 miles
in diameter. We shall soon see whether this spot deserves to be called
“great” or not.
[Illustration: FIG. 9.--SEPT. 23, 1870.]
[Illustration: FIG. 10.--SEPT. 26, 1870.]
Next we have six enlarged views of it on the 19th, 20th, 21st, 22d,
23d, and 26th. On the 19th it is seen very near the eastern limb,
showing like a great hole in the sun, and foreshortened as it comes
into view around the dark edge; for the edge of the sun is really
darker than the central parts, as it is shown here, or as one may see
even through a smoked glass by careful attention. On the 20th we have
the edge still visible, but on the 21st the spot has advanced so far
that the edge cannot be shown for want of room. We see distinctly
the division of the spot into the outer shades which constitute the
penumbra, and the inner darker ones which form the umbra and nucleus.
We notice particularly in this enlarged view, by comparing the
appearances on the 21st, 22d, and 23d, that the spot not only turns
with the sun (as we have already learned), but moves and changes within
itself in the most surprising way, like a terrestrial cloud, which
not only revolves with the rest of the globe, but varies its shape
from hour to hour. This is seen still more plainly when we compare
the appearance on the 23d with that on the 26th, only three days
later, where the process has begun by which the spot finally breaks
up and forever disappears. On looking at all this, the tremendous
scale on which the action occurs must be borne in mind. On the 21st,
for instance, the umbra, or dark central hole, alone was large enough
to let the whole globe of our own earth drop in without touching
the sides! We shall have occasion to recur to this view of the 21st
September again.
[Illustration: FIG. 11.--NASMYTH’S WILLOW LEAVES. (FROM HERSCHEL’S
“OUTLINES OF ASTRONOMY.”)]
In looking at this spot and its striking changes, the reader must not
omit to notice, also, a much less obvious feature,--the vaguely seen
mottlings which show all over the sun’s surface, both quite away from
the spots and also close to them, and which seem to merge into them.
[Illustration: FIG. 12.--THE CACTUS TYPE. (FROM SECCHI’S “LE SOLEIL.”)]
I think if we assign one year rather than another for the birth of the
youthful science of solar physics, it should be 1861, when Kirchhoff
and Bunsen published their memorable research on Spectrum Analysis,
and when Nasmyth observed what he called the “willow-leaf” structure
of the solar surface (see Fig. 11). Mr. Nasmyth, with a very powerful
reflecting telescope, thought he had succeeded in finding what these
faint mottlings really are composed of, and believed that he had
discovered in them some most extraordinary things. This is what he
thought he saw: The whole sun is, according to him, covered with huge
bodies of most definite shape, that of the oblong willow leaf, and of
enormous but uniform size; and the faint mottlings the reader has just
noticed are, according to him, made up of these. “These,” he says,
“cover the whole disk of the sun (except in the space occupied by the
spots) in countless millions, and lie crossing each other in every
imaginable direction.” Sir John Herschel took a particular interest
in the supposed discovery, and, treating it as a matter of established
fact, proceeded to make one of the most amazing suggestions in
explanation that ever came from a scientific man of deserved eminence.
We must remember how much there is unknown in the sun still, and what
a great mystery even yet overhangs many of our relations to that body
which maintains our own vital action, when we read the following words,
which are Herschel’s own. Speaking of these supposed spindle-shaped
monsters, he says:
“The exceedingly definite shape of these objects, their exact
similarity to one another, and the way in which they lie across
and athwart each other,--all these characters seem quite
repugnant to the notion of their being of a vaporous, a cloudy,
or a fluid nature. Nothing remains but to consider them as
separate and independent sheets, flakes, or scales, having some
sort of solidity. And these ... are evidently _the immediate
sources of the solar light and heat_, by whatever mechanism or
whatever processes they may be enabled to develop, and as it were
elaborate, these elements from the bosom of the non-luminous
fluid in which they appear to float. Looked at in this point of
view, we cannot refuse to regard them as _organisms_ of some
peculiar and amazing kind; and though it would be too daring to
speak of such organization as partaking of the nature of life,
yet we do know that vital action is competent to develop at once
heat and light and electricity.”
