TOTAL ECLIPSES OF THE SUN
Primitive peoples indulged in every variety of explanation of mysterious
happenings in the sky. To the Chinese and all through India, a total
eclipse of the sun is caused by "a certain dragon with very black
claws," who, except for their frightening him away by every conceivable
sort of hideous noise, would most certainly "eat up the sun." The
eclipse always goes off, the sun has never been eaten yet. Can you
convince a Chinaman that Rahu, the Dragon, wouldn't have eaten up the
sun, if his unearthly din hadn't frightened him away?
In Japan the eclipse drops poison from the sky into wells, so the
Japanese cover them up. Fontenelle relates that in the middle of the
seventeenth century a multitude of people shut themselves up in cellars
in Paris during a total eclipse.
In the Shu-king, an ancient Chinese work, occurs the earliest record of
a total eclipse of the sun, in the year B. C. 2158. The Nineveh eclipse
of B. C. 763 is perhaps the first of the ancient eclipses of which we
possess a really clear description on the Assyrian eponym tablets in the
British Museum. It is the eclipse possibly referred to in the Book of
Amos, viii.
But of all the ancient eclipses none perhaps exceeds in interest the
famous eclipse of Thales, B. C. 585, May 28. It is the first eclipse to
have been predicted, probably by means of the saros, or 18-year period
of eclipses, which is useful as an approximate method even at the
present day. But the accident of a war between the Lydians and the Medes
has added greatly to the historic interest, because the combatants were
so terrified by the sudden turning of day into night that they at once
concluded a peace cemented by two marriages.
Very many of the ancient eclipses have been of great use to the
historian in verifying dates, and mathematical astronomers have employed
them in correcting the lunar tables, or intricate mathematical data by
which the motion of the moon is predicted.
Coming down to the middle of the sixth century, we find the first
eclipse recorded in England, in the "Saxon Chronicle," A. D. 538. During
the epoch of the Arabian Nights several eclipses were witnessed at
Bagdad, A. D. 829 to 928, and many a century later by Ibu-Jounis, court
astronomer of Hakem, the Caliph of Egypt. Nothing is more interesting
than to search the quaint records of these ancient eclipses. One
occurring in 1560, when Tycho Brahe was but fourteen, had much to do
with turning his permanent interest toward mathematics and astronomy.
The eclipse of 1612 was the first "seen through a tube," the telescope
having been invented only a few years before. "Paradise Lost" was
completed about 1665, and the censorship was still in existence; and it
is matter of record that the oft-quoted passage,
"As when the Sun, new risen,
Looks through the horizontal misty air,
Shorn of his beams; or from behind the Moon,
In dim eclipse, disastrous twilight sheds
On half the nations, and with fear of change
Perplexes monarchs."
_P. L._, i. 594
was strongly urged as sufficient reason for suppressing the entire epic.
London was favored with the outflashing corona, May 3, 1715, and a
pamphlet was issued in prediction, entitled "The Black Day, or a
Prospect of Doomsday."
The first American eclipse expedition was on occasion of the totality of
Oct. 27, 1780, sent out by Harvard College and the American Academy of
Arts and Sciences under Professor Samuel Williams to Penobscot. There
was a fine total eclipse from Albany to Boston on June 16, 1806, and
many important observations of it were made in this country.
But it was not till the European eclipse of 1842 that research got fully
under way, because the germ of the new astronomy, particularly as
applied to the sun, had begun its development; and the significance of
the corona was obvious, if it could be proved a true appendage of the
sun. Photography had not long been discovered, and the corona of 1851
was the first to be automatically registered on a daguerreotype. In 1860
it was proved that prominences and corona both belong to the sun and not
to the moon.
The great Indian eclipse of 1868 brought the important discovery that
the prominences can be observed at any time without an eclipse by means
of the spectroscope. In 1869 bright lines were found in the spectrum of
the corona, one line in the green indicating the presence of an element
not then known on the earth and hence called coronium. In 1870 the
reversing layer or stratum of the sun was discovered. In 1878 a vast
ecliptic extension of the streams of the corona many millions of miles
both east and west of the sun was first seen. This is now known to be
the type of corona characteristic of minimum spots on the sun. In 1882
the spectrum of the corona was first photographed and in 1889 excellent
detail photographs of the corona were taken. In 1893 it was shown that
the corona quite certainly rotates bodily with the sun. In 1896 actual
spectrum photographs of the reversing layer established its existence
beyond doubt--"flash spectrum" it is often called. In 1898 the long
ecliptic streamers of the corona were successfully photographed for the
first time. In 1900 the depth of the reversing layer was found to
average 500 miles, the heat of the corona was first measured by the
bolometer, and many observations showed that the coronal streamers, in
part at least, partake of the nature of electric discharges.
