ow 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 11/2 minutes of arc, an annular
eclipse may sometimes, in some part of its track, become almost or quite
total, and _vice versa_.

[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 o