On Chart I there are six lines drawn across the track: No. 1 near New
Orleans, and No. 6 on the ocean to the east of Norfolk, Va. These
represent the places for which the times of the duration are computed
in the American Nautical Almanac, with the following results:
No. h. m. h. m. m. s.
1. At 1 30 Greenwich M. T. = 7 27. Local M. T. the duration is 1 12.6
2. ” 1 35 ” ” = 7 47. ” ” ” ” ” 1 19.6
3. ” 1 40 ” ” = 8 05. ” ” ” ” ” 1 26.0
4. ” 1 45 ” ” = 8 22. ” ” ” ” ” 1 31.7
5. ” 1 50 ” ” = 8 40. ” ” ” ” ” 1 37.0
6. ” 1 55 ” ” = 8 54. ” ” ” ” ” 1 41.9
[Illustration: CHART III.--FIFTEEN PICTURES OF THE SOLAR CORONA,
ARRANGED IN THE ELEVEN YEAR PERIOD, TO SHOW THE RECURRENCE OF SIMILAR
TYPES DURING THIS PERIOD.]
An observer at the intersection of these cross-lines with the central
line will see the totality during the intervals given in the table.
The mode of the formation of the shadow cones of the moon, called the
penumbra for the partial shadow and the umbra for the total shadow, are
well illustrated in general works on astronomy, and good geometrical
pictures of them can there be found, together with much useful
information regarding the subject of eclipses. As we are here concerned
chiefly with certain practical points about the eclipse of 1900, it
will be well for the reader to consult such works for many details
regarding the astronomical features attending an eclipse of the sun
which must now be omitted.
There are many existing theories to account for the phenomenon of the
sun’s bright appendage, called the corona, which is visible only during
eclipses, on account of the absorbing effects of the earth’s atmosphere
on its light. Is it electrical, or is it magnetic? Is it composed of
fine stuff ejected from the sun, or of meteoric dust falling upon
the sun? Is it merely an optical effect, as some suppose, or is it
a portion of the newly discovered radiant matter streaming off to
enormous distances into space? The answer to these questions is eagerly
sought through observation, photography, and every other possible
means, on the occasion of each total eclipse.
The efforts of astronomers have thus far secured a series of pictures
of the solar corona, which, when compared together, show very
distinctly that the corona, as well as the spots, the protuberances,
and the faculæ, are going through a series of changes which seem to
repeat themselves in the so-called eleven-year period. It has also been
proven, with entire distinctness, that the earth’s magnetic field, as
marked by the changes in the intensity of the magnetic elements, in
the auroral displays, and the earth electric currents show variations
which synchronize closely with those observed on the sun; also that
the weather elements of pressure, temperature, precipitation, and
storm intensity all harmonize with the solar and the earth’s magnetism
in the same synchronism. All attempts of scientists to detect any
variations in the sunshine which falls upon the tropics have been
entirely futile; on the other hand, it has been shown that the magnetic
forces having the characteristics just mentioned impinge upon the
earth in a direction perpendicular to the plane of the earth’s orbit,
just as if the sun, being a magnet, throws out a field of force to
the surface of the earth, which, by its variation depending upon the
internal workings of the sun, produces the changes just enumerated in
the earth’s atmosphere and in its magnetic field, also throughout the
planetary system, being, of course, strongest near the sun. The belief
is gradually growing among scientists that the earth, the sun, and the
planets are all magnetic bodies, and have these bonds of connection
between them in addition to the Newtonian gravitation. This is a most
fascinating field of research, and, though full of difficulties, yet
attracts the attention of many who are convinced that one of the most
pressing duties of the hour is to clear up the problems connected
with the transmission of energy from the sun to the earth in other
forms than the ordinary or sunlight radiation. It is entirely probable
that the secular variations of the weather changes from year to year,
and even from month to month, are bound up with these solar forces,
and that the solution of these questions will carry with them much
information of practical use to civilized man.
The coronas of the past forty years are shown on Chart III, taken
from the report of the eclipse of 1896 (August 9th), by A. Hansky.
It arranges the coronas in the eleven-year period so far as the
dates at which the eclipses occurred permit this to be done, and by
comparing them in vertical lines the similarity is at once seen for the
respective quarters of phases of the period. The forecast there given
for 1900 is seen to resemble 1867, 1878, and 1889, but it differs in
orientation from that on Chart IV, which was prepared by the author.
The four coronas on the left in Chart III are taken at the sun-spot
maximum, and the appearance is that of total confusion in the structure
of the rays; the second and the fourth columns are for the sun’s
medium intensity at about halfway between the maximum and the minimum,
and they show a system of polar rays taking on structural form, the
second column being at a stage of diminishing and the fourth at one of
increasing solar activity; the third column gives the corona when the
spots are at a minimum of frequency and the sun is in a comparatively
quiescent state, wherein the polar rifts are very distinct and the
equatorial wings or extensions greatly developed.
