MEASUREMENT OF LIGHT 105 



tance from it. If the light is shut off from the moon, or from 

 any of the planets, those planets cannot be seen. We know 

 that sometimes the earth passes between the sun and moon, 

 and casts its shadow on the moon, rendering part of the latter 

 invisible. We call this shadow over another heavenly body 

 an eclipse. In the same way we can have eclipses of planets, 

 but because they are so small, they are not noticed except 

 through a telescope. There is one other kind of eclipse, which 

 is the solar eclipse. This is caused by the moon passing be- 

 tween the earth and the sun. In that condition the shadow 

 is upon us, and, if there were people on the moon, they would 

 see that the earth was partially eclipsed. 

 References : 



1. 1002 : 273-275. Conditions for Eclipses. 



2. 1002 : 284-287. Uses of Lunar and Solar Eclipses. 



3. 1304 : 2. Shape of Earth Shown by Lunar Eclipse. 



4. 1803 : 391. Shadow and Eclipses. 



a. 1001 : 161-174. Eclipses Kind and Number. 



6. 1003 : 123-132. Eclipses of the Moon and Sun. 



c. 1004 : 171-181. Eclipses of the Sun and Moon. 



d. 1309 : 18-19. Eclipse of the Moon. 



e. 1312 : 7-8. Eclipses. 



/. 1808 : 375. Umbra and Penumbra. 



g. 1809:276. Shadows. 



78. MEASUREMENT OF LIGHT 



Light can be measured by the direct illumination which it 

 can produce, or by the shadows which it casts. The inten- 

 sity of illumination is estimated in candle power, and the 

 standard unit of measurement is a sperm candle, .three- 

 fourths of an inch in diameter, which burns at the rate of 120 

 grains per hour. 



Before measuring light it is necessary to learn that the in- 



