The discovery of Neptune, as we shall see, was totally different in character from that of Uranus. The latter may be described as the finding of something by an observer who was looking for anything; Neptune was the finding of something definitely sought for, and definitely pointed out by a most successful and brilliant piece of methodical work. But before that time several planets had been found, as the practical result of a definite search, although the guiding principle was such as cannot command our admiration to quite the same extent as in the case of Neptune. To explain it I must say something of the relative sizes of the orbits in which planets move round the sun. These orbits are, as we know, ellipses; but they are very nearly circles, and, excluding refinements, we may consider them as circles, with the sun at the centre of each, so that we may talk of the distance of any planet from the sun as a constant quantity without serious error. Now if we arrange the planetary distances in order, we shall notice a remarkable connection between the terms of the series. Here is a table showing this connection. TABLE OF THE DISTANCES OF THE PLANETS FROM THE SUN, SHOWING "BODE'S LAW." +----------------------------------------------------+ | Name of | Distance from | "Bode's Law" | | Planet. | Sun, taking | (originally formulated | | | that of Earth | by Titius, but brought | | | as 10. | into notice by Bode). | |----------------------------------------------------| | Mercury | 4 | 4 + 0= 4 | | Venus | 7 | 4 + 3= 7 | | The Earth | 10 | 4 + 6= 10 | | Mars | 15 | 4 + 12= 16 | | ( ) | ( ) | 4 + 24= 28 | | Jupiter | 52 | 4 + 48= 52 | | Saturn | 95 | 4 + 96= 100 | | Uranus | 192 | 4 + 192= 196 | +----------------------------------------------------+ [Sidenote: Gap in the series suggesting unknown planet.] [Sidenote: Search for it.] [Sidenote: Accidental discovery.]