| Asteroids |
|
|
The asteroids, or minor planets, are a vast host of very small planets that revolve around sun in orbits that nearly all lie between those Mars and Jupiter, through a few of them transgress these limits. The existence of planet between Mars and Jupiter was suspected before any were found on the basis of an empirical law of planetary distances first put forward by J.D. Titius, a contemporary of Kepler, through it attracted more notice when Johan E. Bode restated it in 1772. The law assigned the following numbers as representing the distances very closely: Mercury 4; Venus 7; earth 10; Mars 16; (blank) 28; Jupiter 52; Saturn 100; (next planet) 196. It will be seen that, except in the first case, each interval is double the preceding one. When in 1781 Sir W. Herschel discovered Uranus, which fitted exactly with the next term in the series after Saturn, conviction was strengthened of the existence of a planet in the gap, and a society of 24 astronomers, with Baron von Zach at its head, was formed to devote itself to the search. Giuseppe Piazzi, director of the Paleumo observatory, was not a member of the society, but on Jan. 1, 1801, while examining a region in Taurus, he observed a small star which he had not sees there before; he soon found that it was moving, and followed it until Feb. 11. In the interval it cased retrograde motion, and began to advance. In the following autumn difficulty was experienced in finding the body again; this served as an incentive to the mathematicians Carl Friedrick Gauss ton improve the existing methods of computing orbits from a few observed positions. As a result of Gauss’s calculations Heinrich W. M. Olbers of Bermen recovered Piazzi’s planet just a year after its discovery. (The three astronomers associated with the finding of the first asteroid have been commemorated by naming the planets numbered 1000, 1001 and 1002 Piazzia, Gausia and Olbersia.) The new planet was named Ceres; its distance agreed exactly with value predicted by the Titus-Bode law, but it was very small (modern measurements give a diameter of 480 mi., about one-fifth of the moon’s) and its orbit was inclined to the ecliptic at the large angle of 10º 37'. Perhaps these circumstances led Olbers to suspect that was one of a group of a small planets, for he continued to sweep the sky after he had recovered it, and three months later he found Pallas, whose distance from the sun proved to be almost the same as that of Ceres. Its orbit was inclined at the very large angle of 34º 43', and the eccentricity was also large. The third and fourth members, Juno and Vesta, were added to the family within six years by C. L. Harding and Olbers respectively. Vesta is the brightest of whole family, some times attaining visibility to the naked eye, but its diameter is only half that of Ceres. It was natural that the discovery of these four little planets, revolving in closely adjacent orbits, should suggest the idea that a larger orb had been rent asunder by explosion. This idea held the field for many years, and indeed was revived in modern times in a modified form. It seems to have been assumed that these four fragments completed the set, and the search was abandoned until 1830. It was then renewed by K.L. Hencke of Drissen who, after 15-yr, found Astraea and began a chain of discoveries that has continued without intermission. By 1890 about 300 small planets had been found, all by visual search at the telescope, a very laborious method, necessitating the charting or memorizing of numbers of faint starts and searching for strangers among them. In 1891 Max Wolf of Königstuhl Heidelberg, introduced the photographic method of search. An equatorial telescope with a photographic plate in the focus was made to follow the stars, which registered as disks, while as asteroid appeared as a short trail owing to its motion during the exposure. This method produced a great acceleration in the rate of discovery, and over 3000 announcements of discoveries were made between 1890 and 1895. Not all of this have been permanently numbered. When number of asteroids grew large there was found to be a danger of mistaking previously discovered asteroid for a new one, and increasingly rigorous conditions were set up that most be satisfied before the asteroid is numbered. In the late 1950’s there were over 1600 number asteroids in the ephemeris. A new asteroid most have been observed during three oppositions to qualify for numbering. Even so, misidentifications in the published records have occurred and a systematic campaign during a decade 1948-58 was necessary to clear up most of the ambiguities. About 15 of the other asteroids were then still unaccounted for; most of them are probably identical with more recently numbered objects. Some views of asteroids: |