CURIOUS HISTORY

Dominion, Volume 24, Issue 240, 7 July 1931, Page 8

CURIOUS HISTORY

Encke’s Comet Returns ASTRONOMER’S WORK “Resisting Medium” Theory Some of the feature# of Encke’s eomet, which is above the horizon now in the early evenings, were described in a paper read last week before the astronomcal section of the Wellington Philosophical Society by Mr. R. C. Hayes, acting-chair-man of the section. The rediscovery of the comet on June 21, on its 37th return since discovery in 1786, said Mr. Hayeg, had suggested that it would be interesting and perhaps profitable to recall some of the comet’s curious history. “It was first discovered at Faria by Mechain in 1786,” Mr. Hayes said, “at which time it was also observed by the celebrated astronomer, Messier. The comet was. reported on that occasion to have a bright nucleus, 1 but no tail. It was seen again in 1795, when it was rediscovered by Miss Caroline Herschel. At its return in 1805 it was kept under observation for about seven weeks, during which it was said to have been visible to the naked eye, and it was on this occasion that Encke, the German astronomer, first gave attention to the comet’s movements. From a fairly complete set of observations, Eneke determined the orbit to be elliptical in shape, and assigned a period of three and a half years. “Encke suspected that the comets seen in 1786 and 1795 were identical with the one he was investigating,” said Mr. Hayes. “After carrying out more detailed calculations, he was assured of that fact. It was on account of Encke’s very thorough Investigations and successful predictions with regard to the comet that his name was finally attached to it, although he had not been the discoverer. At the return in 1828, the comet was said to have been as bright as a star of the fifth magnitude.

Peculiarity in Movement. “The 1838 return of the comet wa# notable from several points of view. It was on this return that a peculiarity in the comet's movements was pointed out by Encke. After every possible disturbing influence had been taken into account, it was found that the comet passed its perihelion position two and a half hours earlier than the predicted time. Later, in 1858, Eneke published a table which clearly indicated a steady decrease in the comet’s periodic time ever since its discovery in 1786. This acceleration, however. diminished in 1868, and Observation's in recent years show that it has now ceased altogether. Observations * show that the periodic time in 1924 wa# actually greater than it was in 1908. , “The theory of a ‘resisting medium' in space was put forward in Encke’s time to account for the apparent shrinking of the comet’s orbit. In connection with thia, Olivier, in his recent book on cornets, ‘Though to-day we no longer believe in the ‘resisting medium' theory, it seems necessary to' postulate some resistance. Perhaps the best we can do is to assume that the critical part of the comet's orbit intersests a region very rich in meteoric material, which, if dense enough, would produce such an effect. An alternate hypothesis is that some physical alteration has taken place in the comet itself. The writer would incline to the first explanation as being the more probable. Soundness of Encke’s Work. “A second interesting event in connection with Encke’s eomet in 1838 arose from its very close approach to Mercury,” said Mr. Hayes. “Encke was able to show that the accepted’.value for the mass of Mercury was much too great for the effect it produced bn the movements of the comet. The soundness of Encke’s work was proved by the fact that a lower value for the mass of Mercury was later adopted. At this same return in 1838, some accurate measurements of its size illustrate to a remarkable degree the property, common to most comets, of decreasing n size when approaching perihelion.’ At the present time the comet is approaching the earth, and receding rapidly from the sun. It will be nearest to the earth on July. 14. If the comet is following closely its predicted path, which seems to be the case, it will move in a south-easterly direction in the sky, through the constellation .Hydra, and passing about 20 degrees to the north of the Southern Cross on, July 20, The times at which it sets in Wellington are as follow:—July 6, 9.1 p.m.; July 10, 10.4 p.m.; July 14, 11122 p.m. For a considerable period after that,, the comet will be situated round about declination 40 degrees south, and will therefore set for only a short time in these latitudes. It is a telescopic object.