ECLIPSES AND KINDRED PHENOMENA
Since some members of the solar system are nearer to us than others, and
all are again much nearer than any of the stars, it must often happen
that one celestial body will pass between us and another, and thus
intercept its light for a while. The moon, being the nearest object in
the universe, will, of course, during its motion across the sky,
temporarily blot out every one of the others which happen to lie in its
path. When it passes in this manner across the face of the sun, it is
said to _eclipse_ it. When it thus hides a planet or star, it is said to
_occult_ it. The reason why a separate term is used for what is merely a
case of obscuring light in exactly the same way, will be plain when one
considers that the disc of the sun is almost of the same apparent size
as that of the moon, and so the complete hiding of the sun can last but
a few minutes at the most; whereas a planet or a star looks so very
small in comparison, that it is always _entirely swallowed up for some
time_ when it passes behind the body of our satellite.
The sun, of course, occults planets and stars in exactly the same manner
as the moon does, but we cannot see these occultations on account of the
blaze of sunlight.
By reason of the small size which the planets look when viewed with the
naked eye, we are not able to note them in the act of passing over stars
and so blotting them out; but such occurrences may be seen in the
telescope, for the planetary bodies then display broad discs.
There is yet another occurrence of the same class which is known as a
_transit_. This takes place when an apparently small body passes across
the face of an apparently large one, the phenomenon being in fact the
exact reverse of an occultation. As there is no appreciable body nearer
to us than the moon, we can never see anything in transit across her
disc. But since the planets Venus and Mercury are both nearer to us than
the sun, they will occasionally be seen to pass across his face, and
thus we get the well-known phenomena called Transits of Venus and
Transits of Mercury.
As the satellites of Jupiter are continually revolving around him, they
will often pass behind or across his disc. Such occultations and
transits of satellites can be well observed in the telescope.
There is, however, a way in which the light of a celestial body may be
obscured without the necessity of its being hidden from us by one
nearer. It will no doubt be granted that any opaque object casts a
shadow when a strong light falls directly upon it. Thus the earth, under
the powerful light which is directed upon it from the sun, casts an
extensive shadow, though we are not aware of the existence of this
shadow until it falls upon something. The shadow which the earth casts
is indeed not noticeable to us until some celestial body passes into it.
As the sun is very large, and the earth in comparison very small, the
shadow thrown by the earth is comparatively short, and reaches out in
space for only about a million miles. There is no visible object except
the moon, which circulates within that distance from our globe, and
therefore she is the only body which can pass into this shadow. Whenever
such a thing happens, her surface at once becomes dark, for the reason
that she never emits any light of her own, but merely reflects that of
the sun. As the moon is continually revolving around the earth, one
would be inclined to imagine that once in every month, namely at what is
called _full moon_, when she is on the other side of the earth with
respect to the sun, she ought to pass through the shadow in question.
But this does not occur every time, because the moon's orbit is not
quite _upon the same plane_ with the earth's. It thus happens that time
after time the moon passes clear of the earth's shadow, sometimes above
it, and sometimes below it. It is indeed only at intervals of about six
months that the moon can be thus obscured. This darkening of her light
is known as an _eclipse of the moon_. It seems a great pity that custom
should oblige us to employ the one term "eclipse" for this and also for
the quite different occurrence, an eclipse of the sun; in which the
sun's face is hidden as a consequence of the moon's body coming directly
_between_ it and our eyes.
The popular mind seems always to have found it more difficult to grasp
the causes of an eclipse of the moon than an eclipse of the sun. As Mr.
J.E. Gore[4] puts it: "The darkening of the sun's light by the
interposition of the moon's body seems more obvious than the passing of
the moon through the earth's shadow."
Eclipses of the moon furnish striking spectacles, but really add little
to our knowledge. They exhibit, however, one of the most remarkable
evidences of the globular shape of our earth; for the outline of its
shadow when seen creeping over the moon's surface is always circular.
[Illustration: FIG. 3.--Total and Partial Eclipses of the Moon. The Moon
is here shown in two positions; i.e. _entirely_ plunged in the earth's
shadow and therefore totally eclipsed, and only _partly_ plunged in it
or partially eclipsed.]
