ECLIPSES
63. THE NATURE OF ECLIPSES.--Every planet has a shadow which travels
with the planet along its orbit, always pointing directly away from the
sun, and cutting off from a certain region of space the sunlight which
otherwise would fill it. For the most part these shadows are invisible,
but occasionally one of them falls upon a planet or some other body
which shines by reflected sunlight, and, cutting off its supply of
light, produces the striking phenomenon which we call an eclipse. The
satellites of Jupiter, Saturn, and Mars are eclipsed whenever they
plunge into the shadows cast by their respective planets, and Jupiter
himself is partially eclipsed when one of his own satellites passes
between him and the sun, and casts upon his broad surface a shadow too
small to cover more than a fraction of it.
But the eclipses of most interest to us are those of the sun and moon,
called respectively solar and lunar eclipses. In Fig. 33 the full moon,
_M´_, is shown immersed in the shadow cast by the earth, and therefore
eclipsed, and in the same figure the new moon, _M_, is shown as casting
its shadow upon the earth and producing an eclipse of the sun. From a
mere inspection of the figure we may learn that an eclipse of the sun
can occur only at new moon--i. e., when the moon is on line between the
earth and sun--and an eclipse of the moon can occur only at full moon.
Why? Also, the eclipsed moon, _M´_, will present substantially the same
appearance from every part of the earth where it is at all visible--the
same from North America as from South America--but the eclipsed sun
will present very different aspects from different parts of the earth.
Thus, at _L_, within the moon's shadow, the sunlight will be entirely
cut off, producing what is called a total eclipse. At points of the
earth's surface near _J_ and _K_ there will be no interference whatever
with the sunlight, and no eclipse, since the moon is quite off the line
joining these regions to any part of the sun. At places between _J_ and
_L_ or _K_ and _L_ the moon will cut off a part of the sun's light, but
not all of it, and will produce what is called a partial eclipse, which,
as seen from the northern parts of the earth, will be an eclipse of the
lower (southern) part of the sun, and as seen from the southern
hemisphere will be an eclipse of the northern part of the sun.
[Illustration: FIG. 33.--Different kinds of eclipse.]
The moon revolves around the earth in a plane, which, in the figure, we
suppose to be perpendicular to the surface of the paper, and to pass
through the sun along the line _M´ M_ produced. But it frequently
happens that this plane is turned to one side of the sun, along some
such line as _P Q_, and in this case the full moon would cut through the
edge of the earth's shadow without being at any time wholly immersed in
it, giving a partial eclipse of the moon, as is shown in the figure.
In what parts of the earth would this eclipse be visible? What kinds of
solar eclipse would be produced by the new moon at _Q_? In what parts of
the earth would they be visible?
64. THE SHADOW CONE.--The shape and position of the earth's shadow are
indicated in Fig. 33 by the lines drawn tangent to the circles which
represent the sun and earth, since it is only between these lines that
the earth interferes with the free radiation of sunlight, and since both
sun and earth are spheres, and the earth is much the smaller of the two,
it is evident that the earth's shadow must be, in geometrical language,
a cone whose base is at the earth, and whose vertex lies far to the
right of the figure--in other words, the earth's shadow, although very
long, tapers off finally to a point and ends. So, too, the shadow of the
moon is a cone, having its base at the moon and its vertex turned away
from the sun, and, as shown in the figure, just about long enough to
reach the earth.
It is easily shown, by the theorem of similar triangles in connection
with the known size of the earth and sun, that the distance from the
center of the earth to the vertex of its shadow is always equal to the
distance of the earth from the sun divided by 108, and, similarly, that
the length of the moon's shadow is equal to the distance of the moon
from the sun divided by 400, the moon's shadow being the smaller and
shorter of the two, because the moon is smaller than the earth. The
radius of the moon's orbit is just about 1/400th part of the radius of
the earth's orbit--i. e., the distance of the moon from the earth is
1/400th part of the distance of the earth from the sun, and it is this
"chance" agreement between the length of the moon's shadow and the
distance of the moon from the earth which makes the tip of the moon's
shadow fall very near the earth at the time of solar eclipses. Indeed,
the elliptical shape of the moon's orbit produces considerable
variations in the distance of the moon from the earth, and in
consequence of these variations the vertex of the shadow sometimes falls
short of reaching the earth, and sometimes even projects considerably
beyond its farther side. When the moon's distance is too great for the
shadow to bridge the space between earth and moon there can be no total
eclipse of the sun, for there is no shadow which can fall upon the
earth, even though the moon does come directly between earth and sun.
