144 



THE MINOR PLANETS. 



ard us, and he is invisible, except in the case which sometimes occurs, in 

 which he is so exactly in line of the direction of the sun as to be between the 

 eye and some portion of the solar disk. In that case the planet is seen as a 

 circular black spot on the dislf of the sun, and the appearance of its motion 

 upon that disk is called a transit of Mercury, 



When the planet, on the other hand, is on the opposite side of the sun, at 

 B, its illuminated hemisphere is presented directly in the line of vision ; but 

 in that case, the planet being in exactly the same quarter of the heavens as the 

 sun is, would necessarily rise and set with the sun, and its appearance being 

 obscured by the immeasurably superior splendor of the sun, it would not be 

 seen. When the planet is in an intermediate position on either side of the sun 

 in its periodical course, its illuminated hemisphere being presented as it always 

 is, directly to the sun, will only be partially turned to the earth, and the planet 



