XIV. PHENOMENA OF LIGHT 



SHADOWS AND ECLIPSES 



The illumination in front of the headlight of a locomotive 

 or of an automobile, or one caused by some powerful search- 

 light, is a familiar sight. The effect is the more startling 

 when the air is filled with minute water particles at times of 

 a fog, or when there is much fine dust or smoke in the air. 

 If into the space thus strongly lit up a large opaque body 

 enters, there will be a region of shadow extending outward 

 from the body and away from the source of illumination. 

 This shadow is the space from which the light is cut off by 

 reason of the opaque body. A cross-section of this shadow 

 region gives a form or outline like that of the body. If a 

 screen is placed outward beyond the opaque body to receive 

 the shadow form (cross-section), and it is moved further and 

 further outward, it will be found that when the opaque body 

 is larger than the light-giving body the shadow reaches 

 outward indefinitely far. The area of its cross-section 

 increases all the time. If the opaque body is smaller than 

 the light-giving body, the cross-section grows smaller and 

 smaller, and the umbra of the shadow has a definite end. 



The astronomer thinks of a body belonging to the solar 

 system, and receiving its light from the sun, as at all times 

 having on the side opposite the sun a shadow region pro- 

 jected outward into space. The shadow of any planet 

 always accompanies it in its course around the sun, as does 

 the shadow of any satellite in its course around a planet. 

 To the astronomer an eclipse of the moon is simply the 

 passage of the moon into the shadow of the earth. In 



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