444 ASTRONOMY. 



sequently, the inhabitants of that part will have a 

 partial eclipse. 



As the moon is not always at the same distance 

 from the earth, it sometimes happens that the coni- 

 cal dense shadow does not reach the earth, as in 

 Plate 24. Fig. 4., and only the penumbral sha- 

 dow falls upon it; this eclipse, consequently, is 

 partial to every part of the earth. Those who are 

 at the centre of the penumbra will lose sight of the 

 centre of the sun, by the interposition of the 

 moon's body, which subtending a smaller angle 

 than the sun, will not entirely cover its surface : 

 so that there will be a ring of light. The eclipse 

 is then said to be annular. 



The satellites, or moons, are often eclipsed by 

 the planets to which they belong. The eclipses of 

 Jupiter's moons are observed with great atten- 

 tion, as they are very useful in ascertaining the 

 longitude. 



When any of the planetary bodies disappear by 

 another coming before it, it is called an occultation. 

 The occultations of the fixed stars by the moon 

 are of great use in determining the longitudes of 

 places. 



Of the Tides. 



The ebbing and flowing of the sea were first 

 shown by Kepler to be owing to the moon's attrac- 

 tion : and Newton demonstrated it upon the prin- 

 ciples of gravitation. The attraction of the moon 

 cannot alter the shape of the solid of the globe, 

 but it has a considerable effect upon the fluid part, 

 which it causes to assume a spheroidal figure, the 

 longest axis being in the direction of the moon. It 

 is, therefore, the highest tide at that place perpen- 



