THE EXPECTED RETURN OF HALLEY'S 



COMET 



By ANDREW C. D. CROMMELIN, B.A., F.R.A.S. 



It is difficult for us now to realise the feelings of terror and 

 dismay that the appearance of a large comet excited in the 

 minds of men. Before the laws of gravitation were discovered, 

 it was impossible to form any true conception of their distance 

 or motion ; from their great size and rapid motion, it was 

 natural to assume that they were in near proximity to our 

 earth — perhaps actually within her atmosphere — so that it was 

 a simple step to associate their coming with plague, famine, 

 and war. Even in the nineteenth century, comet-panics have 

 not been unknown. An anticipated collision with Biela's 

 comet, and the flaming visitants of 1858 and 1874 all caused 

 lively apprehensions among those who had inaccurate ideas 

 of their nature and their great distance from the earth. One 

 reason of this apprehension is doubtless that bright comets 

 generally form an exception to the astronomer's power of 

 accurate prediction. They arrive unexpectedly, at quite 

 irregular intervals, their periods being, as a rule, so long that 

 they have not visited the sun's neighbourhood before during 

 the historic period. Thus, there are sometimes several bright 

 comets within a decade ; but at the present time we have to go 

 back to 1882 for a really brilliant one visible in Europe. There 

 is just one exception to this inability to predict the advent of 

 conspicuous comets, and that is in the case of Halley's comet. 

 The class of periodic comets is indeed a large and increasing 

 one, but most of them are only of interest to the professional 

 astronomer, being faint or invisible to the naked eye. This 

 comet is furthermore of interest from the very long list of 

 observed returns that we are able to trace, which go back to 

 the year b.c. 240. 



In a retrospect of its history it is convenient to begin with 



543 