_Eclipses of the Moon_, or Lunar Eclipses, as they are also called, are
of two kinds--_Total_, and _Partial_. In a total lunar eclipse the moon
passes entirely into the earth's shadow, and the whole of her surface is
consequently darkened. This darkening lasts for about two hours. In a
partial lunar eclipse, a portion only of the moon passes through the
shadow, and so only _part_ of her surface is darkened (see Fig. 3). A
very striking phenomenon during a total eclipse of the moon, is that the
darkening of the lunar surface is usually by no means so intense as one
would expect, when one considers that the sunlight at that time should
be _wholly_ cut off from it. The occasions indeed upon which the moon
has completely disappeared from view during the progress of a total
lunar eclipse are very rare. On the majority of these occasions she has
appeared of a coppery-red colour, while sometimes she has assumed an
ashen hue. The explanations of these variations of colour is to be found
in the then state of the atmosphere which surrounds our earth. When
those portions of our earth's atmosphere through which the sun's rays
have to filter on their way towards the moon are free from watery
vapour, the lunar surface will be tinged with a reddish light, such as
we ordinarily experience at sunset when our air is dry. The ashen colour
is the result of our atmosphere being laden with watery vapour, and is
similar to what we see at sunset when rain is about. Lastly, when the
air around the earth is thickly charged with cloud, no light at all can
pass; and on such occasions the moon disappears altogether for the time
being from the night sky.
_Eclipses of the Sun_, otherwise known as Solar Eclipses, are divided
into _Total_, _Partial_, and _Annular_. A total eclipse of the sun takes
place when the moon comes between the sun and the earth, in such a
manner that it cuts off the sunlight _entirely_ for the time being from
a _portion_ of the earth's surface. A person situated in the region in
question will, therefore, at that moment find the sun temporarily
blotted out from his view by the body of the moon. Since the moon is a
very much smaller body than the sun, and also very much the nearer to us
of the two, it will readily be understood that the portion of the earth
from which the sun is seen thus totally eclipsed will be of small
extent. In places not very distant from this region, the moon will
appear so much shifted in the sky that the sun will be seen only
partially eclipsed. The moon being in constant movement round the earth,
the portion of the earth's surface from which an eclipse is seen as
total will be always a comparatively narrow band lying roughly from west
to east. This band, known as the _track of totality_, can, at the
utmost, never be more than about 165 miles in width, and as a rule is
very much less. For about 2000 miles on either side of it the sun is
seen partially eclipsed. Outside these limits no eclipse of any kind is
visible, as from such regions the moon is not seen to come in the way of
the sun (see Fig. 4 (i.), p. 67).
It may occur to the reader that eclipses can also take place in the
course of which the positions, where the eclipse would ordinarily be
seen as total, will lie outside the surface of the earth. Such an
eclipse is thus not dignified with the name of total eclipse, but is
called a partial eclipse, because from the earth's surface the sun is
only seen _partly eclipsed at the utmost_ (see Fig. 4 (ii.), p. 67).
[Illustration: (i.) Total Eclipse of the Sun.]
[Illustration: (ii.) Partial Eclipse of the Sun.
FIG. 4.--Total and Partial Eclipses of the Sun. From the position A the
Sun cannot be seen, as it is entirely blotted out by the Moon. From B it
is seen partially blotted out, because the Moon is to a certain degree
in the way. From C no eclipse is seen, because the Moon does not come in
the way.
It is to be noted that in a Partial Eclipse of the Sun, the position A
lies _outside_ the surface of the Earth.]
An _Annular eclipse_ is an eclipse which just fails to become total for
yet another reason. We have pointed out that the orbits of the various
members of the solar system are not circular, but oval. Such oval
figures, it will be remembered, are technically known as ellipses. In an
elliptic orbit the controlling body is situated not in the middle of the
figure, but rather towards one of the ends; the actual point which it
occupies being known as the _focus_. The sun being at the focus of the
earth's orbit, it follows that the earth is, at times, a little nearer
to him than at others. The sun will therefore appear to us to vary a
little in size, looking sometimes slightly larger than at other times.
It is so, too, with the moon, at the focus of whose orbit the earth is
situated. She therefore also appears to us at times to vary slightly in
size. The result is that when the sun is eclipsed by the moon, and the
moon at the time appears the larger of the two, she is able to blot out
the sun completely, and so we can get a total eclipse. But when, on the
other hand, the sun appears the larger, the eclipse will not be quite
total, for a portion of the sun's disc will be seen protruding all
around the moon like a ring of light. This is what is known as an
annular eclipse, from the Latin word _annulus_, which means a ring. The
term is consecrated by long usage, but it seems an unfortunate one on
account of its similarity to the word "annual." The Germans speak of
this kind of eclipse as "ring-formed," which is certainly much more to
the point.
