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Page 36
_Comets._
Comets will be most easily understood by the foregoing considerations.
They are often treated as if they were no part of the solar system;
but they are under the control of the same laws, and owe their
existence, motion, and continuance to the same causes as Jupiter and
the rest of the planets. They are really planets of wider wandering,
greater ellipticity, and less density. They have periodic times
less than the earth, and fifty times as great as Neptune. They
are little clouds of gas or meteoric matter, or both, darting into
the solar system from every side, at every angle with the plane
of the ecliptic, becoming luminous with reflected light, passing
the sun, and returning again to outer darkness. Sometimes they
have no tail, having a nucleus surrounded by nebulosity like a
dim sun with zodiacal light; sometimes one tail, sometimes half a
dozen. These follow the comet to perihelion, and precede it afterward
(Fig. 52). The orbits of some comets are enormously elongated; one
end may lie inside the earth's orbit, and the other end be as far
beyond Neptune as that is from the sun. Of course only a small
part of such a curve can be studied by us: the comet is visible
only when near the sun. The same curve around the sun may be an
orbit that will bring it back again, [Page 127] or one that will
carry it off into infinite space, never to return. One rate of speed
on the curve indicates an elliptical orbit that returns; a greater
rate of speed indicates that it will take a parabolic orbit, which
never returns. The exact rate of speed is exceedingly difficult to
determine; hence it cannot be confidently asserted that any comet
ever visible will not return. They may all belong to the solar
system; but some will certainly be gone thousands of years before
their fiery forms will greet the watchful eyes of dwellers on the
earth. A comet that has an elliptic orbit may have it changed to
[Page 128] parabolic by the accelerations of its speed, by
attracting planets; or a parabolic comet may become elliptic, and so
permanently attracted to the system by the retardations of
attracting bodies. A comet of long period may be changed to one of
short period by such attraction, or _vice versa_.
[Illustration: Fig. 52.--Aspects of Remarkable Comets.]
The number of comets, like that of meteor streams, is exceedingly
large. Five hundred have been visible to the naked eye since the
Christian era. Two hundred have been seen by telescopes invented
since their invention. Some authorities estimate the number belonging
to our solar system by millions; Professor Peirce says more than
five thousand millions.
_Famous Comets._
The comet of 1680 is perhaps the one that appeared in A.D. 44, soon
after the death of Julius C�sar, also in the reign of Justinian,
A.D. 531, and in 1106. This is not determined by any recognizable
resemblance. It had a tail 70� long; it was not all arisen when
its head reached the meridian. It is possible, from the shape of
its orbit, that it has a periodic time of nine thousand years, or
that it may have a parabolic orbit, and never return. Observations
taken two hundred years ago have not the exactness necessary to
determine so delicate a point.
On August 19th, 1682, Halley discovered a comet which he soon declared
to be one seen by Kepler in 1607. Looking back still farther, he
found that a comet was seen in 1531 having the same orbit. Still
farther, by the same exact period of seventy-five years, he found
that it was the same comet that had disturbed [Page 129] the
equanimity of Pope Calixtus in 1456. Calculations were undertaken as
to the result of all the accelerations and retardations by the
attractions of all the planets for the next seventy-five years.
There was not time to finish all the work; but a retardation of six
hundred and eighteen days was determined, with a possible error of
thirty days. The comet actually came to time within thirty-three
days, on March 12th, 1759. Again its return was calculated with more
laborious care. It came to time and passed the sun within three days
of the predicted time, on the 16th of November, 1835. It passed from
sight of the most powerful telescopes the following May, and has
never since been seen by human eye. But the eye of science sees it
as having passed its aphelion beyond the orbit of Neptune in 1873,
and is already hastening back to the warmth and light of the sun. It
will be looked for in 1911; and there is good hope of predicting,
long before it is seen, the time of its perihelion within a day.
_Biela's lost Comet._--This was a comet with a periodic time of
six years and eight months. It was observed in January, 1846, to
have separated into two parts of unequal brightness. The lesser
part grew for a month until it equalled the other, then became
smaller and disappeared, while the other was visible a month longer.
At disappearance the parts were 200,000 miles asunder. On its next
return, in 1852, the parts were 1,500,000 miles apart; sometimes
one was brighter and sometimes the other; which was the fragment
and which was the main body could not be recognized. They vanished
in September, 1852, and have never been seen since. Three revolutions
have been made since that time, but no [Page 130] trace of it could
be discovered. Probably the same influence that separated it into
parts, separated the particles till too thin and tenuous to be seen.
There is ground for believing that the earth passed through a part
of it, as before stated under the head of meteors.
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