Zodiacal Light. 285 



contrary the north at his left, and the south at 

 his right. It is plain, from what has been said, 

 that the part of the zodiacal light which is near 

 the sun cannot be seen upon the horizon, because 

 the twilight will cause it to disappear, or at least 

 render its borders very indistinct. It is only 

 a total eclipse of the sun which can show it at 

 the base, and in its densest part ; in that case, 

 as soon as the disc of the moon has completely 

 obscured that of the sun, there appears round 

 the moon an enlightened border, and a kind of 

 beam ; it is more or less dense, according to its 

 distance from the edge of the moon. 



The zodiacal light must be more easily and 

 more frequently perceived in the tropical cli- 

 mates, and particularly near the equator, than it 

 can here; first, because in those parts the ob- 

 liquity of the equator and the zodiac to the 

 horizon is less ; and secondly, because the dura- 

 tion of the twilight is much shorter. This curious 

 light was observed by Cassini in 1683 ; and there 

 is reason to suspect that earlier astronomers ob- 

 served it, but did not describe it with sufficient 

 precision. 