[Illustration: CHART IV.--BIGELOW’S FORECAST OF THE CORONA OF MAY 28,
1900. E, earth’s axis; K, axis of ecliptic; S, axis of sun; C, C, poles
of the solar corona.]
The successful observation of a solar corona depends upon three
conditions: the selection of the instrument, its proper mounting, and
the photographic process, regarding each of which a few suggestions
will be made. The instruments are divided into two classes, for visual
and for photographic work. But in either case the most important
feature is the focal length or the size of the telescope. Since the
photographic image of the corona will not bear magnifying without
dispersing the available light, and thus blurring out the details of
the picture, which is the most important feature to retain to the
utmost, one can not use a short telescope and at the same time a
magnifying eyepiece to enlarge the image by projection on a screen
or on a photographic plate. The only alternative in order to get an
image of large diameter is to use a long-focus lens. The effect of
a difference of focus upon the image of the corona is well shown on
Chart V, which gives a small corona (1) taken with a four-foot lens
(Barnard), (2) with a fifteen-foot lens (Pickering), and (3) with a
forty-foot lens (Schaeberle). The diameter is proportional to the
focal length, but the difference of effect upon the details is very
important. In the small picture the details of the corona near the sun
are completely lost in the general light, while the coronal extensions
from the middle latitudes are seen at a great distance from the
sun--as much as one million miles; at the same time the polar rifts
are distinctly marked, so that the pole or central line from which
they bend is readily located. On the second picture the details of the
polar rays are better brought out, but the extensions are shortened. In
the third the region near the sun’s edge has many interesting details
very clearly defined, while all the extensions are gone. It is evident
that each lens has its advantage, according to the details sought, and
they ought all to be employed in the eclipse. The reproductions on
paper by no means do justice to the original negatives, which make the
distinctions even more pronounced than shown on Chart V.
[Illustration: CHART V.--RELATIVE IMAGES OF THE CORONA AS TAKEN WITH
TELESCOPES RANGING FROM FORTY FEET TO FOUR FEET IN FOCAL LENGTH.]
Some amateur observers have telescopes but no mountings suitable for
eclipse work, and many astronomical telescopes have good equatorial
mountings at home which are yet unavailable in the field for lack
of proper foundations and supports. The ordinary telescope balanced
near the center, with the eye end subject to all sorts of motions
which may happen through jarring and rough handling in the hurry of
shifting photographic plates, makes a very poor eclipse apparatus.
All telescopes of any length should be held firmly at each end, so
as to be perfectly steady, since the least vibration ruins a coronal
picture devoted to delicate photographic effects. There are two ways
of accomplishing this, and only two. Dispensing with an equatorial
mounting, the lens must be set permanently on a base, and light
reflected from a mirror must be utilized, which shall be concentrated
on a plate also placed on a fixed base. This is the method employed
by Schaeberle in Chili, April 16, 1893, to obtain No. III of Chart
V. There is no objection to it if an observer possesses a perfectly
plane mirror, which it is very difficult and also expensive to obtain;
if the reflecting mirror should be imperfect it would distort the
image of the corona. The second method, lacking a good mirror, is
to mount the long-focus lens in a tube and point it directly at the
sun. A forty-foot lens was thus mounted at Cape Ledo for December
22, 1889, and its action was very satisfactory. Of course, it was a
cumbersome arrangement, and could not be employed by a small party.
The foundation for the mounting of the forty-foot tube consisted of
two casks filled with stones and cement, and set firmly in the ground.
These made two good piers, since the narrowing tops of the casks held
the bed plates of the telescope as in a vise. A triangle, whose base
was parallel to the earth’s axis and having the telescope tube itself
for the long side, was fitted with an extension rod for adjustment
in altitude on the third side, and the whole was made to revolve on
ball bearings. This triangular support was rotated by a side rod of
adjustable length, whose end terminated in a sand piston working with
a regulating valve. The sand flowed out steadily like an hour-glass,
and dropped the tube, keeping it central on the sun. The image was
made to follow accurately for twenty minutes without tremor, all the
time holding the solar disk tangent to fixed lines. The principle of a
revolving triangle and a short piston, taking the place of an expensive
reflecting mirror with a delicate clockwork or one carrying a telescope
balanced on its center but subject to jars and side motions, is an
important assistance in field work on account of its ready adaptability
to all sorts of observations. Since time is limited, it is necessary to
provide all operations with automatic arrangements as far as possible,
by using such an apparatus as that described. What can be applied
successfully to a forty-foot lens can serve for shorter telescopes. In
combination with spectroscopes, polariscopes, and special instruments
for photographing, an immense amount of work can be compressed into
the few seconds allowed by mounting them all on such a movable frame
as the triangle. The old-fashioned method of putting one observer to
one telescope ought to be abandoned. Of course, for a rising sun during
the forenoon a modification in the moving support must be employed.
