h as to the nature of the sun, and to which we have already referred
in the last chapter. Even when the moon is placed centrally over the
sun, a thin rim of sunlight is occasionally seen round the margin of the
moon. We then have what is known as an annular eclipse.

It is remarkable that the moon is sometimes able to hide the sun
completely, while on other occasions it fails to do so. It happens that
the average apparent size of the moon is nearly equal to the average
apparent size of the sun, but, owing to the fluctuations in their
distances, the actual apparent sizes of both bodies undergo certain
changes. On certain occasions the apparent size of the moon is greater
than that of the sun. In this case a central passage produces a total
eclipse; but it may also happen that the apparent size of the sun
exceeds that of the moon, in which case a central passage can only
produce an annular eclipse.

[Illustration: Fig. 26.--Form of the Earth's Shadow, showing the
Penumbra, or partially shaded region. Within the Penumbra, the Moon is
visible; in the Shadow it is nearly invisible.]

There are hardly any more interesting celestial phenomena than the
different descriptions of eclipses. The almanac will always give timely
notice of the occurrence, and the more striking features can be observed
without a telescope. In an eclipse of the moon (Fig. 26) it is
interesting to note the moment when the black shadow is first detected,
to watch its gradual encroachment over the bright surface of the moon,
to follow it, in case the eclipse is total, until there is only a thin
crescent of moonlight left, and to watch the final extinction of that
crescent when the whole moon is plunged into the shadow. But now a
spectacle of great interest and beauty is often manifested; for though
the moon is so hidden behind the earth that not a single direct ray of
the sunlight could reach its surface, yet we often find that the moon
remains visible, and, indeed, actually glows with a copper-coloured hue
bright enough to permit several of the markings on the surface to be
discerned.

This illumination of the moon even in the depth of a total eclipse is
due to the sunbeams which have just grazed the edge of the earth. In
doing so they have become bent by the refraction of the atmosphere, and
have thus been turned inwards into the shadow. Such beams have passed
through a prodigious thickness of the earth's atmosphere, and in this
long journey through