544 SCIENCE PROGRESS 



the time when its periodicity was first detected by the great 

 astronomer whose name it bears, and to note that Newton also 

 played an important part in this discovery. About 1679 he 

 showed that a body moving under a central force inversely 

 proportional to the square of the distance would describe an 

 orbit in the form of one of the curves called conic sections, 

 which are four in number — circle, ellipse, parabola, and 

 hyperbola. 



The planets and satellites gave examples of motion in 

 very slightly flattened ellipses, some of them practically circles. 

 It quickly occurred to him that the curious motions of comets 

 might be explained by the hypothesis of their moving round the 

 sun in very long ellipses, or, possibly, in parabolas. Halley 

 entered into these ideas with enthusiasm, and his assistance was 

 invaluable to Newton, who, in spite of his surpassing genius for 

 abstract reasoning of the most profound character, seems not 

 to have been equally successful with long arithmetical com- 

 putations. Between them they studied the motion of the re- 

 markable comet of 1680, which passed extremely near the sun. 

 They found that its path differed very little from a parabola, but 

 was perhaps a very long ellipse, with a period of some six 

 centuries. It has to be remembered that we only see a very 

 small portion of the whole orbit of the comet, since it soon 

 vanishes from sight as it recedes from the sun. Even with 

 the accurate observations that are now made, it is frequently 

 difficult to determine whether an orbit is elliptical or parabolic ; 

 and the latter assumption is always made in the study of the 

 early observations of a new comet, since all parabolas are 

 similar curves, so that tables of parabolic motion can be con- 

 structed which are available for all cases, while ellipses are 

 of all degrees of eccentricity, and each case has to be treated 

 separately. 



In 1682 appeared the object that was destined to make the 

 name of Halley so famous ; he was not, as some imagine, the 

 actual discoverer — indeed, it is doubtful who saw it first. We 

 learn from the record of Flamsteed, the first Astronomer Royal, 

 that the object was seen by some neighbours on August 15 

 (Old Style), by his assistant on the next night, while he himself 

 could not obtain an observation till the 20th. Halley was not 

 then at the Royal Observatory ; but we learn that he saw the 



