y by comparison, for in reality
the dark parts of the spots are brighter than electric light.

[Illustration: _Royal Observatory, Greenwich._

SUN-SPOTS.]

The fact that the spots are in reality depressions or holes is shown by
their change of appearance as they pass over the face of the sun toward
the edge; for the change of shape is exactly that which would be caused
by foreshortening.

It sounds odd to say that the best time for observing the sun is during
a total eclipse, for then the sun's body is hidden by the moon. But yet
to a certain extent this is true, and the reason is that the sun's own
brilliance is our greatest hindrance in observing him, his rays are so
dazzling that they light up our own atmosphere, which prevents us seeing
the edges. Now, during a total eclipse, when nearly all the rays are
cut off, we can see marvellous things, which are invisible at other
times. But total eclipses are few and far between, and so when one is
approaching astronomers make great preparations beforehand.

A total eclipse is not visible from all parts of the world, but only
from that small part on which the shadow of the moon falls, and as the
earth travels, this shadow, which is really a round spot, passes along,
making a dark band. In this band astronomers choose the best
observatories, and there they take up their stations. The dark body of
the moon first appears to cut a little piece out of the side of the sun,
and as it sails on, gradually blotting out more and more, eager
telescopes follow it; at last it covers up the whole sun, and then a
marvellous spectacle appears, for all round the edges of the black moon
are seen glorious red streamers and arches and filaments of marvellous
shapes, continually changing. These are thrown against a background of
pale green light that surrounds the black moon and the hidden sun. In
early days astronomers thought these wonderful coloured streamers
belonged to the moon; but it was soon proved that they really are part
of the sun, and are only invisible at ordinary times, because our
atmosphere is too bright to allow them to be seen. An instrument has
now been invented to cut off most of the light of the sun, and when this
is attached to a telescope these prominences, as they are called, can be
seen at any time, so that there is no need to wait for an eclipse.

[Illustration: THE EARTH AS IT WOULD APPEAR IN COMPARISON WITH THE
FLAMES SHOOTING OUT FROM THE SUN.]

What are thes