There can never be a year without an eclipse of the sun. Indeed there
must be always two such eclipses _at least_ during that period, though
there need be no eclipse of the moon at all. On the other hand, the
greatest number of eclipses which can ever take place during a year are
seven; that is to say, either five solar eclipses and two lunar, or four
solar and three lunar. This general statement refers merely to eclipses
in their broadest significance, and informs us in no way whether they
will be total or partial.
Of all the phenomena which arise from the hiding of any celestial body
by one nearer coming in the way, a total eclipse of the sun is far the
most important. It is, indeed, interesting to consider how much poorer
modern astronomy would be but for the extraordinary coincidence which
makes a total solar eclipse just possible. The sun is about 400 times
farther off from us than the moon, and enormously greater than her in
bulk. Yet the two are relatively so distanced from us as to look about
the same size. The result of this is that the moon, as has been seen,
can often blot out the sun entirely from our view for a short time. When
this takes place the great blaze of sunlight which ordinarily dazzles
our eyes is completely cut off, and we are thus enabled, unimpeded, to
note what is going on in the immediate vicinity of the sun itself.
In a total solar eclipse, the time which elapses from the moment when
the moon's disc first begins to impinge upon that of the sun at his
western edge until the eclipse becomes total, lasts about an hour.
During all this time the black lunar disc may be watched making its way
steadily across the solar face. Notwithstanding the gradual obscuration
of the sun, one does not notice much diminution of light until about
three-quarters of his disc are covered. Then a wan, unearthly appearance
begins to pervade all things, the temperature falls noticeably, and
nature seems to halt in expectation of the coming of something unusual.
The decreasing portion of sun becomes more and more narrow, until at
length it is reduced to a crescent-shaped strip of exceeding fineness.
Strange, ill-defined, flickering shadows (known as "Shadow Bands") may
at this moment be seen chasing each other across any white expanse such
as a wall, a building, or a sheet stretched upon the ground. The western
side of the sky has now assumed an appearance dark and lowering, as if a
rainstorm of great violence were approaching. This is caused by the
mighty mass of the lunar shadow sweeping rapidly along. It flies onward
at the terrific velocity of about half a mile a second.
If the gradually diminishing crescent of sun be now watched through a
telescope, the observer will notice that it does not eventually vanish
all at once, as he might have expected. Rather, it breaks up first of
all along its length into a series of brilliant dots, known as "Baily's
Beads." The reason of this phenomenon is perhaps not entirely agreed
upon, but the majority of astronomers incline to the opinion that the
so-called "beads" are merely the last remnants of sunlight peeping
between those lunar mountain peaks which happen at the moment to fringe
the advancing edge of the moon. The beads are no sooner formed than they
rapidly disappear one after the other, after which no portion of the
solar surface is left to view, and the eclipse is now total (see Fig.
5).
[Illustration: _In a total Eclipse_ _In an annular Eclipse_
FIG. 5.--"Baily's Beads."]
But with the disappearance of the sun there springs into view a new and
strange appearance, ordinarily unseen because of the blaze of sunlight.
It is a kind of aureole, or halo, pearly white in colour, which is seen
to surround the black disc of the moon. This white radiance is none
other than the celebrated phenomenon widely known as the _Solar Corona_.
It was once upon a time thought to belong to the moon, and to be perhaps
a lunar atmosphere illuminated by the sunlight shining through it from
behind. But the suddenness with which the moon always blots out stars
when occulting them, has amply proved that she possesses no atmosphere
worth speaking about. It is now, however, satisfactorily determined that
the corona belongs to the sun, for during the short time that it remains
in view the black body of the moon can be seen creeping across it.
All the time that the _total phase_ (as it is called) lasts, the corona
glows with its pale unearthly light, shedding upon the earth's surface
an illumination somewhat akin to full moonlight. Usually the planet
Venus and a few stars shine out the while in the darkened heaven.
