THE EXPECTED RETURN OF HALLEY'S COMET 545 



comet on at least one night, though clouds prevented him from 

 measuring its place among the stars. It was not, however, this 

 casual glimpse that entitled him to attach his name to the comet, 

 but the remarkable researches, undertaken many years later, 

 in which he found that it is a regular member of the sun's 

 family, returning at intervals of about three-quarters of a 

 century. This was a quite unexpected result of the immense 

 piece of work which he set himself early in the eighteenth 

 century — the calculation of parabolic orbits for which he could 

 find sufficiently accurate observations — his researches including 

 a period of fully two centuries. This was in those days a 

 huge undertaking, for the most suitable methods had not yet 

 been learnt by experience, and he had to devise all the needful 

 machinery for himself as he went along. It must be acknow- 

 ledged that there was some good fortune (though it was 

 thoroughly well deserved) in the fact that the same comet, 

 should occur three times in his list of twenty-four comets. Halley 

 was at once struck with the almost perfect similarity in the 

 elements of their orbits ; the solution of the mystery was not long 

 in coming to him. He saw that it was in all probability the same 

 object returning at regular intervals to the sun — that, to quote 

 his own expression, here was " A Mercury among comets " — 

 comparing the planet of shortest period with what he supposed, 

 though wrongly, to be the comet of shortest period, and drawing 

 the deduction, which was natural, though we now know it 

 to be false, that, as this one had come back, so all the 

 comets would return in longer or shorter periods. We now 

 know a great number of comets with shorter periods than 

 that of Halley, the one that is the true Mercury of comets 

 being Encke's, which returns to perihelion thirty times in a 

 century. 



There was just one feature that made Halley hesitate before 

 definitely announcing his discovery. The three dates that he 

 found for the comet passing nearest to the sun were August 24, 

 1 53 1 ; October 16, 1607; September 4, 1682; the first interval 

 exceeds the second by fifteen months. Now, the periods of the 

 planets are practically the same, revolution after revolution ; but 

 the true explanation was soon seen. He remembered that there 

 was a sensible, though small, variation in the periods of the giant 

 planets, Jupiter and Saturn, due to their mutual attractions, and 

 that not only did the comet pass much nearer to them than 



