PHENOMENA OF LIGHT 173 



By use of a sheet of paper moved back and forth in the 

 shadow cast by the ball, determine (a) the form of the shadow 

 region ; (b) its extent outward (length) ; (c) the form of 

 cross section at right angles to the direction of the shadow ; 

 (d) where the base of the shadow is. Leave the screen 

 supported in some way in the path of the shadow so that 

 cross sections of both umbra and penumbra show upon it. 



4. Making use of a small marble to represent the moon, 

 cause it to move round and round the ball in a path (orbit) 

 that at times passes (a) through the umbra of the shadow; 

 (b) only in part into the umbra ; (c) through the penumbra 

 only, or wholly outside the shadow in the region of illumi- 

 nation. 



State the conditions that apparently must exist (a) for 

 total eclipses of the moon ; (b) for partial eclipses ; (c) for 

 failures to have a lunar eclipse every month. From which 

 side of the earth with reference to the sun are eclipses of 

 the moon to be seen? During which part of the twenty- 

 four hours will this be for the observer? 



5. As the marble (representing the moon) is moved 

 around the ball, try to visualize (a) the revolution of the 

 real moon about the earth, and in a path such that it comes 

 between the sun and an observer ; (b) the onward sweep of 

 the moon's shadow across the earth's surface as it approaches 

 the observer, envelops him in an "eclipse of the sun" and 

 passes on to the eastward beyond him. In so-called eclipses 

 of the sun, what really suffers eclipse, i.e., passes into 

 shadow? What must be true of the duration of a solar 

 eclipse at any station? 



In what part of the moon's shadow must an observer be 