Meantime around the observer animal and plant life behave as at
nightfall. Birds go to roost, bats fly out, worms come to the surface of
the ground, flowers close up. In the Norwegian eclipse of 1896 fish were
seen rising to the surface of the water. When the total phase at length
is over, and the moon in her progress across the sky has allowed the
brilliant disc of the sun to spring into view once more at the other
side, the corona disappears.
There is another famous accompaniment of the sun which partly reveals
itself during total solar eclipses. This is a layer of red flame which
closely envelops the body of the sun and lies between it and the corona.
This layer is known by the name of the _Chromosphere_. Just as at
ordinary times we cannot see the corona on account of the blaze of
sunlight, so are we likewise unable to see the chromosphere because of
the dazzling white light which shines through from the body of the sun
underneath and completely overpowers it. When, however, during a solar
eclipse, the lunar disc has entirely hidden the brilliant face of the
sun, we are still able for a few moments to see an edgewise portion of
the chromosphere in the form of a narrow red strip, fringing the
advancing border of the moon. Later on, just before the moon begins to
uncover the face of the sun from the other side, we may again get a view
of a strip of chromosphere.
The outer surface of the chromosphere is not by any means even. It is
rough and billowy, like the surface of a storm-tossed sea. Portions of
it, indeed, rise at times to such heights that they may be seen standing
out like blood-red points around the black disc of the moon, and remain
thus during a good part of the total phase. These projections are known
as the _Solar Prominences_. In the same way as the corona, the
chromosphere and prominences were for a time supposed to belong to the
moon. This, however, was soon found not to be the case, for the lunar
disc was noticed to creep slowly across them also.
The total phase, or "totality," as it is also called, lasts for
different lengths of time in different eclipses. It is usually of about
two or three minutes' duration, and at the utmost it can never last
longer than about eight minutes.
When totality is over and the corona has faded away, the moon's disc
creeps little by little from the face of the sun, light and heat returns
once more to the earth, and nature recovers gradually from the gloom in
which she has been plunged. About an hour after totality, the last
remnant of moon draws away from the solar disc, and the eclipse is
entirely at an end.
The corona, the chromosphere, and the prominences are the most important
of these accompaniments of the sun which a total eclipse reveals to us.
Our further consideration of them must, however, be reserved for a
subsequent chapter, in which the sun will be treated of at length.
Every one who has had the good fortune to see a total eclipse of the sun
will, the writer feels sure, agree with the verdict of Sir Norman
Lockyer that it is at once one of the "grandest and most awe-inspiring
sights" which man can witness. Needless to say, such an occurrence used
to cause great consternation in less civilised ages; and that it has not
in modern times quite parted with its terrors for some persons, is shown
by the fact that in Iowa, in the United States, a woman died from fright
during the eclipse of 1869.
To the serious observer of a total solar eclipse every instant is
extremely precious. Many distinct observations have to be crowded into a
time all too limited, and this in an eclipse-party necessitates constant
rehearsals in order that not a moment may be wasted when the longed-for
totality arrives. Such preparation is very necessary; for the rarity and
uncommon nature of a total eclipse of the sun, coupled with its
exceeding short duration, tends to flurry the mind, and to render it
slow to seize upon salient points of detail. And, even after every
precaution has been taken, weather possibilities remain to be reckoned
with, so that success is rather a lottery.
Above all things, therefore, a total solar eclipse is an occurrence for
the proper utilisation of which personal experience is of absolute
necessity. It was manifestly out of the question that such experience
could be gained by any individual in early times, as the imperfection
of astronomical theory and geographical knowledge rendered the
predicting of the exact position of the track of totality well-nigh
impossible. Thus chance alone would have enabled one in those days to
witness a total phase, and the probabilities, of course, were much
against a second such experience in the span of a life-time. And even in
more modern times, when the celestial motions had come to be better
understood, the difficulties of foreign travel still were in the way;
for it is, indeed, a notable fact that during many years following the
invention of the telescope the tracks were placed for the most part in
far-off regions of the earth, and Europe was visited by singularly few
total solar eclipses. Thus it came to pass that the building up of a
body of organised knowledge upon this subject was greatly delayed.
Nothing perhaps better shows the soundness of modern astronomical theory
than the almost exact agreement of the time predicted for an eclipse
with its actual occurrence. Similarly, by calculating backwards,
astronomers have discovered the times and seasons at which many ancient
eclipses took place, and valuable opportunities have thus arisen for
checking certain disputed dates in history.