This should be such as to cause the objective of the telescope to rise
from the ground toward the meridian, and it must be accomplished by
attaching a heavy weight which in sinking draws the tube upward.
[Illustration: CHART VI.--TRACKS OF THE EIGHT TOTAL ECLIPSES OF THE SUN
SEEN ON THE NORTH AMERICAN CONTINENT DURING THE NINETEENTH CENTURY.]
Of the two kinds of photographic plates, the wet and the dry, the dry
plates are much more convenient in the field, and are good for certain
purposes. The objections to them are that the rough granulations of
the gelatin film sometimes overpower the fine details of the corona
itself; they burn out very near the disk by over-exposure while the
faint outer extensions are being taken. Wet plates having the requisite
quickness are harder to prepare, but are smoother and hold the coronal
rays better from the base to the outer edge, and there is always plenty
of time to give the necessary exposure. The No. 26 Seed plate requires
from 0.5 to 2.0 seconds only, generally one second being about right;
the wet plate will take the corona in eight seconds or less. The best
time of exposure should be tested on a bright star of about the second
magnitude, by trial before the eclipse. There is no rule about the
photographic focus, except to discover it by a series of exposures
at different distances near the supposed point. Eclipse work is a
practical matter, and many rough-and-ready methods must necessarily
be admitted. A good lens in a wooden light-tight tube, supported at
each end, having the motion of the sun, the photographic focal plane
carefully determined, the time exposures very short, and, finally,
exceedingly slow development of the picture after the eclipse--these
form the prime requisites. Expensive telescopes, clockwork on heavy
iron piers, reflecting mirrors, and such like apparatus are not needed.
Ingenuity in practical details, with great anxiety about the essential
matter of the light itself, is what is needed for a successful eclipse
expedition.
Those persons who have no telescope for viewing the sun, or camera for
photographing it, can yet see the corona to great advantage by means
of a good opera glass, and indeed this is really the most satisfactory
way to thoroughly enjoy the spectacle. The object may be sketched on
paper at once or from memory, and this picture may well be of value to
astronomy.
The tracks of the eight North American eclipses seen since 1800 are
shown on Chart VI. It is noted that three have paths very similarly
located, and that five run in directions about parallel to one another,
but almost at right angles to the first group. This comprises the
eclipse of November 30, 1834, duration two minutes; August 7, 1869,
two minutes and three quarters; July 29, 1878, two minutes and a half,
which stretch from Alaska southeastward in a fan-shape to the South
Atlantic coast. The second group contains the tracks June 16, 1806,
four minutes and a half; July 17, 1860, three minutes; January 11,
1880 thirty-two seconds; January 1, 1889, two minutes and a quarter;
and May 28, 1900, two minutes. These tracks all trend from southwest
to northeast, and cross the North American continent in different
latitudes, that of May 28, 1900, being the most southerly and of rather
short duration; lasting less than two minutes in the United States.
The path of the total eclipse of May 28, 1900, after leaving the United
States, crosses the North Atlantic Ocean to Coimbra, Portugal, and
continues over North Africa to its end at the Red Sea. Stations which
are not situated on the path of the totality will see the sun partially
eclipsed, in proportion to the distance of the locality from the
central line to the northern or the southern limits. Thus New England,
New York, the Ohio Valley, and the southern Rocky Mountain districts
will see the sun about nine tenths covered; the Lake Region, the lower
Missouri Valley, and Southern California will see an eight-tenths
eclipse; and the northern Rocky Mountain region about six tenths or
seven tenths. The best way to view the partially eclipsed sun is to
secure three strips of thin colored glass, one and a half inch wide by
five inches long--red, blue, and green; bind them over the eye end of
a good opera glass, and adjust focus on the sun. This makes the light
safe for the eyes and brings out the spherical aspect of the sun’s
ball. The time of the eclipse can be read by interpolating within the
lines marked on Chart VII.
[Illustration: CHART VII.--THE PATH OF THE TOTAL ECLIPSE OF MAY 28,
1900, WITH TIMES OF BEGINNING AND ENDING AT SEVERAL PLACES AND THE
NORTHERN AND SOUTHERN LIMITS OF THE PARTIAL ECLIPSE.]
No other eclipse track will occur in this country till June 8, 1918,
when one of the first kind will pass from Oregon to Florida, two
minutes in duration. Another will occur in New England, January 24,
1925. Eclipses seven minutes in duration will occur in India in 1955,
and in Africa in 1973, the longest for a thousand years. The remoteness
of the last two, both in time and place, put them out of reckoning for
most of us, but those of 1918 and 1925 give additional zest to the
approaching eclipse of May 28th, as affording further opportunity for
confirming facts and noting differences based upon the observations now
made.