ASTRONOMY. 147 



148. When an opaque body is opposed to one 

 which is luminous, there is a certain space be- 

 hind the former body, from which the latter is 

 only partially visible. This space is called the 

 Penumbra of the opaque body. 



a. If the bodies are spherical, the penumbra is a cone, 

 having for its angle the line joining the centres of 

 the two bodies ; and for its vertex, the point in 

 that line where tangents to the opposite sides of 

 the two bodies intersect one another. 



b. Half the angle of the conical penumbra of the 

 moon, is equal to the apparent semidiameter of the 

 sun, plus the angle which the moon's semidiame- 

 ter subtends at the sun. 



149. To conceive the phenomena of a solar 

 eclipse in general, we may consider the section 

 of the moon's penumbra as advancing on the 

 earth from the west, and as being viewed by an 

 observer in the moon, in the same manner that 

 an eclipse of the moon is viewed by an obser- 

 ver on the earth. 



The observer might be placed any where, providing 

 he saw the whole of that side of the earth which is 

 turned to the sun 5 but it is most convenient to 

 suppose him in the moon, that the arches, compa- 

 red 