It should not be omitted here that the ancients were actually able, _in
a rough way_, to predict eclipses. The Chaldean astronomers had indeed
noticed very early a curious circumstance, _i.e._ that eclipses tend to
repeat themselves after a lapse of slightly more than eighteen years.
In this connection it must, however, be pointed out, in the first
instance, that the eclipses which occur in any particular year are in
no way associated with those which occurred in the previous year. In
other words, the mere fact that an eclipse takes place upon a certain
day this year will not bring about a repetition of it at the same time
next year. However, the nicely balanced behaviour of the solar system,
an equilibrium resulting from æons of orbital ebb and flow, naturally
tends to make the members which compose that family repeat their ancient
combinations again and again; so that after definite lapses of time the
same order of things will _almost exactly_ recur. Thus, as a consequence
of their beautifully poised motions, the sun, the moon, and the earth
tend, after a period of 18 years and 10-1/3 days,[5] to occupy very
nearly the same positions with regard to each other. The result of this
is that, during each recurring period, the eclipses comprised within it
will be repeated in their order.
To give examples:--
The total solar eclipse of August 30, 1905, was a repetition of that of
August 19, 1887.
The partial solar eclipse of February 23, 1906, corresponded to that
which took place on February 11, 1888.
The annular eclipse of July 10, 1907, was a recurrence of that of June
28, 1889.
In this way we can go on until the eighteen year cycle has run out, and
we come upon a total solar eclipse predicted for September 10, 1923,
which will repeat the above-mentioned ones of 1905 and 1887; and so on
too with the others.
From mere observation alone, extending no doubt over many ages, those
time-honoured watchers of the sky, the early Chaldeans, had arrived at
this remarkable generalisation; and they used it for the rough
prediction of eclipses. To the period of recurrence they give the name
of "Saros."
And here we find ourselves led into one of the most interesting and
fascinating by-paths in astronomy, to which writers, as a rule, pay all
too little heed.
In order not to complicate matters unduly, the recurrence of solar
eclipses alone will first be dealt with. This limitation will, however,
not affect the arguments in the slightest, and it will be all the more
easy in consequence to show their application to the case of eclipses of
the moon.
The reader will perhaps have noticed that, with regard to the repetition
of an eclipse, it has been stated that the conditions which bring it on
at each recurrence are reproduced _almost exactly_. Here, then, lies the
_crux_ of the situation. For it is quite evident that were the
conditions _exactly_ reproduced, the recurrences of each eclipse would
go on for an indefinite period. For instance, if the lapse of a saros
period found the sun, moon, and earth again in the precise relative
situations which they had previously occupied, the recurrences of a
solar eclipse would tend to duplicate its forerunner with regard to the
position of the shadow upon the terrestrial surface. But the conditions
_not_ being exactly reproduced, the shadow-track does not pass across
the earth in quite the same regions. It is shifted a little, so to
speak; and each time the eclipse comes round it is found to be shifted a
little farther. Every solar eclipse has therefore a definite "life" of
its own upon the earth, lasting about 1150 years, or 64 saros returns,
and working its way little by little across our globe from north to
south, or from south to north, as the case may be. Let us take an
imaginary example. A _partial_ eclipse occurs, say, somewhere near the
North Pole, the edge of the "partial" shadow just grazing the earth, and
the "track of totality" being as yet cast into space. Here we have the
beginning of a series. At each saros recurrence the partial shadow
encroaches upon a greater extent of earth-surface. At length, in its
turn, the track of totality begins to impinge upon the earth. This track
streaks across our globe at each return of the eclipse, repeating itself
every time in a slightly more southerly latitude. South and south it
moves, passing in turn the Tropic of Cancer, the Equator, the Tropic of
Capricorn, until it reaches the South Pole; after which it touches the
earth no longer, but is cast into space. The rear portion of the partial
shadow, in its turn, grows less and less in extent; and it too in time
finally passes off. Our imaginary eclipse series is now no more--its
"life" has ended.
We have taken, as an example, an eclipse series moving from north to
south. We might have taken one moving from south to north, for they
progress in either direction.
From the description just given the reader might suppose that, if the
tracks of totality of an eclipse series were plotted upon a chart of the
world, they would lie one beneath another like a set of steps. This is,
however, _not_ the case, and the reason is easily found. It depends upon
the fact that the saros does not comprise an exact number of days, but
includes, as we have seen, one-third of a day in addition.
