In addition to the planets and their satellites, the sun is attended by
numerous other bodies, moving with far less regularity, and generally
much less conspicuous in the heavens. These are known as _comets_ and
_meteorites_ or _shooting stars_. One of the most interesting of recent
astronomical discoveries is that an intimate physical connection exists
between these two classes of bodies.
COMETS. Comets have been known from the earliest times, because every
now and then a very large and conspicuous one hastens up to the sun from
the remote regions of space, and perplexes monarchs with the fear of
change. They are called _comets_, from the Latin _coma_, meaning hair,
because when they are bright enough to be seen with the naked eye they
look like stars attended by a long stream of hazy light, which was
thought to resemble a woman’s hair flowing down her back. This train of
light is known as the comet’s _tail_. Such bright comets are sometimes
as brilliant as Venus; their tails have been known to stretch halfway
across the visible sky.
These comets are very beautiful and conspicuous objects, which usually
appear in the sky without any warning from astronomers, and invariably
create a great popular sensation. By far the greater number of comets,
however, are only visible through a telescope, and it is rare that a
year passes without at least half a dozen of these being reported. Up to
the present time nearly a thousand comets of all sizes have been
recorded. Not more than one in five of these visitors is visible to the
naked eye.
COMETARY ORBITS. In all cases in which a comet has been observed
sufficiently often for its orbit to be calculated, it is found that it
moves in one of the curves which are known to the geometer as conic
sections. Less than a hundred of the known comets move like the planets
in _elliptical_ orbits, and consequently their periodical return to
visibility can be predicted. As a rule the eccentricity of these
cometary orbits is very much greater than that of any planetary orbit,
which means that the comet approaches fairly close to the sun at one end
of its orbit, but at the other flies away far beyond the outermost
planet, and for a long period disappears from the view of our most
powerful telescopes.
The great majority of comets have only been seen once, and their orbits
appear to be either _parabolic_ or _hyperbolic_. Neither of these is a
closed curve, and what seems to happen in such cases is that a comet
travelling in such an orbit dashes up to the sun from the remote parts
of space, swings round it, often at very close quarters, and flies away
again forever. Only those comets which have elliptical orbits can be
said to belong to the solar system. The others are visitors from space,
which in the course of their motion come near the sun and are deflected
by it, but then fly away until after a lapse of ages they perhaps come
within the sphere of another star’s attraction. Of the comets which move
in elliptical orbits, about twenty have been observed at more than one
return to the sun. Some of these complete their orbits in quite a short
period, like Encke’s comet, which has the shortest period of all, less
than three and a half years; the longest periodical comet is known as
Halley’s, which returns to the sun after seventy-six years, and last
appeared in 1910; it is a bright and conspicuous object.
THE CONSTITUTION OF COMETS. The nature of comets was long in doubt, and
even today their physical characteristics are not fully understood. They
are certainly formed of gravitational matter, because they move in
orbits which are subject to the same laws as those of the planets. But
they also appear to be acted upon by powerful _repulsive forces_
emanating from the sun, to which is due the remarkable phenomenon of
cometary tails. Perhaps there is not much exaggeration in the statement
once made by a well-known astronomer that the whole material of a comet
stretching halfway across the visible heavens, if properly compressed,
could be placed in a hatbox. The old fear that the earth might suddenly
be annihilated by a comet striking it is thoroughly dispelled by modern
investigation, which leads us to believe that the worst results of such
an encounter would be an extremely beautiful display of shooting stars.
METEORS, or FIREBALLS, are bodies which do not belong to the earth, but
come from other parts of space into our atmosphere, and are seen as
bright balls of fire crossing the sky, with a train of light behind.
Suddenly they are seen to go out, and very often a fall of stones
occurs. Sometimes they are observed to break in two, and loud explosions
like thunder are heard. They move very fast--ten or twelve miles per
second, and are visible when between forty and eighty miles above the
earth.
Other meteors dart across the sky and disappear, all in a very short
time. These are known as shooting stars, and are sometimes big and
bright, like planets. It is estimated that about six or eight meteors
which drop stones come into our atmosphere every year; but some
20,000,000 of small bodies pass through the air every day--these would
all appear as shooting stars if they occurred at night.
At some periods of the year there are so many shooting stars that they
appear like a shower of fire. On November 14th this happens, the shower
being greatest every thirty-three years. A stream of meteors is
travelling round the sun, and every thirty-three years the earth just
comes through them. Meteoric showers also occur about August 9th to
11th, and smaller ones in April.
The luminosity of meteors is due to the intense heat caused by the
resistance of the air to their passage, and in support of this theory it
is found that meteoric stones are always covered, either wholly or in
part, with a crust of cement that has recently been melted.