All subsequent total eclipses have been carefully observed, in whatever
part of the world they may happen, and each has added new results of
significance to our theories of the corona and its relation to the
radiant energy of the sun. In very recent eclipses the cinematograph has
been brought into action as an efficient adjunct of observation; in 1914
the first successful "movie" of the eclipse was secured in Sweden, and
in 1918 Frost of the Yerkes Observatory first applied the cinematograph
to registry of the "flash spectrum," and Stebbins tested out his
photo-electric cell on the corona, making the brightness 0.5 that of the
full moon. In 1914 (Russia) and again in 1919 (on the Atlantic) the
obvious advantages of the aeroplane in ecliptic observation and
photography were sought by the writer, though unsuccessfully. The
photographic tests, however, conducted in preparation for these
expeditions proved the entire practicability of securing eclipse results
of much value, independently of clouds below.
Eclipses in the near future will be total in Australia about six minutes
on September 21, 1922; in California and Mexico about four minutes on
September 10, 1923; and along a line from Toronto to Nantucket about two
minutes on the morning of January 24, 1925.
To all spectators, savage or civilized, scientist or layman, a total
eclipse is wonderful and impressive. Langley said: "The spectacle is one
of which, though the man of science may prosaically state the facts,
perhaps only the poet could render the impression." Very gradually the
moon steals its way across the face of the sun, the lessened light is
hardly noticed. If one is near a tree through whose foliage the sunlight
filters, an extraordinary sight is seen; the ground all about is covered
with luminous crescents, instead of the overlapping disks which were
there before the eclipse came on; in both cases they are images of the
disk of the sun at the time, and the narrowing crescents will be watched
with interest as totality approaches. Then the shadow bands may be seen
flitting across the landscape, like "visible wind." They are probably
related to our atmosphere and the very slender crescent from which true
sunlight still comes.
Then for a few seconds the moon's actual shadow may be caught in its
approach, very suddenly the darkness steals over the landscape
and--totality is on. How lucky if there are no clouds! Every eye is
riveted on "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 splendor."
Then it is now or never with observer and photographer. Months of
diligent preparations at home followed by weeks of tedious journey
abroad, with days of strenuous preparation and rehearsals at the
station--all go for naught unless the whole is tuned up to perfect
operation the instant totality begins. It may last but a minute, or even
less; in 1937, however, total eclipse will last 7 minutes 20 seconds,
the longest ever observed, and within half a minute of the longest
possible. All is over as suddenly as it came on. The first thing is to
complete records, develop plates, and see if everything worked
perfectly.
There is great utility back of all eclipse research, on account of its
wide bearing on meteorology and terrestrial physics, and possibly the
direct use of solar energy for industrial purposes. With this purpose in
view the astronomer devotes himself unsparingly to the acquisition of
every possible fact about the sun and his corona.
Considering the earth as a whole, the number of total eclipses will
average nearly seventy to the century. But at any given place, one may
count himself very fortunate if he sees a single total eclipse, although
he may see several partial ones without going from home. Then, too,
there are annular or ring eclipses, averaging seven in eight years. But
had one been born in Boston or New York in the latter part of the
eighteenth century, he might have lived through the entire nineteenth
century and a long way into the twentieth without seeing more than one
total eclipse of the sun. In London in 1715 no total eclipse had been
visible for six centuries. However, taking general averages, and
recalling the comparatively narrow belt of total eclipse, every part of
the earth is likely to come within range of the moon's shadow once in
about three and a half centuries.
The longest total eclipses always occur near the equator; this is
because an observer on the equator is carried eastward by the earth's
rotation at a velocity of about 1,000 miles per hour, so that he remains
longer in the moon's shadow which is passing over him in the same
direction with a velocity about twice as great.
The general circumstances of total eclipses are readily foretold by
means of the ancient Chaldean period of eclipses known as the saros. It
is 18 years and 10 or 11 days in length (according to the number of leap
years intervening). In one complete saros, forty-one solar eclipses will
generally happen, but only about one-fourth of them will be total. The
saros is a period at the end of which the centers of sun and moon return
very nearly to their relative positions at the beginning of the cycle.