It will be granted, of course, that if the number of days was exact, the
_same_ parts of the earth would always be brought round by the axial
rotation _to front the sun_ at the moment of the recurrence of the
eclipse. But as there is still one-third of a day to complete the saros
period, the earth has yet to make one-third of a rotation upon its axis
before the eclipse takes place. Thus at every recurrence the track of
totality finds itself placed one-third of the earth's circumference to
the _westward_. Three of the recurrences will, of course, complete the
circuit of the globe; and so the fourth recurrence will duplicate the
one which preceded it, three saros returns, or 54 years and 1 month
before. This duplication, as we have already seen, will, however, be
situated in a latitude to the south or north of its predecessor,
according as the eclipse series is progressing in a southerly or
northerly direction.
Lastly, every eclipse series, after working its way across the earth,
will return again to go through the same process after some 12,000
years; so that, at the end of that great lapse of time, the entire
"life" of every eclipse should repeat itself, provided that the
conditions of the solar system have not altered appreciably during the
interval.
We are now in a position to consider this gradual southerly or
northerly progress of eclipse recurrences in its application to the case
of eclipses of the moon. It should be evident that, just as in solar
eclipses the lunar shadow is lowered or raised (as the case may be) each
time it strikes the terrestrial surface, so in lunar eclipses will the
body of the moon shift its place at each recurrence relatively to the
position of the earth's shadow. Every lunar eclipse, therefore, will
commence on our satellite's disc as a partial eclipse at the northern or
southern extremity, as the case may be. Let us take, as an example, an
imaginary series of eclipses of the moon progressing from north to
south. At each recurrence the partial phase will grow greater, its
boundary encroaching more and more to the southward, until eventually
the whole disc is enveloped by the shadow, and the eclipse becomes
total. It will then repeat itself as total during a number of
recurrences, until the entire breadth of the shadow has been passed
through, and the northern edge of the moon at length springs out into
sunlight. This illuminated portion will grow more and more extensive at
each succeeding return, the edge of the shadow appearing to recede from
it until it finally passes off at the south. Similarly, when a lunar
eclipse commences as partial at the south of the moon, the edge of the
shadow at each subsequent recurrence finds itself more and more to the
northward. In due course the total phase will supervene, and will
persist during a number of recurrences until the southerly trend of the
moon results in the uncovering of the lunar surface at the south. Thus,
as the boundary of the shadow is left more and more to the northward,
the illuminated portion on the southern side of the moon becomes at each
recurrence greater and the darkened portion on the northern side less,
until the shadow eventually passes off at the north.
The "life" of an eclipse of the moon happens, for certain reasons, to be
much shorter than that of an eclipse of the sun. It lasts during only
about 860 years, or 48 saros returns.
Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing a
portion of the march of the total solar eclipse of August 30, 1905,
across the surface of the earth. The projection in question has been
employed because it is the one with which people are most familiar. This
eclipse began by striking the neighbourhood of the North Pole in the
guise of a partial eclipse during the latter part of the reign of Queen
Elizabeth, and became total on the earth for the first time on the 24th
of June 1797. Its next appearance was on the 6th of July 1815. It has
not been possible to show the tracks of totality of these two early
visitations on account of the distortion of the polar regions consequent
on the _fiction_ of Mercator's Projection. It is therefore made to
commence with the track of its third appearance, viz. on July 17, 1833.
In consequence of those variations in the apparent sizes of the sun and
moon, which result, as we have seen, from the variations in their
distances from the earth, this eclipse will change from a total into an
annular eclipse towards the end of the twenty-first century. By that
time the track will have passed to the southern side of the equator. The
track will eventually leave the earth near the South Pole about the
beginning of the twenty-sixth century, and the rear portion of the
partial shadow will in its turn be clear of the terrestrial surface by
about 2700 A.D., when the series comes to an end.
[Illustration: FIG. 6.--Map of the World on Mercator's Projection,
showing a portion of the progress of the Total Solar Eclipse of August
30, 1905, across the surface of the earth.]
[4] Astronomical Essays (p. 40), London, 1907.
[5] In some cases the periods between the dates of the corresponding
eclipses _appear_ to include a greater number of days than ten; but this
is easily explained when allowance is made for intervening _leap_ years
(in each of which an _extra_ day has of course been added), and also for
variations in local time.