COMETS. 



which have resulted from ' the calculations of Professor Enck6 r 

 based on all the observations of the comet which have been re- 

 corded. The elements of the great comet of 1843 have resulted 

 from the computations of Mr. Hubbard. Both are subject to- 

 considerable uncertainty, and must be accepted only as the best 

 approximations that carf be obtained. 



What is not subject, however, to the same uncertainty, is the 

 extraordinary proximity of these bodies to the 

 Fig- 6. sun at their respective perihelia. The peri- 



helion distance of the comet of 1680 was about 

 576000 miles, and that of 1843, 538000 miles. 

 Now the semidiameter of the sun being 441000- 

 miles, it follows that the distance of the centres 

 of those comets respectively from the surface 

 of the sun at perihelion must have been only 

 235000 and 97000; so that if the semi- 

 diameter of the nebulous envelope of either of 

 them exceeded this distance, they must have 

 actually grazed the sun. 



The velocity of the orbital motion of these- 

 bodies in perihelion appears by the table to be 

 such, that the comet of 1680 would have re- 

 volved round the sun in a minute, and that of 

 1843 in a little less than two minutes, if they re- 

 tained the same angular motion undiminished. 

 The distance to which the comet of 1680 

 recedes in its aphelion is 28| times greater than 

 that of Neptune. The apparent diameter of the 

 sun seen from that distance would be 2", and 

 the intensity of its light and heat would be 

 730000 times less than at the earth ; while 

 their intensity at the perihelion distance would 

 be 26000 times greater, so that the light and 

 heat received by the comet in its aphelion 

 would be 26000 x 730000=18980 minion times 

 less than in perihelion. 



The greatest aphelion distances in the table 

 are those of Nos. 5, 13, and 17, the comets of 1780, 1830, and 1844, 

 amounting to from 100 to 140 times the distance of Neptune ; the 

 eccentricities differing from unity by less than j^. These orbits, 

 though strictly the results of calculation, must be regarded as 

 subject to considerable uncertainty. 



73. To convey an idea of the form of the orbits of the comets 

 of this group, and of the proportion which their magnitude 

 bears to the dimensions of the solar system, we have drawn in 

 186 



