It is now known that comets
are not wanderers through the celestial spaces from star to star,
but must always have belonged to our system. But their orbits are
so very elongated that thousands, or even hundreds of thousands,
of years are required for a revolution. Sometimes, however, a
comet passing near to Jupiter is so fascinated by that planet
that, in its vain attempts to follow it, it loses so much of its
primitive velocity as to circulate around the sun in a period of a
few years, and thus to become, apparently, a new member of our
system. If the orbit of such a comet, or in fact of any comet,
chances to intersect that of the earth, the latter in passing the
point of intersection encounters minute particles which cause a
meteoric shower.
But all this does not tell us much about the nature and make-up of
a comet. Does it consist of nothing but isolated particles, or is
there a solid nucleus, the attraction of which tends to keep the
mass together? No one yet knows. The spectroscope, if we interpret
its indications in the usual way, tells us that a comet is simply
a mass of hydrocarbon vapor, shining by its own light. But there
must be something wrong in this interpretation. That the light is
reflected sunlight seems to follow necessarily from the increased
brilliancy of the comet as it approaches the sun and its
disappearance as it passes away.
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