An eclipse of the sun is the simpler occurrence, so we shall describe
it first. It happens when the moon comes between the earth and the sun.
Look at our little astronomers shown in Fig. 34. A boy and a girl are
both gazing at the sun, when the moon comes between. To the boy the
moon appears to take a great bite out of the sun, so that it looks like
the left-hand picture in Fig. 35. (I have drawn a line from the end
of the telescope in Fig. 34, which shows how much of the sun is cut
off.) This would be called a partial eclipse of the sun. The almanac
will sometimes describe the eclipses as visible in London, or visible
at Greenwich; but that need not be taken so literally as was supposed
by a Kensington gentleman, who, on noticing that the almanac said an
eclipse was to be visible in London, called a cab and drove into the
city to look for it. His almanac had not mentioned that it would be
visible from his own house. You may usually take for granted that when
an eclipse is said to be visible from London or Greenwich, it will be
more or less visible all over England. Most of these eclipses are only
partial, and though they are interesting to watch they do not teach us
much. By far the most wonderful kind of eclipse is that in which the
whole of the bright part of the sun is blotted out. Then, indeed, we do
see wonders. But such eclipses are rare, and even when they do occur
they only last a very few minutes. The sights that are displayed are so
interesting that astronomers often travel thousands of miles to reach
a suitable locality for making observations.
The girl in Fig. 34 is placed in the best possible position for seeing
the eclipse. There you find her right in the line of the sun and moon;
and I think you will agree that she cannot see any part of the sun, for
the moon is altogether in the way. I have drawn two dotted lines, one
at each side. All that she can see beyond the moon must lie outside
these dotted lines, and she will be in the dark as long as the moon
stays in the way. When the eclipse is complete, comparative darkness
steals over the land. The birds are deceived, and fly home to the trees
to roost. The owls and the bats, thinking their time has arrived,
venture forth on their nocturnal business. Even flowers close their
petals, only to open a few minutes later when the sun again bursts
forth. Other flowers that give forth their fragrance at night are also
sweetly perceptible so long as the sun remains obscured. An unruly
cow, accustomed to break into a meadow at night, was found there after
an eclipse was over; while I learn from the same authority that a man
rushed over in great excitement to see what his chickens were doing,
but came back much disappointed on finding them pecking away as if
nothing had happened.
It will sometimes happen that the moon is so placed that the edge of
the sun can be seen all round it. A case of the kind is shown in the
right-hand picture of Fig. 35. It is called an annular, or ring-shaped
eclipse.
The eclipses of which we have been speaking are, of course, only to be
seen during the day when the sun must be up. The lunar eclipses, which
are visible at night, are due to the interposition of the earth between
the sun and the moon. The sun is at night-time under our feet at the
other side of the earth, and the earth throws a long shadow upwards.
If the moon enter into this shadow, it is plain that the sunlight is
partly or wholly cut off, and since the moon shines by no light of her
own, but only by light borrowed from the sun, it follows that when
she is buried in the shadow all the direct light is intercepted, and
she must lose her brilliancy. Thus we obtain what is called a lunar
eclipse. It is total if the moon be entirely in the shadow. The eclipse
is partial if the moon be only partly in the shadow. The lunar eclipse
is visible to everybody on the dark hemisphere of the earth if the
clouds will keep out of the way, so that usually a great many more
people can see a lunar eclipse than a solar eclipse, which is only
visible from a limited part of the earth. It thus happens that the
lunar eclipse is the more familiar spectacle of the two.
When the moon is entirely in the shadow, one might naturally think
that it would become totally invisible. This is not always the case.
It is a curious fact that in the depth of a total eclipse the moon is
often still visible, for she glows with a copper-colored light, which
is bright enough to render some of the chief marks on her surface
distinctly discernible.