Fig. 584.—Field of view showing motion of minor planets amongst the stars.
Piazzi was surveying the constellation Taurus, where he fancied he had discovered a change of place in a star which he had observed on the 1st of January in that year (1801). He was quite sure of this change next day (the 3rd of January), and he expressed his opinion to Bode and Oriani. But letters took a long time to pass in those days, and when the other astronomers had received the advices the new star had been lost in the sun’s glory. But after a year, on the 31st December, 1801, the planet was again seen and the discovery was proved. The new planet was named Ceres.
The discovery of Ceres led to other discoveries. For, while searching for her, Olbers found other minor planets, and so on to the present day. Now we have nearly two hundred asteroids, and more are probably to be found in the zone beyond Mars.
It would answer no purpose to give a list of the asteroids. We need only remark that the first four were discovered in quick succession, and then a lapse of thirty-eight years occurred before the fifth was found, thus—
| Ceres, | discovered by | Piazzi at Palermo, January 1st, 1801. |
| Pallas, | ” | Olbers at Bremen, March 28th, 1802. |
| Juno, | ” | Harding at Lillienthal, September 2nd, 1804. |
| Vesta, | ” | Olbers at Bremen, March 20th, 1807. |
| Astræa, | ” | Hencke at Driessen, December 8th, 1845. |
Since 1848 there have been numerous minor planets discovered every year.
The hypothesis that all these asteroids are fragments of one large planet which has been destroyed was started by Olbers; and in confirmation of this view it has been determined that the asteroids have essentially the same character. The orbits of these minor planets are different from the larger “wanderers,” and cross each other, as will be seen from the accompanying diagram, so that a collision may one day ensue.
Fig. 585.—Orbit of asteroids.
Planetoids and extra zodiacal planets are titles which have been bestowed upon these bodies, of which Vesta is the first in order in the system, and revolves in 1,325 days, at a mean distance of 225,000,000 of miles from the sun. Juno and Ceres take each about four of our years to revolve in their orbits, at greater distances still, averaging 260,000,000 of miles. Pallas and Ceres are most alike in their periods and distance from the sun; the principal asteroids are only about 300 miles in diameter, while the smaller are very tiny indeed, and one certainly has quite disappeared.