36o 



PHYSIOGRAPHY. 



[chap. 



(p. 354) that, in consequence of the shape of the earth's 

 orbit, we are not always separated from the sun by the 

 same distance ; being, in fact, much nearer in December 

 than in July. This difference of distance causes a corre- 

 sponding difference of apparent magnitude in the solar disc. 

 The apparent size of an object, as every one knows, varies 

 according to the distance at which it is viewed, so that a 

 halfpenny, held at arm's length, may actually seem larger 

 t^ian the entire sun. 



Suppose that an object is situated at A B (Fig. 117) ; its 

 apparent height will be measured by the inclination of the 

 two lines, A O, B O, which are drawn from the opposite ex- 

 tremities of the object to the centre of the eye. A larger 



Fic li;.— Figure to show that apparent magnitude depends upon the ^sual angle. 



object will give a larger angle, and a smaller object a smaller 

 angle. The apparent magnitude of an object will therefore 

 depend upon the angle which it forms, or subtends, at the 

 eye. If a small object, C D, be interposed in the line of 

 sight, it may be so placed as to subtend precisely the same 

 angle. Hence, a small object, near to the eye, may appear 

 Just as big as a large object which is a great way off. 



It is easy to see from such a diagram as Fig. 117, how the 

 actual size of the sun may be measured. Let a circular 

 disc, say one inch in diameter, be cut out of cardboard ; or 

 take the halfpenny mentioned above, for this has exactly 



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