GENERAL IDEAS.
The primary meaning of the word "Eclipse" ([Greek: ekleipsis]) is a
forsaking, quitting, or disappearance. Hence the covering over of
something by something else, or the immersion of something in something;
and these apparently crude definitions will be found on investigation to
represent precisely the facts of the case.
Inasmuch as the Earth and the Moon are for our present purpose
practically "solid bodies," each must cast a shadow into space as the
result of being illuminated by the Sun, regarded as a source of light.
What we shall eventually have to consider is: What results arise from
the existence of these shadows according to the circumstances under
which they are viewed? But before reaching this point, some other
preliminary considerations must be dealt with.
The various bodies which together make up the Solar system, that is to
say, in particular, those bodies called the "planets"--some of them
"primary," others "secondary" (_alias_ "Satellites" or "Moons")--are
constantly in motion. Consequently, if we imagine a line to be drawn
between any two at any given time, such a line will point in a different
direction at another time, and so it may occasionally happen that three
of these ever-moving bodies will come into one and the same straight
line. Now the consequences of this state of things were admirably well
pointed out nearly half a century ago by a popular writer, who in his
day greatly aided the development of science amongst the masses. "When
one of the extremes of the series of three bodies which thus assume a
common direction is the Sun, the intermediate body deprives the other
extreme body, either wholly or partially, of the illumination which it
habitually receives. When one of the extremes is the Earth, the
intermediate body intercepts, wholly or partially, the other extreme
body from the view of the observers situate at places on the Earth which
are in the common line of direction, and the intermediate body is seen
to pass over the other extreme body as it enters upon or leaves the
common line of direction. The phenomena resulting from such
contingencies of position and direction are variously denominated
_Eclipses_, _Transits_, and _Occultations_, according to the relative
apparent magnitudes of the interposing and obscured bodies, and
according to the circumstances which attend them."[1]
The Earth moves round the Sun once in every year; the Moon moves round
the Earth once in every lunar month (27 days). I hope everybody
understands those essential facts. Then we must note that the Earth
moves round the Sun in a certain plane (it is nothing for our present
purpose what that plane is). If the Moon as the Earth's companion moved
round the Earth in the same plane, an eclipse of the Sun would happen
regularly every month when the Moon was in "Conjunction" ("New Moon"),
and also every month at the intermediate period there would be a total
eclipse of the Moon on the occasion of every "Opposition" (or "Full
Moon"). But inasmuch as the Moon's orbit does not lie in quite the same
plane as the Earth's, but is inclined thereto at an angle which may be
taken to average about 5-1/8°, the actual facts are different; that is to
say, instead of there being in every year about 25 eclipses (solar and
lunar in nearly equal numbers), which there would be if the orbits had
identical planes, there are only a very few eclipses in the year, never,
under the most favourable circumstances, more than 7, and sometimes as
few as 2. Nor are the numbers equally apportioned. In years where there
are 7 eclipses, 5 of them _may_ be of the Sun and 2 of the Moon; where
there are only 2 eclipses, both _must_ be of the Sun. Under no
circumstances can there be in any one year more than 3 eclipses of the
Moon, and in some years there will be none. The reasons for these
diversities are of a technical character, and a full elucidation of them
would not be of interest to the general reader. It may here be added,
parenthetically, that the occasions will be very rare of there being 5
solar eclipses in one year. This last happened in 1823,[2] and will
only happen once again in the next two centuries, namely in 1935. If a
total eclipse of the Sun happens early in January there may be another
in December of the same year, as in 1889 (Jan. 1 and Dec. 22). This will
not happen again till 2057, when there will be total eclipses on Jan. 5
and Dec. 26. There is one very curious fact which may be here
conveniently stated as a bare fact, reserving the explanation of it for
a future page, namely, that eclipses of the Sun and Moon are linked
together in a certain chain or sequence which takes rather more than 18
years to run out when the sequence recurs and recurs _ad infinitum_. In
this 18-year period, which bears the name of the "Saros," there usually
happen 70 eclipses, of which 41 are of the Sun and 29 of the Moon.
Accordingly, eclipses of the Sun are more numerous than those of the
Moon in the proportion of about 3 to 2, yet at any given place on the
Earth more lunar eclipses are visible than solar eclipses, because the
former when they occur are visible over the whole hemisphere of the
Earth which is turned towards the Moon whilst the area over which a
total eclipse of the Sun is visible is but a belt of the Earth no more
than about 150 to 170 miles wide. Partial eclipses of the Sun, however,
are visible over a very much wider area on either side of the path
traversed by the Moon's shadow.