[Illustration: FIG. 13.--EQUATORIAL TELESCOPE AND PROJECTION.]
Such are his words; and when we consider that each of these solar
inhabitants was supposed to extend about two hundred by one thousand
miles upon the surface of the fiery ocean, we may subscribe to Mr.
Proctor’s comment, that “Milton’s picture of him who on the fires of
hell ‘lay floating many a rood,’ seems tame and commonplace compared
with Herschel’s conception of these floating monsters, the least
covering a greater space than the British Islands.”
[Illustration: FIG. 14.--POLARIZING EYE-PIECE.]
I hope I may not appear wanting in respect for Sir John Herschel--a man
whose memory I reverence--in thus citing views which, if his honored
life could have been prolonged, he would have abandoned. I do so
because nothing else can so forcibly illustrate the field for wonder
and wild conjecture solar physics presented even a few years ago; and
its supposed connection with that “Vital Force,” which was till so
lately accepted by physiology, serves as a kind of landmark on the way
we have come.
This new science of ours, then, youthful as it is, has already had its
age of fable.
After a time Nasmyth’s observation was attributed to imperfect
definition, but was not fairly disproved. He had, indeed, a basis of
fact for his statement, and to him belongs the credit of first pointing
out the existence of this minute structure, though he mistook its true
character. It will be seen later how the real forms might be mistaken
for leaves, and _in certain particular cases_ they certainly do take on
a very leaf-like appearance. Here is a drawing (Fig. 12) which Father
Secchi gives of some of them in the spot of April 14, 1867, and which
he compares to a branch of cactus. He remarks somewhere else that they
resemble a crystallization of sal-ammoniac, and calls them veils of
most intricate structure. This was the state of our knowledge in 1870,
and it may seem surprising that such wonderful statements had not been
proved or disproved, when they referred to mere matters of observation.
But direct observation is here very difficult on account of the
incessant tremor and vibration of our own atmosphere.
[Illustration: FIG. 15.--SPOT OF SEPT. 21, 1870. (REDUCED FROM AN
ORIGINAL DRAWING BY S. P. LANGLEY.)]
[Illustration: FIG. 16.--SPOT OF MARCH 5, 1873. (REDUCED FROM AN
ORIGINAL DRAWING BY S. P. LANGLEY.)]
The surface of the sun may be compared to an elaborate engraving,
filled with the closest and most delicate lines and hatchings, but an
engraving which during ninety-nine hundredths of the time can only be
seen across such a quivering mass of heated air as makes everything
confused and liable to be mistaken, causing what is definite to look
like a vaguely seen mottling. It is literally true that the more
delicate features we are about to show, are only distinctly visible
even by the best telescope during less than one-hundredth of the
time, coming out as they do in brief instants when our dancing air is
momentarily still, so that one who has sat at a powerful telescope all
day is exceptionally lucky if he has secured enough glimpses of the
true structure to aggregate five minutes of clear seeing, while at all
other times the attempt to magnify only produces a blurring of the
image. This study, then, demands not only fine telescopes and special
optical aids, but endless patience.
[Illustration: FIG. 17.--SUN ON MARCH 5, 1873. (FROM A DRAWING BY S. P.
LANGLEY.)]