So, in general, the eclipse of any year will be a repetition of one
which took place 18 years before, and another very similar in
circumstances will happen 18 years in the future. Three periods of the
saros, or 54 years and 1 month, will usually bring about a return of any
given eclipse to any particular part of the earth, so far as longitude
is concerned, though the returning track will lie about 600 miles to the
north or south of the one 54 years earlier.
Paths of total eclipses frequently intersect, if large areas like an
entire country are considered; Spain, for instance, where total eclipses
have occurred in 1842, 1860, 1870, 1900 and 1905. Besides crossing
Spain, the tracks of totality on May 28, 1900, and August 30, 1905, were
unique in intersecting exactly over a large city--Tripoli in Barbary, on
both of which occasions the writer's expeditions to that city were
rewarded with perfect observing conditions in that now Italian province
on the edge of the great desert.
Kepler was the first astronomer to calculate eclipses with some approach
to scientific form, as exemplified in his Rudolphine Tables. His method
was of course geometrical. But La Grange, who applied the methods of
more refined analysis to the problem, was the first to develop a method
by which an eclipse and all its circumstances could be accurately
predicted for any part of the earth. To many minds, the prediction of an
eclipse affords the best illustration of the superior knowledge of the
astronomer: it seems little short of the marvelous. But recalling that
the motion of the moon follows the law of gravitation, and that its
position in the sky is predictable for years in advance with a high
degree of precision, it will readily be seen how the arrival of the
moon's shadow, and hence the total eclipses of the sun, can be foretold
for any place over which the shadow passes.
All these data derived by the mathematician are known as the elements of
the eclipse, and they are prepared many years in advance and published
in the nautical almanacs and astronomical ephemerides issued by the
leading nations. Buchanan's "Treatise on Eclipses" will supply all the
technical information regarding the prediction of eclipses that anyone
desirous of inquiring into this phase of the problem may desire.
So important are total eclipses in the scheme of modern solar research,
and so necessary are clear skies in order that expeditions may be
favored with success, that every effort is now made to ascertain the
weather chances at particular stations along the line of eclipse many
years in advance. This method of securing preliminary cloud observations
for a series of years has proved especially useful for the eclipses of
1893, 1896, 1900, and 1918; and had it been employed in Russia for
totality of 1914, many well-equipped expeditions might have been spared
disaster. The California and Mexico totality of 1923 does not require
this forethought, as the regions visited are quite likely to be free
from cloud; but observations are now in process of accumulation for the
total eclipse of 1925. The out-look for clear skies on that occasion,
the total eclipse nearest New York for more than a century, is not very
promising. The path of totality passes over Marquette, Michigan,
Rochester and Poughkeepsie, New York, Newport, Rhode Island, and
Nantucket about nine in the morning.
Everyone who saw it will remember the last total eclipse in this part of
the world--on June 8, 1918, visible from Oregon to Florida. Many will
recall the last total eclipse that was visible before that in the
eastern part of the United States, on May 28, 1900, visible in a narrow
path from New Orleans to Norfolk. One's father or grandfather will
perhaps remember the total eclipse of July 29, 1878, which passed over
the United States from Pike's Peak to Texas (it was the writer's maiden
eclipse), and another on August 7, 1869, which passed southeasterly over
Iowa and Kentucky. On all these occasions the paths of total eclipse
were dotted with numerous observing parties, many of them equipped with
elaborate apparatus for studying and photographing the solar corona and
prominences, together with a multitude of other phenomena which are seen
only when total eclipses take place.
Looking forward rather than backward, a striking series, or family, of
eclipses happens in the future: it is the series of May, 1901 and 1919,
recurring again on June 8, 1937 (over the Pacific Ocean), June 20, 1955
(through India, Siam, and Luzon), and June 30, 1973 (visible in Sahara,
Abyssinia, and Somali). Already in 1919 this totality was 6 minutes 50
seconds in duration; in 1937, as already mentioned, it will be 7 minutes
20 seconds, and at the subsequent returns even longer yet, approaching
the estimated maximum of 7 minutes 58 seconds which has never been
observed. This remarkable series of total eclipses is longer in duration
than any others during a thousand years. Its next subsequent return is
in 1991, occurring with the eclipsed sun practically at noon in the
zenith of Mount Popocatepetl in Mexico.
Whatever may be the progress of solar research during the intervening
years, it is impossible to imagine the alert astronomer of that remote
day without incentive for further investigation of the sun's corona, in
which are concealed no doubt many secrets of the sun's evolution from
nebula to star.