A FAMOUS ASTRONOMICAL PROBLEM 495 



accurate observations of its positions could be secured only when the 

 planet was near its greatest angular distances from the sun and 

 on the rare occasions when the planet passed between us and the sun's 

 disk. Consequently, observations of the highest accuracy were few 

 in number; and, 



2. There were large discrepancies between Mercury's predicted and 

 observed positions, certainly not due to the attractions of any known 

 members of our solar system. 



Le Verrier, of Neptunian fame, undertook a systematic investiga- 

 tion of Mercury's orbit, making use of all available observations. 

 His results were derived and published in 1859. His work established 

 that there were peculiarities in the planet's orbital motion which could 

 not be due to the attractions of known masses of matter. Chief among 

 the peculiarities was a slow rotation of the orbit itself. It is best 

 described as a forward motion of the orbit's perihelion amounting to 

 38 seconds of arc per century. 



Le Verrier announced that the outstanding difEerences between 

 prediction and observation could be produced and explained by the 

 disturbing attractions of an undiscovered planet closer to the sun 

 than Mercury and revolving around the sun in an orbit lying nearly 

 in the plane of Mercury's orbit. The mass of (the quantity of matter 

 in) the hypothetical planet would depend upon its distance from 

 Mercury: if half way between Mercury and the sun, its mass would 

 be two thirds that of Mercury; if further from Mercury, the necessary 

 mass would be greater ; if nearer, smaller. A group or " ring " of 

 small planets, instead of one large planet, would serve equally well, 

 provided the total mass of the planetoids were of the same order of 

 magnitude. Le Verrier did not say that such an undiscovered planet 

 or ring of planetoids did exist, but simply that it would account for 

 the observed anomalies. The accuracy of his computations, published 

 in detail, could not be questioned. The recognition of his masterly 

 skill, and the memory of his entirely similar discovery of Neptune, 

 assisted in convincing astronomers quite generally that a planet or 

 group of planets existed. The discovery of the disturbing mass 

 became at once a noted problem. 



A body traveling around the sun in a circular orbit whose radius 

 is only one half Mercury's average solar distance would never be 

 more than 12° from the sun as viewed by terrestrial observers. A 

 search for it by ordinary methods would accordingly be fruitless. A 

 body large enough to shine brilliantly on a dark-sky background would 

 be hopelessly lost in the bright sky near the sun. Mercury itself, 

 though running out between 20° and 30° from the sun every few 

 weeks, is seldom seen by any save astronomers; and they know where 

 to look for it in the twilight sky. 



Two special methods of discovery were applicable: (1) To detect 