My attention was first particularly directed to the subject in 1870
(shortly after the regular study of the Photosphere was begun at the
Allegheny Observatory by means of its equatorial telescope of thirteen
inches’ aperture), with the view of finding out what this vaguely seen
structure really is. Nearly three years of constant watching were given
to obtain the results which follow. The method I have used for it is
indicated in the drawing (Fig. 13), which shows the preliminary step
of projecting the image of the sun directly upon a sheet of paper,
divided into squares and attached to the eye-end of a great equatorial
telescope. When this is directed to the sun in a darkened dome, the
solar picture is formed upon the paper as in a camera obscura, and this
picture can be made as large or as small as we please by varying the
lenses which project it. As the sun moves along in the sky, its image
moves across the paper; and as we can observe how long the whole sun
(whose diameter in miles is known) takes to cross, we can find how many
miles correspond to the time it is in crossing one of the squares, and
so get the scale of the future drawing, and the true size in miles of
the spot we are about to study. Then a piece of clock-work attached to
the telescope is put in motion, and it begins to follow the sun in the
sky, and the spot appears fixed on the paper. A tracing of the spot’s
outline is next made, but the finer details are not to be observed by
this method, which is purely preliminary, and only for the purpose
of fixing the scale and the points of the compass (so to speak) on
the sun’s face. The projecting apparatus is next removed and replaced
by the polarizing eye-piece. Sir William Herschel used to avoid the
blinding effects of the concentrated solar light by passing the rays
through ink and water, but the phenomena of “polarization” have been
used to better advantage in modern apparatus. This instrument, one
of the first of its kind ever constructed, and in which the light is
polarized with three successive reflections through the three tubes
seen in the drawing (Fig. 14), was made in Pittsburgh as a part of the
gift of apparatus by one of its citizens to the Observatory, and has
been most useful. By its aid the eye can be safely placed where the
concentrated heat would otherwise melt iron. In practice I have often
gazed through it at the sun’s face without intermission from four to
five hours, with no more fatigue or harm to the eye than in reading a
book. By its aid the observer fills in the outline already projected on
the paper.
[Illustration: FIG. 18.--“THE PLUME” SPOT OF MARCH 5 AND 6, 1873. (FROM
AN ORIGINAL DRAWING BY S. P. LANGLEY.)]
The photograph has transported us already so near the sun’s surface
that we have seen details there invisible to the naked eye. We have
seen that what we have called “spots” are indeed regions whose actual
vastness surpasses the vague immensity of a dream, and it will not
cause surprise that in them is a temperature which also surpasses
greatly that of the hottest furnace. We shall see later, in fact, that
the whole surface is composed largely of metals turned into vapor in
this heat, and that if we could indeed drop our great globe itself upon
the sun, it would be dissipated as a snow-flake. Now, we cannot suppose
this great space is fully described when we have divided it into the
penumbra, umbra, and nucleus, or that the little photograph has shown
us all there is, and we rather anticipate that these great spaces must
be filled with curious things, if we could get near enough to see them.
We cannot advantageously enlarge our photograph further; but if we
could really come closer, we should have the nearer view that the work
at Allegheny, I have just alluded to, now affords. The drawing (Fig.
15) of the central part of the same great spot, already cited, was made
on the 21st of September, 1870, and may be compared with the photograph
of that day. We have now a greatly more magnified view than before, but
it is not blurred by the magnifying, and is full of detail. We have
been brought within two hundred thousand miles of the sun, or rather
less than the actual distance of the moon, and are seeing for ourselves
what was a few years since thought out of the reach of any observer.
See how full of intricate forms that void, black, umbral space in
the photograph has become! The penumbra is filled with detail of the
strangest kind, and there are two great “bridges,” as they are called,
which are almost wholly invisible in the photograph. Notice the line
in one of the bridges which follows its sinuosities through its whole
length of twelve thousand miles, making us suspect that it is made up
of smaller parts as a rope is made up of cords (as, in fact, it is);
and look at the end, where the cords themselves are unravelled into
threads fine as threads of silk, and these again resolved into finer
fibres, till in more and more web-like fineness it passes beyond the
reach of sight! I am speaking, however, here rather of the wonderful
original, as I so well remember it, than of what my sketch or even the
engraver’s skill can render.
[Illustration: FIG. 19.--TYPICAL SUN SPOT OF DECEMBER, 1873.
(REDUCED FROM AN ORIGINAL DRAWING BY S. P. LANGLEY.)]
[Illustration: FIG. 20.--FROST CRYSTAL.]
Next we have quite another “spot” belonging to another year (1873).
