PARALLAX. 



CHAPTER III. 



Parallax. 



" The broad circumference 



Hung on his shoulders like the moon, whose orb 

 Through optic glass the Tuscan artist views, 

 At evening, from the top of Fesole' 

 Or in Valdarno, to descry new lands, 

 Rivers or mountains, in her spotty globe." Milton. 



WE have now shown that our earth is revolving around the 

 sun, which is the grand central luminary, and that within- its 

 orbit are the orbits of Venus and Mercury, while exterior are the 

 orbits of Mars, Jupiter, and Saturn. We have learned to look 

 upon these bodies as orbs, or balls like our own earth, and suppose 

 them to revolve like our earth upon an axis. We now desire to 

 know something of their distance from us, and the actual velocity 

 with which both we and they are moving. The diameter of our 

 earth we have assumed at 8000 miles, or equal lengths, we can, 

 from knowing this, ascertain the distance of the moon from the 

 earth, and of the earth from the sun. Every one is familiar 

 with the fact, that every change of position of a spectator, 

 causes an apparent change of place in the object viewed. Thus, 

 if while in a certain position, we observe a particular house to be in 

 the range, or same line with a distant tree, then upon changing 

 our position, the house will no longer be in a* line with the tree, 

 but will appear to have moved in the contrary direction. This 

 apparent change of place of the object, due to a real change of 

 place in the observer, is called parallax, and by its means, we can 

 determine the distances of the heavenly bodies. Thus, supposing 

 spectators on opposite portions of the earth's surface, as at A and 

 B, to view the moon or a planet, at c, the observer at A, will see 

 the object c, apparently at a, while the observer at B will per- 

 ceive it at the same time at b. Here is an apparent change of 



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