ASTRONOMY* 7$ 



would be seen from the centre of the earth, 

 and as it is seen from a point on the surface. 



The parallax of an object, is therefore the same 

 with the angle which the distance between the 

 centre and a given point on the surface subtends 

 at the object. 



Though an object to have no parallax, ought, strict- 

 ly speaking, to be at an infinite distance, yet it will 

 have no sensible parallax, if its distance is very 

 great compared with the diameter of the earth. An 

 angle of one-fourth of a second may be considered 

 as insensible ; so that if the radius of the earth 

 subtend an angle, at the distance of any object, less 

 than one-fourth of a second, that object will be 

 seen, from all points of the earth's surface, in the 

 same position. 



Now, an arch of I" is .000004-&4S of the radius ; and 

 the fourth of a second is therefore 



00001212 = 



and therefore, if a body is distant from the earth 

 by 825082 of its semidiameters, it can have no sen- 

 sible parallax. 



Though the centre of the earth is a point from which 

 no observations can be made, yet as it is equally 

 related to all the points on the surface, the posi- 

 tions of the heavenly bodies may be most conveni- 

 ently 