But there is then produced a peculiar kind of partial eclipse called
_annular_, or ring-shaped, because the moon, although eclipsing the
central parts of the sun, is not large enough to cover the whole of it,
but leaves the sun's edge visible as a ring of light, which completely
surrounds the moon. Although, strictly speaking, this is only a partial
eclipse, it is customary to put total and annular eclipses together in
one class, which is called central eclipses, since in these eclipses the
line of centers of sun and moon strikes the earth, while in ordinary
partial eclipses it passes to one side of the earth without striking it.
In this latter case we have to consider another cone called the
_penumbra_--i. e., partial shadow--which is shown in Fig. 33 by the
broken lines tangent to the sun and moon, and crossing at the point _V_,
which is the vertex of this cone. This penumbral cone includes within
its surface all that region of space within which the moon cuts off any
of the sunlight, and of course it includes the shadow cone which
produces total eclipses. Wherever the penumbra falls there will be a
solar eclipse of some kind, and the nearer the place is to the axis of
the penumbra, the more nearly total will be the eclipse. Since the moon
stands about midway between the earth and the vertex of the penumbra,
the diameter of the penumbra where it strikes the earth will be about
twice as great as the diameter of the moon, and the student should be
able to show from this that the region of the earth's surface within
which a partial solar eclipse is visible extends in a straight line
about 2,100 miles on either side of the region where the eclipse is
total. Measured along the curved surface of the earth, this distance is
frequently much greater.
Is it true that if at any time the axis of the shadow cone comes within
2,100 miles of the earth's surface a partial eclipse will be visible in
those parts of the earth nearest the axis of the shadow?
65. DIFFERENT CHARACTERISTICS OF LUNAR AND SOLAR ECLIPSES.--One marked
difference between lunar and solar eclipses which has been already
suggested, may be learned from Fig. 33. The full moon, _M´_, will be
seen eclipsed from every part of the earth where it is visible at all at
the time of the eclipse--that is, from the whole night side of the
earth; while the eclipsed sun will be seen eclipsed only from those
parts of the day side of the earth upon which the moon's shadow or
penumbra falls. Since the point of the shadow at best but little more
than reaches to the earth, the amount of space upon the earth which it
can cover at any one moment is very small, seldom more than 100 to 200
miles in length, and it is only within the space thus actually covered
by the shadow that the sun is at any given moment totally eclipsed, but
within this region the sun disappears, absolutely, behind the solid body
of the moon, leaving to view only such outlying parts and appendages as
are too large for the moon to cover. At a lunar eclipse, on the other
hand, the earth coming between sun and moon cuts off the light from the
latter, but, curiously enough, does not cut it off so completely that
the moon disappears altogether from sight even in mid-eclipse. The
explanation of this continued visibility is furnished by the broken
lines extending, in Fig. 33, from the earth through the moon. These
represent sunlight, which, entering the earth's atmosphere near the edge
of the earth (edge as seen from sun and moon), passes through it and
emerges in a changed direction, refracted, into the shadow cone and
feebly illumines the moon's surface with a ruddy light like that often
shown in our red sunsets. Eclipse and sunset alike show that when the
sun's light shines through dense layers of air it is the red rays which
come through most freely, and the attentive observer may often see at a
clear sunset something which corresponds exactly to the bending of the
sunlight into the shadow cone; just before the sun reaches the horizon
its disk is distorted from a circle into an oval whose horizontal
diameter is longer than the vertical one (see § 50).
