NEW PLANETS NEAR THE SUN. 35 



In some respects the discovery of a planet nearer to the 

 sun than Mercury may seem to many far inferior in interest 

 to the detection of the remote giant Neptune. Between 

 Mercury and the sun there intervenes a mean distance of 

 only thirty-six millions of miles, a distance seeming quite in- 

 significant beside those which have been dealt with in describ- 

 ing the discovery of Uranus and Neptune. Again it is 

 quite certain that any planet between Mercury and the sun 

 must be far inferior to our own earth in size and mass, where- 

 as Neptune exceeds the earth 105 times in size and 17 times 

 in mass. Thus a much smaller region has to be searched 

 over for a much smaller body. Moreover, while mathema- 

 tical calculation cannot deal nearly so exactly with an intra- 

 Mercurial planet as with Neptune, for there are no perturba- 

 tions of Mercury which give the slightest information as to 

 the orbital position of his disturber, the part of the heavens 

 occupied by the intra-Mercurial planet is known without cal- 

 culation, seeing that the planet must always lie within six or 

 seven degrees or so of the sun, and can never be very far 

 from the ecliptic. 



Yet in reality the detection of an intra-Mercurial planet 

 is a problem of far greater difficulty than that of such a 

 planet as Neptune, while even now when most astronomers 

 consider that an intra-Mercurial planet has been detected, 

 the determination of its orbit is a problem which seems to 

 present almost insuperable difficulties. 



I may remark, indeed, with regard to Neptune, that he 

 might have been successfully searched for withouta hundredth 

 part of the labour and thought actually devoted to his detec- 

 tion. It may sound rather daring to assert that any fairly 

 good geometrician could have pointed after less than an hour's 

 calculation, based on the facts known respecting Uranus in 

 1842, to a region within which the disturbing planet must 

 certainly lie, a region larger considerably no doubt than 

 that to which Adams and Leverrier pointed, yet a region 

 which a single observer could have swept over adequately in 

 half-a-dozen favourable evenings, two such survevs sufficing 



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