Page:Popular Science Monthly Volume 87.djvu/224

220 predicted times and places. About 60 such comets have been observed with periods less than 100 years. There is the great Jupiter family of comets, about 30 in the family, so-called because their aphelions—the points of the orbits farthest from the Sun—lie near Jupiter's orbit. Their periods vary from 3 to 8 years, their motions around the Sun are all from west to east, as in the case of the planets, and their orbit planes make small angles with the plane of the solar system. In a similar way there are two Saturn comets, three Uranus comets, and six Neptune comets, one of the latter being Halley's. Halley's comet is revolving around the Sun from east to west; that is, in a retrograde direction; and the motions of two comets which disappeared many years ago were likewise from east to west. The motions of all the other short-period comets are from west to east.

The origin of the periodic comets is an interesting question. Newton, of Yale, who was the chief student of the subject, gave practical certainty to the view that the periodic comets have been captured, so to speak, by the major planets, and especially by Jupiter; that is, that comets approaching the Sun in their elongated orbits and passing close to the major planets have had their orbits converted, either during one visit or cumulatively during several visits, into the forms we now observe. Perhaps the strongest doubt as to the sufficiency of the explanation arises from the fact that 95 per cent. of the motions appear to be from west to east. Newton's theory seems to demand that about 25 per cent. of Jupiter's comets should move in retrograde orbits, whereas none of Jupiter's comets, nor the two Saturn comets, do so move. Three of the eleven comets related to Uranus and Neptune, namely, Halley's comet and two lost comets, travel in the retrograde direction. The capture theory is technical and we must not pursue it. Fortunately, there is another avenue of approach. Barnard has noted that the short-period comets differ in appearance from those which come to our system unexpectedly, in that the former are the more diffuse in appearance; that is, they have larger diameters in proportion to their total brightness. There is reason to believe that the head of a comet consists principally of separate small bodies. Now in a collection of small bodies the gravitational forces holding them together are extremely slight. When the group approaches the center of the solar system the Sun's attractions upon the nearer members of the group are appreciably stronger than upon the members which are farthest from the Sun. The orbital motions of the nearer particles are relatively quickened and those of the farther particles relatively delayed. If the comet is traveling upon a very elongated orbit the mutual attractions within the head can again be effective while the comet is in the outer parts of the orbit, and a condensing process probably occurs; but, if the orbit extends out only as far as Jupiter or other major planets, there is little opportunity for the internal attractions to re-condense the particles, and