First, there is a view (Fig. 17) of the sun’s disk with the spot on
it (as it would appear in a small telescope), to show its relative
size, and then a larger drawing of the spot itself (Fig. 16), on a
scale of twelve thousand miles to the inch, so that the region shown
to the reader’s eyes, though but a “spot” on the sun, covers an area
of over one billion square miles, or more than five times the entire
surface of the earth, land, and water. To help us to conceive its
vastness, I have drawn in one corner the continents of North and South
America on the same scale as the “spot.” Notice the evidence of
solar whirlwinds and the extraordinary “plume” (Fig. 16), which is a
something we have no terrestrial simile for. The appearance of the
original would have been described most correctly by such incongruous
images as “leaf-like” and “crystalline” and “flame-like;” and even in
this inadequate sketch there may remain some faint suggestion of the
appearance of its wonderful archetype, which was indeed that of a great
flame leaping into spires and viewed through a window covered with
frost crystals. Neither “frost” nor “flame” is really there, but we
cannot avoid this seemingly unnatural union of images, which was fully
justified by the marvellous thing itself. The reader must bear in mind
that the whole of this was actually in motion, not merely turning with
the sun’s rotation, but whirling and shifting within itself, and that
the motion was in parts occasionally probably as high as fifty miles
per second,--per _second_, remember, not per hour,--so that it changed
under the gazer’s eyes. The hook-shaped prominence in the lower part
(actually larger than the United States) broke up and disappeared in
about twenty minutes, or while the writer was engaged in drawing it.
The imagination is confounded in an attempt to realize to itself the
true character of such a phenomenon.
[Illustration: FIG. 21.--CYCLONE SPOT. (DRAWN BY FATHER SECCHI.)]
On page 19 is a separate view of the plume (Fig. 18), a fac-simile of
the original sketch, which was made with the eye at the telescope. The
pointed or flame-like tips are not a very common form, the terminals
being more commonly clubbed, like those in Father Secchi’s “branch
of cactus” type given on page 12. It must be borne in mind, too, if
the drawing does not seem to contain all that the text implies, that
there were but a few minutes in which to attempt to draw, where even a
skilled draughtsman might have spent hours on the details momentarily
visible, and that much must be left to memory. The writer’s note-book
at the time contains an expression of despair at his utter inability to
render most of what he saw.
Let us now look at another and even more wonderful example. Fig. 19
shows part of a great spot which the writer drew in December, 1873,
when the rare coincidence happened of a fine spot and fine terrestrial
weather to observe it in. In this, as well as in the preceding drawing,
the pores which cover the sun’s surface by millions may be noted. The
luminous dots which divide them are what Nasmyth imperfectly saw, but
we are hardly more able than he to say what they really are. Each of
these countless “dots” is larger than England, Scotland, and Ireland
together! The wonderful “crystalline” structure in the centre cannot
be a real crystal, for it is ten times the area of Europe, and changed
slowly while I drew it; but the reader may be sure that its resemblance
to some crystallizations has not been in the least exaggerated. I have
sought to study various actual crystals for comparison, but found none
quite satisfactory. That of sal-ammoniac in some remote way resembles
it, as Secchi says; but perhaps the frost crystals on a window-pane
are better. Fig. 20 shows one selected among several windows I had
photographed in a preceding winter, which has some suggestions of the
so-called crystalline spot-forms in it, but which lacks the filamentary
thread-like components presently described. Of course the reader will
understand that it is given as a suggestion of the appearance merely,
and that no similarity of nature is meant to be indicated.
[Illustration: FIG. 22.--SPOT OF MARCH 31, 1875. (FROM AN ORIGINAL
DRAWING BY S. P. LANGLEY.)]
There were wonderful fern-like forms in this spot, too, and an
appearance like that of pine-boughs covered with snow; for, strangely
enough, the intense whiteness of the solar surface in the best
telescopes constantly suggests cold. I have had the same impression
vividly in looking at the immense masses of molten-white iron in a
great puddling-furnace. The salient feature here is one very difficult
to see, even in good telescopes, but one which is of great interest. It
has been shown in the previous drawings, but we have not enlarged on
it. Everywhere in the spot are long white threads, or filaments, lying
upon one another, tending in a general sense toward the centre, and
each of which grows brighter toward its inner extremity. These make up,
in fact, as we now see, the penumbra, or outer shade, and the so-called
“crystal” is really affiliated to them. Besides this, on closer looking
we see that the inner shade, or umbra, and the very deepest shades, or
nuclei, are really made of them too. We can look into the dark centre,
as into a funnel, to the depth of probably over five thousand miles;
but as far as we may go down we come to no liquid or solid floor, and
see only volumes of whirling vapor, disposed not vaguely like our
clouds, but in the singularly definite, fern-like, flower-like forms
which are themselves made of these “filaments,” each of which is from
three to five thousand miles long, and from fifty to two hundred miles
thick, and each of which (as we saw in the first spot) appears to be
made up like a rope of still finer and finer strands, looking in the
rare instants when irradiation makes an isolated one visible, like a
thread of gossamer or the finest of cobweb. These suggest the fine
threads of spun glass; and here there is something more than a mere
resemblance of form, for both appear to have one causal feature in
common, due to a viscous or “sticky” fluid; for there is much reason to
believe that the solar atmosphere, even where thinner than our own air,
is rendered viscous by the enormous heat, and owes to this its tendency
to pull out in strings in common with such otherwise dissimilar things
as honey, or melted sugar, or melted glass.
