ELLIPTIC COMETS. 



proper proportions and relative positions, so as to exhibit to the 

 eye their several ellipticities, and the relative directions of their 

 axes.* All these bodies, without one exception, revolve in the 

 common direction of the planets. 



50. It is not alone, however, in the direction of their motions 

 that the orbits of these bodies have an analogy to those of the 

 planets. Their inclinations, with one exception, are within the 

 limits of those of the planets. Their eccentricities, though incom- 

 parably greater than those of the planets, are, as will presently 

 appear, incomparably less than those of all other comets yet dis- 

 covered. Their mean distances and periods (with the exception 

 of the last two in the Table just referred to) are within the limits 

 of those of the planetoids. 



The comparison of the numbers given in this Table with those 

 in the Tables of the elements of other elliptic comets, and the 

 comparison of the diagrams of their orbits with those of others, 

 will show in a striking manner, to how great an extent the orbits 

 of this group of comets possess the planetary character. Besides 

 moving round the sun in the common direction, their inclinations, 

 with a single exception, are within the limits of those of the 

 planets. It is true that their eccentricities have an order of 

 magnitude much greater ; but on the other hand, it will be seen 

 presently that they are incomparably less than the eccentricities 

 of all other periodic comets yet discovered. Their mean distances 

 and periods place them in direct analogy with the planetoids. 



Moderate as are the eccentricities as compared with those of other 

 comets, they are sufficiently great to impart a decided oval form 

 to the orbits, and to produce considerable differences between the 

 perihelion and aphelion distances, as will be apparent by inspecting 

 the Table. It appears that while the perihelion of Encke's comet 

 lies within the orbit of Mercury, its aphelion lies outside the orbit 

 of the most remote of the planetoids, and not far within that of 

 Jupiter. The perihelion of Biela's comet, in like manner, lies 

 between the orbits of the earth and Yenus, while its aphelion 

 lies outside that of Jupiter. In the case of Faye's comet, the 

 least eccentric of the group, the perihelion lies near the orbit of 

 Mars, and the aphelion outside that of Jupiter. 



It must be remembered that the elliptic form of these orbits 

 has only been verified by observations on the successive returns 

 to perihelion of the first five comets in the Table. The elliptic 

 elements of the others may, so far as is at present known, have 

 been effaced by disturbing causes. 



* In the diagram, to prevent confusion, the orbits of the different comets 

 are indicated by dotted or broken lines of different kinds. 



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