When the earth is between the moon and the sun in a line, the moon lies
in the shadow of the earth, and so suffers temporary obscuration; a
_lunar eclipse_ then takes place. When the moon passes between the earth
and the sun, the latter is at certain places on the earth obscured by
the dark body of the moon, and a _solar eclipse_ takes place.
LUNAR ECLIPSES. The shadow cast by the earth is conical, and may be
shown to extend about one million miles from its surface. At a distance
of a quarter of a million miles away the width of this shadow is about
six thousand miles; and if the moon passes into it at that approximate
distance from the earth, its disc of two thousand miles diameter may be
partially or totally obscured. The moon and sun may be on opposite sides
of the earth, and yet the former not in shadow. This is due to the fact
that the moon’s orbit round the earth is not exactly in the same plane
as that of the earth’s orbit round the sun. If it were so, we should
have total eclipses at every full moon; but since the two planes are
inclined to each other at an angle of 5° 9′, eclipses will occur when
the moon is at or near its _nodes_ or positions of coincidence with the
plane of the ecliptic. Partial eclipses are produced when only a portion
of the moon passes into shadow; annular eclipses such as are sometimes
observed in the case of the sun cannot occur with the moon.
[Illustration: =GIANT SHADOWS CAST BY THE EARTH AND MOON=
=HOW THE MOON THROWS ITS SHADOW ON THE EARTH, SHUTTING OFF THE LIGHT OF
THE SUN=]
[Illustration: =HOW THE MOON COMES BETWEEN THE EARTH AND SUN, CAUSING
THE SHADOW SHOWN ABOVE=]
[Illustration: =HOW THE EARTH THROWS ITS SHADOW ACROSS THE MOON=
On its way through space the moon passes sometimes between the sun and
the earth, shutting off the sunlight from the earth, as shown in the top
picture. The drawing in the middle shows us that the moon does not hide
the sunlight from the whole of the earth, but only from a part of it.
But in the part from which the sun is hid the moon’s shadow makes day so
dark that we can see the stars. We call this an eclipse of the sun.
Sometimes, too, the earth passes between the moon and the sun so as to
cut off all sunlight from the moon, as shown in the bottom picture. We
call this an eclipse of the moon.]
SOLAR ECLIPSES. The shadow cast by the moon is also conical, and extends
over a slightly varying distance of about a quarter of a million miles
from the moon’s surface. This being the approximate distance of the moon
from the earth, it is seen that when the moon is between the earth and
the sun the shadow may reach the earth. The extreme limit of the shadow
may range from twenty-three thousand miles short of the earth, in which
case an entire eclipse of the sun is impossible, to fifteen thousand
miles beyond the earth. In the latter case a circular shadow will be
projected on the surface of the globe, travelling onwards slowly in the
direction of the motion of the moon. Within this shadow or _umbra_ the
body of the sun cannot be observed, and a total eclipse prevails. A
circular region exists round this shadow, in which only part of the sun
is visible; this region is therefore partly in shadow, and is called the
_penumbra_. Outside the penumbra the whole sun may be viewed; the moon’s
shadow is not nearly large enough to render a solar eclipse co-existent
over all parts of the earth’s face towards the sun.