EFFECTS OF PARALLAX IN GENERAL. 



year is, then, two hundred millions of miles from the station, 

 from which we view it at another. 



7. Now it is a fact, within the familiar experience of every one, 

 that the relative position of objects will depend upon the point 

 from which they are viewed. If we stand upon the bank of a 

 river, along the margin of which a multitude of ships are sta- 

 tioned, and view the masts of the vessels, they will have among 

 each other a certain relative arrangement. If we change our 

 position, however, through the space of a few hundred yards, the 

 relative position of these masts will not be the same as before. 

 Two which before lay in line will now be seen separate ; and two 

 which before were separated are now brought into line. Two, 

 one of which was to the right of the other, are now reversed ; 

 that which was to the right, is at the left, and vice versa ; nor 

 are these changes produced by any change of position of the 

 ships themselves, for they are moored in stationary positions. The 

 changes of appearance are the result of our own change of posi- 

 tion ; and the greater that change of position is, the greater will 

 be the relative change of these appearances. Let us suppose, 

 however, that we are moved to a much greater distance from the 

 shipping; any change in our position will produce much less 

 effect upon the relative position of the masts; perhaps it will 

 require a very considerable change to produce a perceivable effect 

 upon them. In fine, in proportion as our distance from the masts 

 is increased, so in proportion will it require a greater change in 

 our own position to produce the same apparent change in their 

 position. 



8. Thus it is with all visible objects. When a multitude of 

 stationary objects are viewed from a distance, their relative posi- 

 tion will depend upon the position of the observer ; and if the 

 station of the observer be changed, a change in the relative 

 position of the objects must be expected ; and if no perceptible 

 change is produced, it must be inferred that the distance of 

 the objects is incomparably greater than the change of position 

 of the observer. 



Let us now apply these reflections to the case of the earth and 

 the stars. The stars are analogous to the masts of the ships, and 

 the earth is the station on which the observer is placed. It might 

 have been expected that the magnitude of the globe, being eight 

 thousand miles in diameter, would produce a change of position 

 of the observer sufficient to cause a change in the relative position 

 of the stars, but we find that such is not the case. The stars, 

 viewed from opposite sides of the globe, present exactly the 

 same appearance ; we must, therefore, infer that the diameter of 

 the earth is absolutely nothing compared to their distance. 



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