1910] 0)1 HalUifs Comet. 759 



accurate results that Dr. Max Wolf of Heidelberg was enabled to 

 pick up the faint comet on one of his photographs on September 11, 

 1909, close to the place predicted. 



It may not be quite clear wherein Hes the chief difficulty of the 

 work, or the chief merit of the accuracy attained. Is not the orbit 

 of the comet well known after so many revolutions have been 

 observed ? Let us suppose for a moment that this were so ; that 

 there were rails laid on the track round the sun on which the comet 

 must travel like a train, without deviating so much as a hair's 

 breadth. There would still remain one element of uncertainty, viz., 

 the time at which it would arrive. The disturbances of the planets 

 would now be limited to delays and accelerations, such as attend the 

 passage of a train through wayside stations. We must add together 

 all the delays and subtract any time saved. What accuracy may we 

 expect in the final result ? Remember that the whole journey takes 

 75 years, and that there may be unknown disturbing causes. We 

 shall be fortunate if the time is predicted within a week ; the pre- 

 diction of a famous French astronomer differed by a whole month 

 from that of Messrs. Cowell and Crommelin, and that of another 

 unknown calculator by two months. It was therefore gratifying to 

 them to be correct Avithin three days. 



It may be remarked here that this difference, small though it is, 

 nevertheless considerably alters the apixirent imth of the comet as seen 

 from the earth. We have supposed the actual path round the sun 

 to be prescribed ; but the apparent path as seen by us who are 

 moving also is a different matter. The paths of the planets round 

 the sun are nearly circles ; but as seen from the earth, their motions 

 are very different — there are times when they seem to stand still and 

 then move backwards. Hence we must be prepared to find the 

 apjjarent path of the comet, as seen by us, modified considerably by 

 a delay in its orbit, even though that orbit may be accurately pre- 

 scribed ; and in searching for the comet we depend, of course, on 

 the apparent path, and not the real. To see the kind of difference 

 that may result, let us compare the two ephemerides given by Messrs. 

 Cowell and Crommehu, one in March 1908, when they had made 

 approximate calculations only, and the other now that they have 

 made their laborious and accurate calculations, and corrected them 

 finally by the latest news brought by the comet itself. 



The change is for our advantage in the northern hemisphere, 

 but the possibility that the comet might be visible during the total 

 eclipse of the sun on May 8 in Tasmania has now disappeared. The 

 comet will have set before the eclipse begins. One peculiarity of 

 the present return, to which Mr. Crommelin has drawn attention, 

 is that the interval since the last appearance is the shortest on record 

 by some five and a half months. It is also noteworthy that this quick 

 return is the paradoxical result of special delays by Jupiter. The 

 orbits of the comet and of Jupiter cross (approximately) at two 



