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 Caesar, 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 deg. 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