LE YERRIER AND ADAMS PLANET. 



disturbing planet could Jae nearly assigned about 1822, it could 

 not be determined with the desired precision. 



Assuming, however, as nearly as was practicable, the longitude 

 of Uranus at the moment of heliocentric conjunction * with the 

 disturbing planet, this, combined with the mean motion of the 

 sought planet, inferred from its period, would give a rough 

 approximation to its place for any given time. 



10. Rough approximations were not, however, what MM. Le 

 Yerrier and Adams sought. They aimed at more exact results ; 

 and, after investigations involving all the resources and ex- 

 hausting all the vast powers of analysis, these eminent 

 geometers arrived at the following elements of the undiscovered 

 planet : 



11. On the 23rd September, 1846, Dr. Galle, one of the astrono- 

 mers of the Royal Observatory at Berlin, received a letter from 

 M. Le Yerrier, announcing to him the principal results of his 

 calculations, informing him that the longitude of the sought 

 planet must then be 326, and requesting him to look for it. 

 Dr. Galle, assisted by Professor Encke, accordingly did "look for 

 it," and found it that very night. It appeared as a star of the 

 8th magnitude, having the longitude of 326 52', and consequently 

 only 52' from the place assigned by M. Le Yerrier. The calcula- 

 tions of Mr. Adams, reduced to the same date, gave for its place 

 329 19', being 2 27' from the place where it was actually 

 found. 



To illustrate the relative proximity of these remarkable predic- 

 tions to the actual observed place, let the arc of the ecliptic, 

 from long. 323 to long. 330, be represented in fig. 3. The 

 place assigned by M. Le Yerrier for the sought planet is indi- 



* Two objects are said to be in heliocentric conjunction, when they are 

 so placed that they would be seen in the same direction by an observer 

 looking at them from the sun. 

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