We may compare those mysterious things, the filaments, to long grasses
growing in the bed of a stream, which show us the direction and the
eddies of the current. The likeness holds in more ways than one. They
are not lying, as it were, flat upon the surface of the water, but
_within_ the medium; and they do not stretch along in any one plane,
but they bend down and up. Moreover, they are, as we see, apparently
rooted at one end, and their tips rise above the turbid fluid and grow
brighter as they are lifted out of it. But perhaps the most significant
use of the comparison is made if we ask whether the stream is moving in
an eddy like a whirlpool or boiling up from the ground. The question
in other words is, “Are these spots themselves the sign of a mere
chaotic disturbance, or do they show us by the disposition of these
filaments that each is a great solar maelstrom, carrying the surface
matter of the sun down into its body? or, finally, are they just the
opposite,--something comparable to fiery fountains or volcanoes on the
earth, throwing up to the surface the contents of the unknown solar
interior?”
[Illustration: FIG. 23.--CIRROUS CLOUD. (FROM A PHOTOGRAPH.)]
Before we try to answer this question, let us remember that the
astonishing rapidity with which these forms change, and still more the
fact that they do not by any means always change by a bodily removal
of one part from another, but by a dissolving away and a fading out
into invisibility, like the melting of a cloud into thin air,--let us
remember that all this assimilates them to something cloud-like and
vaporous, rather than crystalline, and that, as we have here seen,
we can ourselves pronounce from such results of recent observation
that these are not lumps of scoriæ floating on the solar furnace (as
some have thought them), and still less, literal crystals. We can see
for ourselves, I believe, that so far there is no evidence here of
any solid, or even liquid, but that the surface of the sun is purely
vaporous. Fig. 23 shows a cirrous cloud in our own atmosphere, caught
for us by photography, and which the reader will find it interesting to
compare with the apparently analogous solar cloud-forms.
[Illustration: FIG. 24.--SPOT OF MARCH 31, 1875. (FROM AN ORIGINAL
DRAWING BY S. P. LANGLEY.)]
“Vaporous,” we call them, for want of a better word, but without
meaning that it is like the vapor of our clouds. There is no exact
terrestrial analogy for these extraordinary forms, which are in fact,
as we shall see later, composed of iron and other metals--not of solid
iron nor even of liquid, but iron heated beyond even the liquid state
to that of iron-steam or vapor.
With all this in mind, let us return to the question, “Are the spots,
these gigantic areas of disturbance, comparable to whirlpools or to
volcanoes?” It may seem unphilosophical to assume that they are one
or the other, and in fact they may possibly be neither; but it is
certain that the surface of the sun would soon cool from its enormous
temperature, if it were not supplied with fresh heat, and it is almost
certain that this heat is drawn from the interior. As M. Faye has
pointed out,[1] there _must_ be a circulation up and down, the cooled
products being carried within, heated and brought out again, or the
sun would, however hot, grow cold outside; and, what is of interest
to us, the earth would grow cold also, and we should all die. No one,
I believe, who has studied the subject, will contradict the statement
that if the sun’s surface were absolutely cut off from any heat supply
from the interior, organic life in general upon the earth (and our
own life in particular) would cease much within a month. This solar
circulation, then, is of nearly as much consequence to us as that of
our own bodies, if we but knew it; and now let us look at the spots
again with this in mind.
