4, 7, 10, 16, 28, 52, 100, 196, 388;
which numbers approximately represent the distances of the planets from the sun expressed in radii of the Earth’s orbit. A little table will make the matter more clear.
| Planets. | Distance: Bode’s Law. | True distance from Sun. | |
| Mercury | 4 | 3.9 | |
| Venus | 7 | 7.2 | |
| Earth | 10 | 10.0 | |
| Mars | 16 | 15.2 | |
| [Ceres] | [28] | [27.7] | |
| Jupiter | 52 | 52.0 | |
| Saturn | 100 | 95.4 | |
| [Uranus] | [196] | [191.8] | |
| [Neptune] | [388] | [300.0] |
Bode having examined these relations and noticing the void between 16 and 52 (Ceres and the other minor planets, and Uranus and Neptune also, being then unknown) ventured to predict the discovery of new planets, and this idea stimulated him to organise a little company of astronomers to hunt for new planets. Before, however, this scheme was got into working order, Piazzi, director of the Observatory at Palermo, on January 1, 1801, noted an 8th magnitude star in Taurus, which on the next and succeeding nights he saw again, and found had moved. He observed the strange object for 6 weeks, when illness interrupted him. However he wrote letters announcing what he had seen, one of them to Bode himself; but this letter, though dated Jan. 24, did not reach Bode at Berlin, till March 20—a striking illustration of the state of the Postal service on the Continent less than 100 years ago. The new body, at first assumed to be a tailless comet, was eventually recognised to be a new planet; and the name of Ceres, the tutelary goddess of Sicily, was at Piazzi’s instance bestowed upon it.
Looking for Ceres in March, 1802, Olbers at Bremen, came upon another new planet, which was afterwards named Pallas. At first he thought he had got hold of a new variable star, but two hours sufficed to show that the object under notice was in motion. The two new bodies were found to be so much alike in size and appearance, and in their orbits, that Olbers suggested both were but fragments of some larger body which had been shattered by some great convulsion of nature. The idea was a daring one, and it was an attractive one, though now regarded as untenable. However it served the purpose of stimulating research, and the discovery of Pallas was followed by that of Juno, by Harding, at Lilienthal 1804; and of Vesta, by Olbers, at Bremen in 1807.
The organised search for minor planets was relinquished in 1816, presumably because no more planets seemed to be forthcoming, and it does not appear that any further attempts were made by anybody till about 1830, when a Prussian amateur, named Hencke of Driessen, profiting by the publication of some new star maps put forth by the Berlin Academy, commenced a methodical search for small planets. These Berlin maps, one for each hour of R. A., were only completed in 1859, and, therefore, Hencke had only a small number of them at his command during the early years of his labours. Still it is strange that 15 years elapsed before his zeal and perseverance were rewarded, his first discovery, the planet Astræa, not taking place till December 1845. Once however the ice was broken new planets followed with considerable rapidity, and beginning with 1847, no year has elapsed without several or many having been found. During the last decade the number detected annually has been very great—sometimes as many as 20 in a year, but this has been the result of photography being brought to bear on the work. It is obvious that if a photograph of a given field taken on any one day is compared with a photograph taken a few days earlier or later, and any of the objects photographed have moved, their change of place will soon be noticed and will be a distinct proof of their planetary nature.
It seems quite certain that all the larger of these planets have now been found, for the average brilliancy (and this no doubt means the average size) of those recently discovered has been steadily diminishing year by year, and it looks as if the limit of visibility will soon be reached, if it has not been reached already.
The three largest of these bodies, in order of size, have generally been thought to be Vesta, Ceres, and Pallas; but Barnard, from observations made in 1894, concluded that Ceres is 520 miles in diameter; Pallas, 304 miles; and Vesta, 241 miles. As to all the rest of the minor planets, excepting Juno, Hornstein is of opinion that those having a greater diameter than 25 geographical miles are few in number, and that the majority of them are no larger than from 5 to 15 miles in diameter.
From what has gone before the reader will readily infer that these minor planets are of no sort of interest to the casual amateur who dabbles in Astronomy; and indeed that they are of very little interest to anybody. With a few general statistics, therefore, this chapter may be concluded. The total number of minor planets now known nearly reaches 500, and every year increases the list; but not, however, at as rapid a rate as was once the case, because the German mathematicians, who alone latterly have been willing to trouble themselves with the computation of the orbits, are understood to have announced that they are no longer able to keep pace with the discoveries made. Those who care to investigate in detail the circumstances of these planets will find great extremes in the nature of the orbits. Whilst the planet nearest to the Sun has a period of only 3 years, the most distant occupies nearly 9 years in performing its journey round the Sun. So, also, there are great differences in the eccentricities of the orbits and in their inclinations to the ecliptic. Whilst one planet revolves almost in the plane of the ecliptic, another (Pallas) has an orbit which is inclined no less than 34° to the ecliptic. One word, in conclusion, as to the names applied to these bodies. At the outset the names given were, without exception, chosen from the mythologies of ancient Greece and Rome, but, latterly, the most fantastic and ridiculous names have in many cases been selected, names which in too many instances have served no other purpose than that of displaying the national or personal vanity of the astronomers who applied them to the several planets. The French are great offenders in this matter.