THE SUN AND HIS FAMILY 



183 



Uranus. All the planets described thus far have 

 been known for many centuries. It was thought that 

 Saturn was the most distant planet until 1781, when 

 William Herschel, a German musician, discovered 

 Uranus. 



Because of the great distance of Uranus from us, 

 it has been difficult to make observations of its fea- 

 tures. Iteeems, however, to resemble Saturn and Jupi- 

 ter. Four moons are known to' revolve about this 

 planet. 



Neptune. The discovery of Neptune is one of the 

 most dramatic events in the history of science. Uranus 

 was discovered accidentally, but Neptune was discov- 

 ered by the aid of a mathematician in his study. Soon 

 after the discovery of Uranus the size and position of 

 its orbit were determined and its motion was carefully 

 followed by observers. As years went by, astronomers 

 noticed to their surprise that Uranus did not stay in 

 its computed orbit. By 1820 it was suggested that the 

 peculiarities in the motion of Uranus might be due to 

 the gravitational attraction of some unseen, distant 

 body, and the problem was to calculate where the 

 trouble maker was. Because of the great mathematical 

 difficulties involved, some astronomers thought the 

 problem could not be solved. But the problem was at- 

 tacked and solved independently in 1846 by Adams, 



Courtesy Mount Wilson Observatory 



FIG. 301. TWELVE EXPOSURES OF SATURN 



an Englishman, and Leverrier, a Frenchman. Al- 

 though Adams finished his work first, the work of Le- 

 verrier led to the discovery of Neptune. Leverrier 

 communicated his work to Galle, a German astrono- 

 mer who in the evening of February 23, 1846, pointed 

 his telescope toward the sky and found the planet 

 Neptune in almost the exact place Adams and Lever- 

 rier had predicted it would be. The discovery caused 

 a sensation. 



Neptune is about 2,800 million miles from the sun 

 and because of its great distance from the earth can 

 be seen only through a telescope. Even the large tele- 

 scopes do not reveal its surface clearly. It is approxi- 

 mately 36,000 miles in diameter. It moves in its orbit 

 at the rate of three and one third miles a second and 

 requires about 164 years to revolve in its orbit around 

 the sun. 



Pluto. After the discovery of Neptune in 1846 as- 

 tronomers considered the possibility of the existence, 

 of planets still more remote from the sun. Percival 

 Lowell, an American astronomer, became intensely 

 interested in the problem of finding more planets, and 

 he eventually calculated mathematically the probable 

 position of an unknown body. After twenty-five years 

 of searching, this planet was finally located on Janu- 

 ary 21, 1930, by C. W. Tombaugh, a young astronomer 

 working in the Lowell astronomical observatory. The 

 planet was found in almost the exact direction that 

 Lowell predicted it would be. 



The orbit of Pluto is different from those of the 

 other planets. The orbit is inclined to the plane of the 

 orbits of the other planets. The diameter of Pluto is 

 estimated to be -much less than that of the earth. Not 

 much is known about its physical characteristics. 



Exercise. From your study of the conditions on the 

 various planets as now known by the astronomers, list 

 those planets upon which there might or might not be 

 life, giving supporting reasons in each case. 



What are the characteristics of other members of 

 our solar system? The planetoids or asteroids. Look- 

 again at Figure 298 and notice the wide space between 

 the orbits of Mars and Jupiter. Notice also the hun- 

 dreds of small bodies revolving about the sun in this 

 space. These bodies are called planetoids by some 

 writers and asteroids by others. 



Most of the planetoids are very small in compari- 

 son to the planets. Only a few of them have diameters 

 exceeding one hundred miles. Astronomers are not yet 

 able to determine the masses of the planetoids, but 

 certain facts indicate that they are solid bodies. 



Comets. All the members of the solar system that 

 we have considered thus far move in more or less cir- 

 cular orbits around the sun, and they all have spheri- 

 cal or nearly spherical shapes. Comets are also mem- 

 bers of our solar system because they move under the 



