In January, 1846, Biela’s comet was observed to be divided.  At its next return, in 1852, the parts were 1,500,000 miles apart.  They could not be found on their periodic returns in 1859, 1865, and 1872; but it [Page 125] should have crossed the earth’s orbit early in September, 1872.  The earth itself would arrive at the point of crossing two or three months later.  If the law of revolution held, we might still expect to find some of the trailing meteoroids of the comet not gone by on our arrival.  It was shown that the point of the earth that would strike them would be toward a certain place in the constellation of Andromeda, if the remains of the diluted comet were still there.  The prediction was verified in every respect.  At the appointed time, place, [Page 126] and direction, the streaming lights were in our sky.  That these little bodies belonged to the original comet none can doubt.  By the perturbations of planetary attraction, or by different original velocities, a comet may be lengthened into an invisible stream, or an invisible stream agglomerated till it is visible as a comet.

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.