2 2o THE POPULAR SCIENCE MONTHLY 



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 famity, 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 simi- 

 lar way there are two Saturn comets, three Uranus comets, and six Nep- 

 tune comets, one of the latter being Halley's. Halley's comet is re- 

 volving 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. New- 

 ton, 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 ob- 

 serve. Perhaps the strongest doubt as to the sufficiency of the explana- 

 tion 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 cap- 

 ture theory is technical and we must not pursue it. Fortunately, there 

 is another avenue of approacb. 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 bright- 

 ness. There is reason to believe that the head of a comet consists prin- 

 cipally 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 trav- 

 eling 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 ex- 

 tends out only as far as Jupiter or other major planets, there is little 

 opportunity for the internal attractions to re-condense the particles, and 



