LIBRAI 



THE MINOR PLANET MT 191 1: AND ON MINOR\J$ 



PLANETS IN GENERAL. *y^lA£ 



By Robert T. A. Innes, F.RA.S. 



It is commonly known that the number of minor planets 

 \or asteroids as they were called by Sir William Herschel) is 

 \ ery large and constantly increasing, and as the Union Observa- 

 tory devotes some considerable portion of its time to these small 

 bodies, it will not be unfitting- to discuss briefly the present state 

 of our knowledge of these minute astronomical objects. 



Before any minor planet was known, the existence of at least 

 one had been speculated upon. If the orbits of the six planets 

 known to the ancients are plotted, it is seen that the gap between 

 those of Mars and Jupiter is larger than a certain symmetry 

 required. The idea that such a planet existed with its orbit 

 between those of Mars and Jupiter was first mooted by Kepler. 

 Guided by profound, but totally inaccurate wisdom, the philoso- 

 pher Kant explained the disproportionate magnitude of the 

 interval as due to the large mass of Jupiter, the zone in which 

 each planet moved being the storehouse from which it was built 

 up. The so-called Bode's law which was discovered by Titius. 

 followed as it was by the discovery of Uranus, whose distance 

 from the Sun agreed well enough with it, clearly indicated that 

 a planet should be looked for between the orbits of Mars and 

 Jupiter ; and this opinion got to be held so firmly that, late in 

 the eighteenth century, a co-operative scheme for its search was 

 formed, and one of the chosen observers was Piazzi. But before 

 Piazzi heard of his appointment two remarkable events occurred. 

 One was the qualification of the since world-renowned philoso- 

 pher Hegel as a private-do sent at the Jena University. His dis- 

 sertation was on the planetary orbits, and again pure philosophy 

 was able to state that it might be a mistake to look for a planet 

 between Mars and Jupiter. This weighty pronouncement would 

 no doubt have stilled astronomical activity, but, unfortunately 

 for the philosopher, a planet had already been discovered by 

 Piazzi. the tardiness of postal communications in those days being 

 just sufficient to let Hegel's publication and appointment pre-date 

 the news of Piazzi's discovery of some months before by a few 

 days. The first minor planet, Ceres, was thus discovered by 

 Piazzi on January 1, 189 1. It created a profound sensation, not 

 unmixed with alarm lest this addition to the solar family should 

 become " lost " before its orbit could be calculated. As is well- 

 known, the mathematician. Gauss, earned his spurs by computing 

 the orbit. It will be seen later how history repeated itself in 

 the case of the planet 191 1 MT. Three other minor planets, all 

 travelling between the orbits of Mars and Jupiter, were found 

 by 1807. I think astronomers then considered the zone exhausted. 

 No additions were made until 1845, when Hencke discovered 

 Astrea ; henceforward discoveries were made every year until, at 

 the end of November, 1891, no less than 322 of these bodies 



