CHAMBERS'S INFORMATION FOR THE PEOPLE. 



The Solar System, to the orbit of Neptune. 



The astronomer finds the distance between 

 objects, not by measuring lines, but by measuring 

 angles. Even distances on the earth are most 

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accurately measured in this way, as in trigono- 

 metrical surveying. Suppose that an observer at 

 A wishes to ascertain the distance of an inacces- 

 sible object at B ; it could never be found by 

 looking at it from A alone. He chooses another 

 station, C, to which he has access, in a direction 

 perpendicular, we shall say, to the direction of B, 

 and at a convenient distance, say 100 yards. If 

 the object looked at from A lay directly east, it 

 will no longer be east when looked at from C ; its 

 direction is now CB, not CE, and the difference 

 of direction is seen in the greater or less wideness 



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of the opening between those two lines, or in the- 

 angle BCE, as it is called. The size of such an 

 angle is ascertained by describing a circle from the 

 point C, and measuring how many degrees (that 

 is, 36oth parts of the circle) the arc be contains. 



By considering the figure, it will be seen that 

 the more distant B is, the smaller must be the angle 

 BCE, and also ABC, which is evidently equal to 

 BCE. For every size of the angle ABC, there is 

 a certain fixed length of AB (the length of AC 

 remaining the same) ; and trigonometry teaches,, 

 from knowing the base AC and the angle opposite, 

 to find the length of AB. Everything depends on 

 measuring the angles of direction, or the angular 

 distances, accurately. Degrees are divided each 

 into 60 parts, called minutes, and these again into 

 60 parts, called seconds ; and astronomical instru- 

 ments are now so delicate that a difference of 

 direction of one second can be measured. 



The difference of direction in which a body 

 is seen when viewed from two places at a dis- 

 tance from one another, is called by astronomers- 



