144 OUTLINES OF NATURAL PHILOSOPHY. 



guished through the shade ; the nearer the 

 moon happens to be to the earth, or the farther 

 she is from the apex of the shadow, the dark- 

 ness is the greater. 



The light that, by the refraction of the earth's atmo- 

 sphere, is made to enter within the limits of ihe 

 conical shadow, is no doubt the cause of this phe* 

 nomenon. 



In some instances the moon has disappeared entirely, 

 as in that mentioned by KEPLER in June 1620. 

 HEVELIUS has taken notice of another, where the 

 moon could not be seen even with a telescope, 

 though the night was remarkably clear. 



146. As an eclipse of the moon happens at 

 the same instant of absolute time to all obser- 

 vers, it is one of the phenomena, from the ob- 

 servation of which the longitudes of places may 

 be most directly inferred, 67* 



On account of the ill-defined boundary of the sha- 

 dow, this method of ascertaining the longitude 

 does not admit of great precision. It is difficult 

 to determine the beginning or end to less than a 



/* . oo 



minute of time. 



The arrival of the boundary of the shadow, at the dif- 

 ferent spots, can be more accurately ascertained 

 than the beginning or end ; and, therefore, as many 

 5 observations 



