The gap being thus filled up at the distance 28 by the discovery of Ceres, no one thought that other planets might exist there; and if Piazzi had supposed so, he might have at once discovered a dozen of the small bodies which revolve in this region. An astronomer of Bremen, Olbers, observed this planet on the evening of March 28, 1802, when he perceived in the constellation of the Virgin a star of the seventh magnitude which was not marked on Bode’s chart, which he used. The following day he found it had changed its place, and recognized by this fact that it was a second planet. But it was much more difficult to give citizenship to it than to its elder sister, because, the gap being filled up, it was not required, and it was more inconvenient than agreeable. They looked upon it, then, as a comet until its motion proved that it revolved in the same region as Ceres at the distance 2.77, and in 1,685 days (the period of Ceres is 1,681 days). They gave it the name of _Pallas_. The unexpected discoveries of Ceres and Pallas led astronomers to revise the catalogues of stars and celestial charts. Harding was of the number of the zealous revisers. He was soon rewarded for his trouble. On September 1, 1804, at ten o’clock in the evening, he saw in the constellation of Pisces a star of the eighth magnitude which was not noted in the _Histoire Céleste_ of Lalande. On September 4, he found it had perceptibly changed its place: it was a new planet. It received the name of _Juno_. Its distance from the sun is expressed by the number 2.67, and its revolution is performed in 1,592 days. After these three discoveries, Olbers, noticing that the orbits of these planets crossed each other in the constellation of the Virgin, advanced the hypothesis that they might be nothing else but fragments of a large shattered planet. Mechanics show that, in this case, the fragments would again pass every year—that is to say, at each of their revolutions—through the spot where the catastrophe took place. Olbers then set himself to explore the constellation Virgo carefully, and found on March 29, 1807, a fourth small planet, to which he gave the name of _Vesta_. Its distance is but 2.36, and its revolution only 1.326 days. This is the brightest of the small planets, and it is sometimes seen with the naked eye (when we know where it is), like a star of the sixth magnitude.