PARALLAX OF STARS. 



To comprehend the effect which might be expected to be pro- 

 duced upon the apparent place of a distant object by such a motion, 

 let E E' E" E'", fig. 1, represent the earth's annual course round 

 the sun as seen in perspective, and let FJ l 



o be any distant object visible from 

 the earth. The extremity E of the line 

 E o, which is the visual direction of the 

 object, being carried with the earth 

 round the circle E E' E" E"', will annually 

 describe a cone of which the base is the 

 path of the earth, and the vertex is the 

 place of the object o. While the earth 

 moves round the circle E E", the line of 

 visual direction would therefore have a 

 corresponding motion, and the apparent 

 place of the object would be successively 

 changed with the change of direction of 

 this line. If the object be imagined to 

 be projected by the eye upon the firma- 

 ment, it would trace upon it a path o 

 o' o" o'", which would be circular or 

 elliptical, according to the direction of 

 the object. "When the earth is at E, the 

 object would be seen at o; and when 

 the earth is at E", it would be seen at o". 

 The extent of this apparent displacement 

 of the object would be measured by the 

 angle E o E", which the diameter E E" 

 of the earth's path or orbit would sub- 

 tend at the object o. 



It has been stated that, in general, 

 the apparent displacement of a distant 

 visible object produced by any change 

 in the station from which it is viewed 

 is called PARALLAX. That which is pro- 

 duced by the change of position due to 

 the diurnal motion of the earth being 

 called DIURNAL PARALLAX, the corresponding displacement 

 due to the annual motion of the earth is called the ANNUAL 



PARALLAX. 



The greatest amount, therefore, of the annual parallax for any 

 proposed object is the angle which the semidiameter of the earth's 

 orbit subtends at such object, as the greatest amount of the diurnal 

 parallax is the angle which the semidiameter of the earth itself 

 subtends at the object. 



