On the Difappearance of the Ring of Saturn. 223 



vented a very ingenious kind of projeftion, a reprefentation 

 of which was given by profeffor Bode, of Berlin, in his Aftro- ' 

 nomical Ahnanac for 1789, in which year thefe phaenomena 

 took place in the fame manner as they will do in 1803 ; and 

 this reprefentation has been repealed in the fame work for the 

 prefent year: but as this diagram, in my opinion, is too dif- 

 ficult for thofe not well verfed in aftronomy, I flatter mylclf 

 that the following figure will render the phsenomenon ealier 

 to be miderllood by amateurs ; though Lambert's method is 

 more exaft, and better fuited to aftronomers. 



The circle (Plate IV. fig. i.) reprefents the earth's orbit, 

 and exhibits the points where the earth is placed at the com- 

 mencement of each month. The vernal equinox is marked 

 OT, the line SZ drawn through the fun; and the place of 

 Saturn on the 15th of June is the dire£lion of the plane of 

 the rino;, Saturn, at the four periods announced, is in the 

 lines drawn parallel to SZ, though at a diftance from the 

 fun five times greater than is here reprefented for want of 

 room. The plane of the paper is that of the ecliptic, with 

 which the one-half of Saturn's ring makes an angle of 31°. 



I have already obferved that the ring twice difappears, and 

 then twice becomes aojain vifible. This phaenomenon arifes 

 partly from the fituation of Saturn in regard to the fun, and 

 partly from that of the earth ; and each of thefe caufes muft 

 be confidered fcparatelv to have a clear idea of thefe phaeno- 

 mena. In regard to the firft, it is evident that that fide only 

 of Saturn's ring which is turned towards the fun can be illu- 

 minated ; and this illumination is fl^rongeft when the planet 

 is in 20^ of n and 20'' of f , the whole furface of the ring 

 being then oppofite to the fun. Now as the direftion of the 

 ring'is always the fame, that is to fay, parallel to itfelf, 

 the rays of the fun, as the planet approaches to 20° of nj» 

 and K, mull fall more obrKpiely on the ring, by which 

 means it will be more faintly lighted, and at the fame time 

 appear fmaller; and when Saturn has attained to this longi- 

 tude, no part of the upper furface, but merely the edge of the 

 ring, the plane of which palTes through the fun, will be illu- 

 minated ; fo that the ring will then have the appearance of a 

 thin luminous line, but will not be vifible without the help 

 of good glaffes. For the lall feven years the ring has been 

 obterved to decreafe in light and in breadth ; and this de- 

 creafe will continue till the 15th of June, after which both 

 will again increafe. During the lafi fifteen years the foutliern 

 furface of the ring has been illuminated, but in the next fif-- 

 teen the northern furface alone will receive the rays of the 

 fun. In whatever part of our folar fyfleni this jdiicnome- 



iiun 



