SECT. XXXV. HALLEY'S COMET. 363 



that it would pass its perihelion on the 18th of April, 1759. The 

 comet did arrive at that point of its orbit on the 12th of March, 

 which was thirty-seven days before the time assigned. Clairaut 

 subsequently reduced the error to twenty-three days ; and La 

 Place has since shown that it would only have been thirteen days 

 if the mass of Saturn had been as well known as it is now. It 

 appears, from this, that the path of the comet was not quite 

 known at that period ; and, although many observations were 

 then made, they were far from attaining the accuracy of those of 

 the present day. Besides, since the year 1759, the orbit of the 

 comet has been altered by the attraction of Jupiter in one 

 direction, and that of Saturn, Uranus, and Neptune in the other; 

 yet, notwithstanding these sources of uncertainty, and our igno- 

 rance of all the possible causes of derangement from unknown 

 bodies on the confines of our system, or in the regions beyond it, 

 the comet appeared exactly at the time, and not far from the 

 place, assigned to it by astronomers ; and its actual arrival at 

 its perihelion a little before noon on the 16th of November, 1835, 

 only differed from the computed time by a very few days, which 

 was probably owing to the attraction of Neptune. 



The fulfilment of this astronomical prediction is truly wonder- 

 ful, if it be considered that the comet is seen only for a few 

 weeks during its passage through our system, and that it wan- 

 ders from the sun for seventy-five years to twice the distance of 

 Uranus. This enormous orbit is four times longer than it is 

 broad ; its length is about 3420 millions of miles, or about 

 thirty-six times the mean distance of the earth from the sun. 

 At its perihelion the comet comes within nearly fifty-seven 

 millions of miles of the sun, and at its aphelion it is sixty times 

 more distant. On account of this extensive range it must ex- 

 perience 3600 times more light and heat when nearest to the sun 

 than in the most remote point of its orbit. In the one position 

 the sun will seem to be four times larger than he appears to us, 

 and at the other he will not be apparently larger than a star 

 (N. 228.) 



On the first appearance of Halley's comet, early in August 

 1835, it seemed to be merely a globular mass of dim vapour, 

 without a tail. A concentration of light, a little on one side of 

 the centre, increased as the comet approached the sun and earth, 

 and latterly looked so like the disc of a small planet, that it 



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