Astronomy. [Lecture 18. 



the observance of them has produced a discovery 

 of some importance. It was early observed that 

 they ceased to be visible at certain intervals, and 

 again at stated periods reappeared. The apparent 

 motion of the spots is from the eastern to the 

 western side of the sun ; and as they are observed 

 to move quicker when they are near the central 

 region than when they are near the limb, it fol- 

 lows that the sun must be a spherical body, and 

 that he revolves on his axis from west to east. 

 The time in which he performs this revolution, 

 as observed by Cassini, is twenty-five days, 

 fourteen hours, and eight minutes; and from 

 the time of the motion of the spots, which is 

 sometimes straight, but more frequently curved 

 or elliptical, it is discovered that his axis is not 

 perpendicular to the plane of the ecliptic, but 

 inclined to it, so as to make an angle with the 

 perpendicular of about seven degrees and a half. 

 The zodiacal light, as it is called, is another 

 striking phenomenon connected with this glo- 

 rious luminary. In explaining it the sun is 

 supposed to be enveloped with a fluid matter, 

 luminous in itself, or only enlightened by the 

 solar rays, and which constitutes a higher atmo- 

 sphere. This matter is more abundant and 

 more extended round his equator than else- 

 where, and gives to the solar atmosphere an 

 appearance resembling that of a double convex 

 lens, the diameter of which is in the plane of the 

 sun's equator. It was discovered in 1683, by 



