ASTRONOMY. 



different times. These moons are some- 

 times eclipsed by the shadow of Jupiter 

 falling 1 upon them; and the eclipses have 

 been found of great use in determining 

 the longitudes of different places on the 

 earth: E-v. Suppose two observers of an 

 eclipse, one at London, the other at the 

 Cape of Good Hope, the eclipse will ap- 

 pear at the same instant of time to both ; 

 but being situated under different meri- 

 dians, they count different hours, accord- 

 ing to which the difference of their longi- 

 tude is found. Thus, if an emersion of a 

 satellite is observed at London 9 h 33' 12", 

 and at another place 10 h 46' 45", the dif- 

 ference of time is l h 13' 33" ; of course 

 that other place is 18 23' 15" east of 

 London. 



The eclipses of Jupiter's satellites have 

 been applied also to measure the velocity 

 of light: by comparing the times of the 

 apparent entrance and emersion of the 

 satellites with tables calculated for the 

 mean distances of the earth from the sa- 

 tellite, the visible emersion at the least 

 distance is found to happen about eight 

 minutes sooner ; and at the greatest dis- 

 tance about eight minutes later than the 

 tables .- consequently, a ray of light is 

 about 16 minutes in passing through the 

 earth's orbit, or eight minutes in coming 

 from the sun to the earth. If therefore 

 the distance be 95,000,000 of miles, the 

 velocity of light per second is equal to 



95,000,000 10Q+l , ., . 



198 thousand miles in a se- 



8x 60 

 cond nearly. 



Saturn can hardly be seen by the naked 

 eye. When examined by a telescope, it 

 exhibits a very remarkable appearance. 

 It is surrounded by a thin, flat, broad, lu- 

 minous ring, which surrounds the body of 

 the planet, but does not touch it. This 

 ring casts a strong shadow upon the pla- 

 net, and is divided into two, by a distinct 

 line in the middle of its breadth. The 

 rings are circular, bvit appear elliptical 

 from being viewed obliquely. 



According to Dr. Herschel, the dimen- 

 sions of the rings, and the space between, 

 are as follows. 



Miles. 

 Inner diameter of the smaller 



ring 146,345 



Outside diameter of ditto . . 184,393 

 Inner diameter of the larger ring 190,248 

 Outside diameter of ditto . . 204,883 

 Breadth of the inner ring . . 20,000 

 Ditto . . . outer ring . . 7,200 

 Ditto of vacant space .... 2,839 



Besides this ring, Saturn has seven 

 moons of different sizes, and its body is 

 surrounded also by belts, like those of 

 Jupiter. 



The Herschel planet, with its six satel- 

 lites, have been entirely discovered by 

 Dr. Herschel. It cannot be seen without 

 a telescope, but it does not require a 

 powerful one. The satellites cannot be 

 seen without the most powerful tele- 

 scopes. We shall subjoin in the opposite 

 page a table, which will contain a number 

 of particulars relating to the planets, that 

 will be found of great utility to the reader 