QUERY.--At a total lunar eclipse what would be the effect upon the
appearance of the moon if the atmosphere around the edge of the earth
were heavily laden with clouds?
66. THE TRACK OF THE SHADOW.--We may regard the moon's shadow cone as a
huge pencil attached to the moon, moving with it along its orbit in the
direction of the arrowhead (Fig. 34), and as it moves drawing a black
line across the face of the earth at the time of total eclipse. This
black line is the path of the shadow and marks out those regions within
which the eclipse will be total at some stage of its progress. If the
point of the shadow just reaches the earth its trace will have no
sensible width, while, if the moon is nearer, the point of the cone will
be broken off, and, like a blunt pencil, it will draw a broad streak
across the earth, and this under the most favorable circumstances may
have a breadth of a little more than 160 miles and a length of 10,000 or
12,000 miles. The student should be able to show from the known distance
of the moon (240,000 miles) and the known interval between consecutive
new moons (29.5 days) that on the average the moon's shadow sweeps past
the earth at the rate of 2,100 miles per hour, and that in a general way
this motion is from west to east, since that is the direction of the
moon's motion in its orbit. The actual velocity with which the moon's
shadow moves past a given station may, however, be considerably greater
or less than this, since on the one hand when the shadow falls very
obliquely, as when the eclipse occurs near sunrise or sunset, the
shifting of the shadow will be very much greater than the actual motion
of the moon which produces it, and on the other hand the earth in
revolving upon its axis carries the spectator and the ground upon which
he stands along the same direction in which the shadow is moving. At the
equator, with the sun and moon overhead, this motion of the earth
subtracts about 1,000 miles per hour from the velocity with which the
shadow passes by. It is chiefly on this account, the diminished velocity
with which the shadow passes by, that total solar eclipses last longer
in the tropics than in higher latitudes, but even under the most
favorable circumstances the duration of totality does not reach eight
minutes at any one place, although it may take the shadow several hours
to sweep the entire length of its path across the earth.
According to Whitmell the greatest possible duration of a total solar
eclipse is 7m. 40s., and it can attain this limit only when the eclipse
occurs near the beginning of July and is visible at a place 5° north of
the equator.
The duration of a lunar eclipse depends mainly upon the position of the
moon with respect to the earth's shadow. If it strikes the shadow
centrally, as at _M´_, Fig. 33, a total eclipse may last for about two
hours, with an additional hour at the beginning and end, during which
the moon is entering and leaving the earth's shadow. If the moon meets
the shadow at one side of the axis, as at _P_, the total phase of the
eclipse may fail altogether, and between these extremes the duration of
totality may be anything from two hours downward.
[Illustration: FIG. 34.--Relation of the lunar nodes to eclipses.]
67. RELATION OF THE LUNAR NODES TO ECLIPSES.--To show why the moon
sometimes encounters the earth's shadow centrally and more frequently at
full moon passes by without touching it at all, we resort to Fig. 34,
which represents a part of the orbit of the earth about the sun, with
dates showing the time in each year at which the earth passes the part
of its orbit thus marked. The orbit of the moon about the earth, _M M´_,
is also shown, with the new moon, _M_, casting its shadow toward the
earth and the full moon, _M´_, apparently immersed in the earth's
shadow. But here appearances are deceptive, and the student who has
made the observations set forth in Chapter III has learned for himself
a fact of which careful account must now be taken. The apparent paths of
the moon and sun among the stars are great circles which lie near each
other, but are not exactly the same; and since these great circles are
only the intersections of the sky with the planes of the earth's orbit
and the moon's orbit, we see that these planes are slightly inclined to
each other and must therefore intersect along some line passing through
the center of the earth. This line, _N´ N´´_, is shown in the figure,
and if we suppose the surface of the paper to represent the plane of the
earth's orbit, we shall have to suppose the moon's orbit to be tipped
around this line, so that the left side of the orbit lies above and the
right side below the surface of the paper. But since the earth's shadow
lies in the plane of its orbit--i. e., in the surface of the paper--the
full moon of March, _M´_, must have passed below the shadow, and the new
moon, _M_, must have cast its shadow above the earth, so that neither a
lunar nor a solar eclipse could occur in that month. But toward the end
of May the earth and moon have reached a position where the line
_N´ N´´_ points almost directly toward the sun, in line with the shadow
cones which hide it. Note that the line _N´ N´´_ remains very nearly
parallel to its original position, while the earth is moving along its
orbit. The full moon will now be very near this line and therefore very
close to the plane of the earth's orbit, if not actually in it, and must
pass through the shadow of the earth and be eclipsed. So also the new
moon will cast its shadow in the plane of the ecliptic, and this shadow,
falling upon the earth, produced the total solar eclipse of May 28,
1900.
