SECT. VIII. ASTRONOMICAL TABLES. 63 



Neptune has a diameter of 39,793 miles, consequently he is 

 nearly 200 times larger than the earth, and may be seen with a 

 telescope of moderate power. His motion is retrograde at pre- 

 sent, and six times slower than that of the earth. At so great a 

 distance from the sun it can only have the Tabth part of the light 

 and heat the earth receives ; but having a satellite, the deficiency 

 of light may in some measure be supplied. 



The prediction may now be transferred from Uranus to Nep- 

 tune, whose perturbations may reveal the existence of a planet 

 still further removed, which may for ever remain beyond the 

 reach of telescopic vision yet its mass, the form and position of its 

 orbit, and all the circumstances of its motion may become known, 

 and the limits of the solar system may still be extended hundreds 

 of millions of miles. 



The mean distance of Neptune from the sun has subsequently 

 proved to be only 2893 millions of miles, and the period of his 

 revolution 166 years, so that Baron Bode's law, of the interval 

 between the orbits of any two planets being twice as great as the 

 inferior interval and half of the superior, fails in the case of 

 Neptune, though it was useful on the first approximation to his 

 motions ; and since Bode's time it has led to the discovery of fifty- 

 five telescopic planets revolving between the orbits of Mars and 

 Jupiter, some by chance, others by a systematic search on the 

 faith that these minute planets are fragments of a larger body 

 that has exploded, because their distances from the sun are nearly 

 the same ; the lines of the nodes of some of their orbits terminate 

 in the same points of the heavens, and the inclinations of their 

 orbits are such as might have taken place from their mutual 

 disturbances at the time of the explosion, and while yet they 

 were near enough for their forms to affect their motions. The 

 orbits of the more recently discovered asteroids show that this 

 hypothesis is untenable. 



The tables of Mars, Venus, and even those of the sun, have 

 been greatly improved, and still engage the attention of our 

 Astronomer Eoyal, Mr. Airy, and other eminent astronomers. We 

 are chiefly indebted to the German astronomers for tables of 

 the four older telescopic planets, Vesta, Juno, Ceres, and Pallas ; 

 the others have only been discovered since the year 1845. 



The determination of the path of a planet when disturbed by 

 all the others, a problem which has employed the talents of the 