[1] To Mr. Herbert Spencer must be assigned the earliest
suggestion of the necessity of such a circulation.
[Illustration: FIG. 25.--TYPICAL ILLUSTRATION OF FAYE’S THEORY.]
Fig. 21 shows a drawing by Father Secchi of a spot in 1854; and it is,
if unexaggerated, quite the most remarkable case of distinct cyclonic
action recorded. I say “if unexaggerated” because there is a strong
tendency in most designers to select what is striking in a spot, and
to emphasize that unduly, even when there is no conscious disposition
to alter. Every one who sketches may see a similar unconscious
tendency in himself or herself, shown in a disposition to draw all the
mountains and hills too high,--a tendency on which Ruskin, I think,
has remarked. In drawings of the sun there is a strong temptation to
exaggerate these circular forms, and we must not forget this in making
up the evidence. There is great need of caution, then, in receiving
such representations; but there certainly are forms which seem to be
clearly due to cyclonic action. They are usually scattered, however,
through larger spots, and I have never, in all my study of the sun,
seen one such complete type of the cyclone spot as that first given
from Secchi. Instances where spots break up into numerous subdivisions
by a process of “segmentation” under the apparent action of separate
whirlwinds are much more common. I have noticed, as an apparent effect
of this segmentation, what I may call the “honeycomb structure” from
its appearance with low powers, but which with higher ones turns out to
be made up of filamentary masses disposed in circular and ovoid curves,
often apparently overlying one another, and frequently presenting a
most curious resemblance to vegetable forms, though we appear to see
the real agency of whirlwinds in making them. I add some transcripts
of my original pencil memoranda themselves, made with the eye at the
telescope, which, though not at all finished drawings, may be trusted
the more as being quite literal transcripts at first hand.
[Illustration: FIG. 26.--SPOT OF OCT. 13, 1876. (FROM ORIGINAL DRAWING
BY S. P. LANGLEY.)]
Figs. 22 and 24, for instance, are two sketches of a little spot,
showing what, with low powers, gives the appearance I have called the
honeycomb structure, but which we see here to be due to whirls which
have disposed the filaments in these remarkable forms. The first was
drawn at eleven in the forenoon of March 31, 1875, the second at
three in the afternoon of the same day. The scale of the drawing is
fifteen thousand miles to the inch, and the changes in this little
spot in these few hours imply a cataclysm compared with which the
disappearance of the American continent from the earth’s surface would
be a trifle.
The very act of the solar whirlwind’s motion seemed to pass before my
eyes in some of these sketches; for while drawing them as rapidly as
possible, a new hole would be formed where there was none before, as if
by a gigantic invisible auger boring downward.
[Illustration: FIG. 27.--PHOTOGRAPH OF EDGE OF SUN. (BY PERMISSION OF
WARREN DE LA RUE, LONDON.)]
M. Faye, the distinguished French astronomer, believes that, owing to
the fact that different zones of the sun rotate faster than others,
whirlwinds analogous to our terrestrial cyclones, but on a vaster
scale, are set in motion, and suck down the cooled vapors of the solar
surface into its interior, to be heated and returned again, thus
establishing a circulation which keeps the surface from cooling down.
He points out that we should not conclude that these whirlwinds are not
acting everywhere, merely because our bird’s-eye view does not always
show them. We see that the spinning action of a whirlpool in water
becomes more marked as we go below the surface, which is comparatively
undisturbed, and we often see one whirl break up into several minor
ones, but all sucking downward and never upward. According to M. Faye,
something very like this takes place on the sun, and in Fig. 25 he
gives this section to show what he believes to occur in the case of a
spot which has “segmented,” or divided into two, like the one whose
(imaginary) section is shown above it. This theory is to be considered
in connection with such drawings as we have just shown, which are
themselves, however, no way dependent on theory, but transcripts from
Nature.
I do not here either espouse or oppose the “cyclonic” theory, but it is
hardly possible for any one who has been an eyewitness of such things
to refuse to regard some such disturbance as a real and efficient cause
in such instances as this.