72 



Halley's Comet, 



By Col. E. E. Markwick, C.B., F.R.A.S. 



{Read as a General Lecture on 13th March^ 1909)' 



The following is a much condensed account of the original 

 lecture on Halley's Comet, as exigencies of space will only allow 

 certain portions to be given : — 



Halley's Comet interests us in several respects, because it is 

 the first periodic comet whose return was ever predicted ; because 

 its period is 76 years, about the average duration of a healthy man's 

 life ; because it has returned so many times in the past to the 

 vicinity of the sun, on some of which occasions it is connected with 

 historical events ; because it illustrates the marvellous accuracy, 

 and complexity also, of the laws of universal gravitation enounced 

 by Newton ; because of the brilliant feats of calculation which it 

 has been the cause of evolving from some of the finest mathematical 

 intellects ; and, lastly, it is interesting because its return to perihelion 

 is due next year (19 10), and we may all reasonably hope to get a 

 view of this celestial visitant, which has been an object of such deep 

 study to astronomers. 



This comet had been observed by Newton, Halley, and their 

 contemporaries in 1682. Halley investigated the orbit, and found 

 that it was identical with comets which had appeared in 1607 and 

 1 53 1, and he was able to announce boldly This body moves in an 

 elliptic orbit round the sun, and will return again to the sun in the 

 year 1759." The calculation of the first cometic orbit ever worked 

 out by man was therefore effected by Halley, one of our own 

 countrymen, and in the glory of such a feat posterity all the world 

 over, English or Continental, have always called this body Halley's 

 Comet. Halley did not live to see the fulfilment of his prophecy, 

 but the comet returned to perihelion on the 13th March, 1759, thus 

 verifying the correctness of his work. 



The orbit, or track of the comet, is a very elongated ellipse, 

 with the sun in one of the foci. The length of the major axis is 

 about 3,400 millions of miles, so that the comet when in aphelion 

 is a considerable distance outside the orbit of Neptune, the outer- 

 most planet of the Solar system. The perihelion distance from the 

 sun is rather more than half the earth's distance from the sun. 

 The plane of the ellipse, or orbit, is slightly inclined at an angle of 

 17^ to the ecliptic. 



Plate VIII, illustrates the general position of the orbit in 

 connection with the sun and the orbits of the planets. Certain 

 dates are marked along the com.et's track which has been followed 

 since the last nearest approach to the sun in 1335. A selection of 

 these as connected with some historical events follows : — 



1835. Comet's perihelion passage on i6th November, the 

 5th year of King William IV., and i}^ years 

 before the accession of the late Queen Victoria. 



