ECLIPSES. 



BY JACOB B. BROWN. 



The calculation of eclipses is a difficult and recondite 

 matter which the present article has no intention of attacking. 

 From the known intricacy of the calculations it is generally 

 supposed, by those who have not much occasion to think 

 about it, that the physical explanation of an eclipse is in like 

 manner involved and difficult. The contrary is the fact, and 

 an attempt will be made to show as much. 



When the earth and the sun are both in the plane of the 

 ecliptic, the opaque and spherical earth intercepts the rays of 

 the sun and casts a conical shadow — conical because the sun 

 is so much larger than the earth. 



There may be those who do not know, or, at all events, 

 have not reflected that the plane of the ecliptic means neither 

 more nor less than the plane where eclipses take place. It is 

 obvious that in order that one body be eclipsed by another for 

 a third, the three must be in the same straight line, or nearly' 

 so. If two of the bodies be in a given plane, the third must 

 be in the same plane. The moon can eclipse the sun when 

 she is "in her nodes" or passing through the plane of the 

 ecliptic, and at no other time; for the sun and the earth are 

 there already as matter of course. There can be a transit of 

 Venus when Venus is in her nodes, or crosses the plane of the 

 ecliptic, and only then ; for the transit is a kind of eclipse. 

 And the conical shadow cast by the earth is always on the 

 surface of the ecliptic plane, and at interv^als exactly calcul- 

 able, catches and more or less completely darkens the slow 

 revolving moon. 



This does not mean that all eclipses take place in the 

 ecliptic plane. There are those seen from the earth with 

 which the sun and moon have no connection. 



* Through the kimhiess of Miss Anne Knapp Whitney, the Institute 

 is enabled to pulilish this article from the pen of its deceased member, 

 Jacob 1>. lirown. 