_N´ N´´_ is called the line of nodes of the moon's orbit (§ 39), and the
two positions of the earth in its orbit, diametrically opposite each
other, at which _N´ N´´_ points exactly toward the sun, we shall call
the _nodes_ of the lunar orbit. Strictly speaking, the nodes are those
points of the sky against which the moon's center is projected at the
moment when in its orbital motion it cuts through the plane of the
earth's orbit. Bearing in mind these definitions, we may condense much
of what precedes into the proposition: Eclipses of either sun or moon
can occur only when the earth is at or near one of the nodes of the
moon's orbit. Corresponding to these positions of the earth there are in
each year two seasons, about six months apart, at which times, and at
these only, eclipses can occur. Thus in the year 1900 the earth passed
these two points on June 2d and November 24th respectively, and the
following list of eclipses which occurred in that year shows that all of
them were within a few days of one or the other of these dates:
_Eclipses of the Year 1900_
Total solar eclipse May 28th.
Partial lunar eclipse June 12th.
Annular (solar) eclipse November 21st.
68. ECLIPSE LIMITS.--If the earth is exactly at the node at the time of
new moon, the moon's shadow will fall centrally upon it and will produce
an eclipse visible within the torrid zone, since this is that part of
the earth's surface nearest the plane of its orbit. If the earth is near
but not at the node, the new moon will stand a little north or south of
the plane of the earth's orbit, and its shadow will strike the earth
farther north or south than before, producing an eclipse in the
temperate or frigid zones; or the shadow may even pass entirely above or
below the earth, producing no eclipse whatever, or at most a partial
eclipse visible near the north or south pole. Just how many days' motion
the earth may be away from the node and still permit an eclipse is shown
in the following brief table of eclipse limits, as they are called:
_Solar Eclipse Limits_
If at any new moon the earth is
Less than 10 days away from a node, a central eclipse is certain.
Between 10 and 16 days " " " some kind of eclipse is certain.
Between 16 and 19 days " " " a partial eclipse is possible.
More than 19 days " " " no eclipse is possible.
_Lunar Eclipse Limits_
If at any full moon the earth is
Less than 4 days away from a node, a total eclipse is certain.
Between 4 and 10 days " " " some kind of eclipse is certain.
Between 10 and 14 days " " " a partial eclipse is possible.
More than 14 days " " " no eclipse is possible.
From this table of eclipse limits we may draw some interesting
conclusions about the frequency with which eclipses occur.
69. NUMBER OF ECLIPSES IN A YEAR.--Whenever the earth passes a node of
the moon's orbit a new moon must occur at some time during the 2 × 16
days that the earth remains inside the limits where some kind of eclipse
is certain, and there must therefore be an eclipse of the sun every time
the earth passes a node of the moon's orbit. But, since there are two
nodes past which the earth moves at least once in each year, there must
be at least two solar eclipses every year. Can there be more than two?
On the average, will central or partial eclipses be the more numerous?
