observing solar prominences.

The other is the great amount of "anticipation." Copernicus, as we have
seen, was anticipated by the Greeks; Kepler was not actually the first
who thought of elliptic orbits; others before Newton had imagined an
attractive force.

Both these points furnish much food for thought!

CHAPTER XIX

COMETS

The reader has, no doubt, been struck by the marked uniformity which
exists among those members of the solar system with which we have dealt
up to the present. The sun, the planets, and their satellites are all
what we call solid bodies. The planets move around the sun, and the
satellites around the planets, in orbits which, though strictly
speaking, ellipses, are yet not in any instance of a very oval form. Two
results naturally follow from these considerations. Firstly, the bodies
in question hide the light coming to us from those further off, when
they pass in front of them. Secondly, the planets never get so far from
the sun that we lose sight of them altogether.

With the objects known as Comets it is, however, quite the contrary.
These objects do not conform to our notions of solidity. They are so
transparent that they can pass across the smallest star without dimming
its light in the slightest degree. Again, they are only visible to us
during a portion of their orbits. A comet may be briefly described as an
illuminated filmy-looking object, made up usually of three portions--a
head, a nucleus, or brighter central portion within this head, and a
tail. The heads of comets vary greatly in size; some, indeed, appear
quite small, like stars, while others look even as large as the moon.
Occasionally the nucleus is wanting, and sometimes the tail also.

[Illustration: FIG. 18.--Showing how the Tail of a Comet is directed
away from the Sun.]

These mysterious visitors to our skies come up into view out of the
immensities beyond, move towards the sun at a rapidly increasing speed,
and, having gone around it, dash away again into the depths of space. As
a comet approaches the sun, its body appears to grow smaller and
smaller, while, at the same time, it gradually throws out behind it an
appendage like a tail. As the comet moves round the central orb this
tail is always directed _away_ from the sun; and when it departs again
into space the tail goes in advance. As the comet's distance from the
sun increases, the tail gradually shrinks away and the head once more
grows in size (see