494 THE POPULAR SCIENCE MONTHLY 



THE CLOSING OF A FAMOUS ASTEONOMICAL PROBLEM 



Bx Professor W. W. CAMPBELL 



DIKECTOR OF THE LICK OBSERVATORY 



THERE is perhaps no more striking illustration of the power of 

 scientific method than that relating to the discovery of Neptune 

 in 1846. The planet Uranus, until then the outermost known mem- 

 ber of our solar system, refused to follow the path computed for it by 

 mathematical astronomers. With the progress of time the discrepancies 

 between its predicted and observed positions grew constantly larger 

 until, in the early eighteen-forties, the discordance amounted to fully 

 75 seconds of arc. This is a small angle — not more than one twenty- 

 fifth the angular diameter of our moon — yet a very large angle to 

 refined astronomy, for a discrepancy of two seconds would have been 

 detected with ease. The opinion gradually developed that Uranus 

 was drawn from its natural course by the attractions of an undiscovered 

 planet still farther from the sun than itself. Adams in 1843 and 

 Le Verrier in 1845, independently, and each without knowledge of 

 the other's plans, attacked the then extremely difficult problem of 

 determining the approximate orbit, mass and position of an undis- 

 covered body whose attractions should produce the perturbations ob- 

 served. Regrettable and avoidable delays occurred in searching for 

 the planet after Adams's results were communicated to the astronomer 

 royal, in October, 1845. Le A^errier's results were communicated to 

 the Berlin Observatory in September, 1846, with the request that a 

 search be made. The disturbing planet, later named Neptune, was 

 found on the first evening that it was looked for, less than one degree 

 of arc from the position assigned by Le Verrier. If an energetic 

 search had been made in England the year before, the planet would 

 have been discovered within two degrees of the position assigned by 

 Adams. 



The above resume of this unsurpassed achievement of the human 

 mind forms a natural prelude to the present article, as it was the im- 

 mediate forerunner of another problem, famous for half a century, 

 which has now been brought to a satisfactory conclusion. 



The determination of the orbit of the planet Mercury gave great 

 difficulty to its investigators, principally from two causes : 



1. Being the innermost known planet in our system, remaining 

 always near the sun, and usually lost to view in the sun's glare, fairly 