A similar line of reasoning will not hold true for eclipses of the moon,
since it is quite possible that no full moon should occur during the 20
days required by the earth to move past the node from the western to the
eastern limit. This omission of a full moon while the earth is within
the eclipse limits sometimes happens at both nodes in the same year, and
then we have a year with no eclipse of the moon. The student may note in
the list of eclipses for 1900 that the partial lunar eclipse of June
12th occurred 10 days after the earth passed the node, and was therefore
within the doubtful zone where eclipses may occur and may fail, and
corresponding to this position the eclipse was a very small one, only a
thousandth part of the moon's diameter dipping into the shadow of the
earth. By so much the year 1900 escaped being an illustration of a year
in which no lunar eclipse occurred.
A partial eclipse of the moon will usually occur about a fortnight
before or after a total eclipse of the sun, since the full moon will
then be within the eclipse limit at the opposite node. A partial eclipse
of the sun will always occur about a fortnight before or after a total
eclipse of the moon.
[Illustration: FIG. 35.--The eclipse of May 28, 1900.]
70. ECLIPSE MAPS.--It is the custom of astronomers to prepare, in
advance of the more important eclipses, maps showing the trace of the
moon's shadow across the earth, and indicating the times of beginning
and ending of the eclipses, as is shown in Fig. 35. While the actual
construction of such a map requires much technical knowledge, the
principles involved are simple enough: the straight line passed through
the center of sun and moon is the axis of the shadow cone, and the map
contains little more than a graphical representation of when and where
this cone meets the surface of the earth. Thus in the map, the "Path of
Total Eclipse" is the trace of the shadow cone across the face of the
earth, and the width of this path shows that the earth encountered the
shadow considerably inside the vertex of the cone. The general direction
of the path is from west to east, and the slight sinuousities which it
presents are for the most part due to unavoidable distortion of the
map caused by the attempt to represent the curved surface of the earth
upon the flat surface of the paper. On either side of the Path of Total
Eclipse is the region within which the eclipse was only partial, and the
broken lines marked Begins at 3h., Ends at 3h., show the intersection of
the penumbral cone with the surface of the earth at 3 P. M., Greenwich
time. These two lines inclose every part of the earth's surface from
which at that time any eclipse whatever could be seen, and at this
moment the partial eclipse was just beginning at every point on the
eastern edge of the penumbra and just ending at every point on the
western edge, while at the center of the penumbra, on the Path of Total
Eclipse, lay the shadow of the moon, an oval patch whose greatest
diameter was but little more than 60 miles in length, and within which
lay every part of the earth where the eclipse was total at that moment.
The position of the penumbra at other hours is also shown on the map,
although with more distortion, because it then meets the surface of the
earth more obliquely, and from these lines it is easy to obtain the time
of beginning and end of the eclipse at any desired place, and to
estimate by the distance of the place from the Path of Total Eclipse how
much of the sun's face was obscured.
Let the student make these "predictions" for Washington, Chicago,
London, and Algiers.
The points in the map marked First Contact, Last Contact, show the
places at which the penumbral cone first touched the earth and finally
left it. According to computations made as a basis for the construction
of the map the Greenwich time of First Contact was 0h. 12.5m. and of
Last Contact 5h. 35.6m., and the difference between these two times
gives the total duration of the eclipse upon the earth--i. e., 5 hours
23.1 minutes.
[Illustration: FIG. 36.--Central eclipses for the first two decades of
the twentieth century. OPPOLZER.]
71. FUTURE ECLIPSES.--An eclipse map of a different kind is shown in
Fig. 36, which represents the shadow paths of all the central eclipses
of the sun, visible during the period 1900-1918 A. D., in those parts of
the earth north of the south temperate zone. Each continuous black line
shows the path of the shadow in a total eclipse, from its beginning, at
sunrise, at the western end of the line to its end, sunset, at the
eastern end, the little circle near the middle of the line showing the
place at which the eclipse was total at noon. The broken lines represent
similar data for the annular eclipses. This map is one of a series
prepared by the Austrian astronomer, Oppolzer, showing the path of every
such eclipse from the year 1200 B. C. to 2160 A. D., a period of more
than three thousand years.
