180 OUTLINES OF NATURAL PHILOSOPHY. 



SECT. X. 



. 



OF THE SECONDARY PLANETS. 



179- JUPITER, when viewed through a tele- 

 scope, is found to be accompanied by four small 

 stars, ranged nearly in a straight line, parallel 

 almost to the plane of the ecliptic ; and occa- 

 sionally on the same, or on opposite sides of the 

 planet. 



a. Sometimes these small stars pass between us and 

 Jupiter, and their shadows are then seen traver- 

 sing his disk. Hence it is evident, that both Ju- 

 piter and they are opaque bodies, which derive 

 their light from the Sun. 



#. Jupiter, accordingly, projects behind him a coni- 

 cal shadow, in which the little stars just mention- 

 ed are often immersed ; so that they disappear, 

 and are eclipsed, as the Moon is, by falling into 

 the shadow of the Earth. 



c. Hence these little stars are to be considered as 

 moons, which circulate round Jupiter; and as 

 they are always observed to move eastward when 

 they are eclipsed, and westward when they pass 

 over the disk, it is evident that their motion is 



progressive, 



