2 OUR PHYSICAL WORLD 



moon, a welcome relief from the fearsome gloom. He was prone 

 to identify all these things as the dwelling-places or the very 

 incarnations of his gods, easily believing that they exerted a 

 potent influence for good or evil over his daily life. So astron- 

 omy, or its earlier prototype, astrology, is the oldest of the 

 natural sciences. 



The early astrologers knew most of the planets, too, as bril- 

 liant bodies that are not fixed in their positions with relation to 

 each other as are the stars, but are constantly changing their 

 locations, apparently pursuing somewhat erratic courses among 

 the stars. Indeed, the Greek word from which the name planet 

 is derived means "a wanderer." The paths of these wandering 

 bodies the ancients knew with remarkable accuracy and they 

 even foretold their positions with certainty. The names of these 

 bodies still indicate their identification with the ancient gods. 



These planets we now know revolve about the sun. The earth 

 is simply one of them. In order they are: Mercury, Venus, the 

 earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury 

 is nearest to the sun, Neptune the farthest. Uranus and Neptune 

 were unknown to the ancients, for they are visible only with the 

 aid of the telescope. Uranus was discovered by accident by 

 Sir William Herschel in 1781, while he was making a systematic 

 survey of all the stars. The size, motion, and position of Nep- 

 tune were calculated before its discovery, for Uranus did not 

 move as it should, and astronomers felt certain it must be influ- 

 enced by some, as yet, undiscovered planet. Adams and 

 Leverrier, respectively an English and a French astronomer, 

 made the very difficult calculations to determine its position, 

 and Galle, a German astronomer, was the first to see it, Septem- 

 ber 23, 1846. 



Between the orbits of Mars and Jupiter there are more than 

 500 small bodies, similar to planets except for their size. They 

 also revolve about the sun. These are known as planetoids. 

 The first of them was discovered the first day of the nineteenth 

 century by Piazzi at Palermo, Italy. 