If we examine the dates of the eclipses shown in this map we shall find
that they are not limited to the particular seasons, May and November,
in which those of the year 1900 occurred, but are scattered through all
the months of the year, from January to December. This shows at once
that the line of nodes, _N´ N´´_, of Fig. 34, does not remain in a fixed
position, but turns round in the plane of the earth's orbit so that in
different years the earth reaches the node in different months. The
precession has already furnished us an illustration of a similar change,
the slow rotation of the earth's axis, producing a corresponding
shifting of the line in which the planes of the equator and ecliptic
intersect; and in much the same way, through the disturbing influence of
the sun's attraction, the line _N´ N´´_ is made to revolve westward,
opposite to the arrowheads in Fig. 34, at the rate of nearly 20° per
year, so that the earth comes to each node about 19 days earlier in each
year than in the year preceding, and the eclipse season in each year
comes on the average about 19 days earlier than in the year before,
although there is a good deal of irregularity in the amount of change in
particular years.
72. RECURRENCE OF ECLIPSES.--Before the beginning of the Christian era
astronomers had found out a rough-and-ready method of predicting
eclipses, which is still of interest and value. The substance of the
method is that if we start with any eclipse whatever--e. g., the eclipse
of May 28, 1900--and reckon forward or backward from that date a period
of 18 years and 10 or 11 days, we shall find another eclipse quite
similar in its general characteristics to the one with which we started.
Thus, from the map of eclipses (Fig. 36), we find that a total solar
eclipse will occur on June 8, 1918, 18 years and 11 days after the one
illustrated in Fig. 35. This period of 18 years and 11 days is called
_saros_, an ancient word which means cycle or repetition, and since
every eclipse is repeated after the lapse of a saros, we may find the
dates of all the eclipses of 1918 by adding 11 days to the dates given
in the table of eclipses for 1900 (§ 67), and it is to be especially
noted that each eclipse of 1918 will be like its predecessor of 1900 in
character--lunar, solar, partial, total, etc. The eclipses of any year
may be predicted by a similar reference to those which occurred eighteen
years earlier. Consult a file of old almanacs.
The exact length of a saros is 223 lunar months, each of which is a
little more than 29.5 days long, and if we multiply the exact value of
this last number (see § 60) by 223, we shall find for the product
6,585.32 days, which is equal to 18 years 11.32 days when there are four
leap years included in the 18, or 18 years 10.32 days when the number of
leap years is five; and in applying the saros to the prediction of
eclipses, due heed must be paid to the number of intervening leap years.
To explain why eclipses are repeated at the end of the saros, we note
that the occurrence of an eclipse depends solely upon the relative
positions of the earth, moon, and node of the moon's orbit, and the
eclipse will be repeated as often as these three come back to the
position which first produced it. This happens at the end of every
saros, since the saros is, approximately, the least common multiple of
the length of the year, the length of the lunar month, and the length of
time required by the line of nodes to make a complete revolution around
the ecliptic. If the saros were exactly a multiple of these three
periods, every eclipse would be repeated over and over again for
thousands of years; but such is not the case, the saros is not an exact
multiple of a year, nor is it an exact multiple of the time required for
a revolution of the line of nodes, and in consequence the restitution
which comes at the end of the saros is not a perfect one. The earth at
the 223d new moon is in fact about half a day's motion farther west,
relative to the node, than it was at the beginning, and the resulting
eclipse, while very similar, is not precisely the same as before. After
another 18 years, at the second repetition, the earth is a day farther
from the node than at first, and the eclipse differs still more in
character, etc. This is shown in Fig. 37, which represents the apparent
positions of the disks of the sun and moon as seen from the center of
the earth at the end of each sixth saros, 108 years, where the upper row
of figures represents the number of repetitions of the eclipse from the
beginning, marked _0_, to the end, _72_. The solar eclipse limits, 10,
16, 19 days, are also shown, and all those eclipses which fall between
the 10-day limits will be central as seen from some part of the earth,
those between 16 and 19 partial wherever seen, while between 10 and 16
they may be either total or partial. Compare the figure with the
following description given by Professor Newcomb: "A series of such
eclipses commences with a very small eclipse near one pole of the earth.