[Illustration: FIG. 2.--THEORY OF A TOTAL ECLIPSE OF THE SUN.]
Confining our attention in the first instance to eclipses of the Sun,
the diagrams fig. 2 and fig. 3 will make clear, with very little verbal
description, the essential features of the two principal kinds of
eclipses of the Sun. In these figures S represents the Sun, M the Moon
and E the Earth. They are not, of course, even approximately drawn to
scale either as to the size of the bodies or their relative distances,
but this is a matter of no moment as regards the principles involved. M
being in sunshine receives light on, as it were, the left hand side,
which faces S the Sun. The shadow of the Moon cast into space is, in the
particular case, thrown as regards its tip on to the Earth and is
intercepted by the Earth. Persons at the moment situated on the Earth
within the limits of this shadow will not see any part of the Sun at
all; they will see, in fact, nothing but the Moon as a black disc with
only such light behind and around it as may be reflected back on to the
sky by the illuminated (but to the Earth invisible) hemisphere of the
Moon, or as may proceed from the Sun's Corona (to be described
presently). The condition of things therefore is that known as a "total"
eclipse of the Sun so far as regards the inhabitants of the narrow strip
of Earth primarily affected.
[Illustration: FIG. 3.--THEORY OF AN ANNULAR ECLIPSE OF THE SUN.]
Fig. 3 represents nearly but not quite the same condition of things.
Here the Earth and the Moon are in those parts of their respective
orbits which put the two bodies at or near the maximum distance
possible from the Sun and from one another. The Moon casts its usual
shadow, but the tip does not actually reach any part of the Earth's
surface. Or, in other words, to an observer on the Earth the Moon is not
big enough to conceal the whole body of the Sun. The result is this; at
the instant of central coincidence the Moon covers up only the centre of
the Sun, leaving the outer edge all round uncovered.
This outer edge shows as a bright ring of light, and the eclipse is of
the sort known as an "annular" eclipse of the Sun.[3] As the greatest
breadth of the annulus can never exceed 1½ minutes of arc, an annular
eclipse may sometimes, in some part of its track, become almost or quite
total, and _vice versâ_.
[Illustration: FIG. 4.--ANNULAR ECLIPSE OF THE SUN.]
The idea will naturally suggest itself, what exactly does happen to the
inhabitants living outside (on the one side or the other) of the strip
of the Earth where the central line of shadow falls? This depends in
every case on circumstances, but it may be stated generally that the
inhabitants outside the central line but within 1000 to 2000 miles on
either side, will see a larger or smaller part of the Sun concealed by
the Moon's solid body, simultaneously with the total concealment of the
Sun to the favoured individuals who live, or who for the moment are
located, within the limits of the central zone.
[Illustration: FIG. 5.--PARTIAL ECLIPSE OF THE SUN.]
Now we must advance one stage in our conceptions of the movements of the
Earth and the Moon, so far as regards the bearing of those movements on
the question of eclipses. The Earth moves in a plane which is called the
"Plane of the Ecliptic," and correspondingly, the Sun has an _apparent_
annual motion in the same plane. The Moon moving in a different plane,
inclined to the first mentioned one to the extent of rather more than
5°, the Moon's orbit will evidently intersect the ecliptic in two
places. These places of intersection are called "Nodes," and the line
which may be imagined to join these Nodes is called the "Line of Nodes."
When the Moon is crossing the ecliptic from the S. to the N. side
thereof, the Moon is said to be passing through its "Ascending Node"
([Symbol: Ascending node]); the converse of this will be the Moon
passing back again from the N. side of the ecliptic to the S. side,
which is the "Descending Node" ([Symbol: Descending node]). Such changes
of position, with the terms designating them, apply not only to the Moon
in its movement round the Earth, but to all the planets and comets
circulating round the Sun; and also to satellites circulating round
certain of the planets, but with these matters we have no concern now.
FOOTNOTES:
[Footnote 1: D. Lardner, _Handbook of Astronomy_, 3rd ed., p. 288.]
[Footnote 2: But not one of them was visible at Greenwich.]
[Footnote 3: Latin _Annulus_, a ring.]