284 Astronomy. [Lecture 18. 



only the superior parts of the phenomenon, 

 with respect to the horizon of the observer, are 

 apparent. For, as the sun rises and approaches 

 the horizon, or again before he has descended 

 many degrees below it, it becomes lost in the 

 twilight. This circumstance is usually thus 

 explained Let IKOA (PL XXVI. fig. 111.) 

 be the zodiacal light in one of the most favour- 

 able positions for observing it, that is about 

 the latter end of February or beginning of March, 

 when the first point of Aries may be supposed 

 in K, upon the plane of the horizon HR, and 

 the sun being in S, about the 10th degree of 

 Pisces upon the boundary CP of twilight, 18 

 degrees below the horizon. The ecliptic TKZ 

 is here confounded with the axis AZ of the 

 zodiacal light, and forms with the horizon an 

 angle of about 64 degrees. The point A of 

 this light falls between the stars of the neck and 

 head of Taurus, and terminates about the 10th 

 degree of Gemini, whence it follows that the 

 distance from its point to its base at the sun is 

 about 90 degrees. 



The same figure represents the situation AEZ 

 which this light would assume, the morning of 

 the same day just before day-break. The angle 

 N t z of the ecliptic with the horizon being 

 about 26 degrees, supposing only that the spec- 

 tator, who had in the evening the north-pole B 

 on his right, and the meridian M at his left, 

 being turned towards the east, shall have on the 