Gradually increasing for about eleven recurrences, it will become
central near the same pole. Forty or more central eclipses will then
recur, the central line moving slowly toward the other pole. The series
will then become partial, and finally cease. The entire duration of the
series will be more than a thousand years. A new series commences, on
the average, at intervals of thirty years."
[Illustration: FIG. 37.--Graphical illustration of the saros.]
A similar figure may be constructed to represent the recurrence of lunar
eclipses; but here, in consequence of the smaller eclipse limits, we
shall find that a series is of shorter duration, a little over eight
centuries as compared with twelve centuries, which is the average
duration of a series of solar eclipses.
One further matter connected with the saros deserves attention. During
the period of 6,585.32 days the earth has 6,585 times turned toward the
sun the same face upon which the moon's shadow fell at the beginning of
the saros, but at the end of the saros the odd 0.32 of a day gives the
earth time to make about a third of a revolution more before the eclipse
is repeated, and in consequence the eclipse is seen in a different
region of the earth, on the average about 116° farther west in
longitude. Compare in Fig. 36 the regions in which the eclipses of 1900
and 1918 are visible.
Is this change in the region where the repeated eclipse is visible, true
of lunar eclipses as well as solar?
73. USE OF ECLIPSES.--At all times and among all peoples eclipses, and
particularly total eclipses of the sun, have been reckoned among the
most impressive phenomena of Nature. In early times and among
uncultivated people they were usually regarded with apprehension, often
amounting to a terror and frenzy, which civilized travelers have not
scrupled to use for their own purposes with the aid of the eclipse
predictions contained in their almanacs, threatening at the proper time
to destroy the sun or moon, and pointing to the advancing eclipse as
proof that their threats were not vain. In our own day and our own land
these feelings of awe have not quite disappeared, but for the most part
eclipses are now awaited with an interest and pleasure which, contrasted
with the former feelings of mankind, furnish one of the most striking
illustrations of the effect of scientific knowledge in transforming
human fear and misery into a sense of security and enjoyment.
But to the astronomer an eclipse is more than a beautiful illustration
of the working of natural laws; it is in varying degree an opportunity
of adding to his store of knowledge respecting the heavenly bodies. The
region immediately surrounding the sun is at most times closed to
research by the blinding glare of the sun's own light, so that a planet
as large as the moon might exist here unseen were it not for the
occasional opportunity presented by a total eclipse which shuts off the
excessive light and permits not only a search for unknown planets but
for anything and everything which may exist around the sun. More than
one astronomer has reported the discovery of such planets, and at least
one of these has found a name and a description in some of the books,
but at the present time most astronomers are very skeptical about the
existence of any such object of considerable size, although there is
some reason to believe that an enormous number of little bodies, ranging
in size from grains of sand upward, do move in this region, as yet
unseen and offering to the future problems for investigation.
But in other directions the study of this region at the times of total
eclipse has yielded far larger returns, and in the chapter on the sun we
shall have to consider the marvelous appearances presented by the solar
prominences and by the corona, an appendage of the sun which reaches out
from his surface for millions of miles but is never seen save at an
eclipse. Photographs of the corona are taken by astronomers at every
opportunity, and reproductions of some of these may be found in Chapter
X.
Annular eclipses and lunar eclipses are of comparatively little
consequence, but any recorded eclipse may become of value in connection
with chronology. We date our letters in a particular year of the
twentieth century, and commonly suppose that the years are reckoned from
the birth of Christ; but this is an error, for the eclipses which were
observed of old and by the chroniclers have been associated with events
of his life, when examined by the astronomers are found quite
inconsistent with astronomic theory. They are, however, reconciled with
it if we assume that our system of dates has its origin four years after
the birth of Christ, or, in other words, that Christ was born in the
year 4 B. C. A mistake was doubtless made at the time the Christian era
was introduced into chronology. At many other points the chance record
of an eclipse in the early annals of civilization furnishes a similar
means of controlling and correcting the dates assigned by the historian
to events long past.