APPLETONS' 



POPULAR SCIENCE 



MONTHLY. 



JANUARY, 1900. 



ADVANCE OF ASTKOXOMY DUKIXG THE IsllsE- 

 TEENTH CENTURY. 



By Sib ROBERT BALL, 



LOWNDEAN PEOFE980E OF ASTEOKOMY AT TUB TJNIVEE6ITY OF CAMBRIDGE, ENGLAND. 



ONE of the most remarkable chapters in the astronomy of the 

 past century was commenced on the very first night with 

 which that century began. It was, indeed, on the 1st of January, 

 1801, that the discovery of a new planet was announced. The five 

 great orbs — Jupiter, Saturn, Mercury, Mars, and Venus — had been 

 known from the earliest times of which we have records, and the 

 planet Uranus had been discovered nearly twenty years before the 

 previous century closed. The solar system was thus thought to 

 consist of these six planets and, of course, the earth. On the 

 memorable night to which I have referred, Piazzi, the astronomer, 

 made a remarkable advance. He discovered yet another planet — 

 the seventh, or eighth, if the earth be included. The new body 

 was a small object in comparison with those which were previously 

 known. It was invisible to the unaided eye, and seemed no more 

 than a starlike point even when viewed through a telescope. It 

 revolved around the sun in the wide region between the orbits of 

 Mars and Jupiter. This discovery was speedily followed by others 

 of the same kind, and, as the century has advanced to its close, the 

 numbers of these planets — asteroids, as they are generally called — 

 has been gradually increasing, so much so that now, of these little 

 bodies known to astronomers, the number amounts to about four 

 hundred and fifty. 



But just as the beginning of the century was heralded by the 

 discovery of the first of these asteroids, so the close of the century 



VOL. LTI. — 23 



