[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 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.