THE MOON, PLANETS, AND COMETS 259 



11,000 miles across. The inner edge of this ring is about 

 6000 miles from the surface of the planet. Although these 

 rings are of vast extent in the plane of the equator they 

 are very thin, and when they are seen edgewise to us they 

 are barely visible even in the largest telescopes. It is cer- 

 tain that they are not more than 100 miles thick, and probably 

 not more than 50. 



262. The discovery of Uranus. Only six planets were known 

 until the year 1781, when William Herschel, a German musi- 

 cian who had migrated to England, discovered the planet 

 Uranus. Herschel secured a book on astronomy and became 

 greatly interested in the science. He immediately resolved to 

 procure a telescope and explore the heavens for himself. In 

 those days (about 1760) telescopes could not be purchased as 

 at present, and it was necessary for Herschel to learn to make 

 them. He first studied mathematics in order to understand 

 the theory of making the curved surfaces for his mirrors. In 

 spite of this difficult study, which was necessary before he 

 could begin to construct a telescope, he mastered the task. 

 He then began to grind mirrors, making one after another, 

 each better than its predecessor. With a telescope of his 

 own make, on an evening in 1781, Herschel detected an ob- 

 ject in the sky which was invisible to the unaided eye and 

 which appeared slightly different from a star. After follow- 

 ing it a few nights he found that it was moving with respect 

 to the stars, which proved that it was a member of the solar 

 system. At first he thought it was a comet, but in a few 

 weeks it was determined that it was a previously unknown 

 planet. This aroused the greatest interest, not only among 

 scientific men but among people in general. Herschel was 

 recognized by King George III and made director of the 

 Royal Observatory of England. 



The planet Uranus has a diameter of a little more than 

 30,000 miles. Four moons have been discovered revolving 

 around it in planes which are almost at right angles to the 



