THE STELLAR UNIVERSE. 



nents, has teen the absence of all apparent effects of parallax 

 among the fixed stars, those objects which are scattered in such 

 countless numbers over every part of the firmament. From what 

 has been explained, it will be perceived that, supposing these 

 bodies to be, as they evidently must be, placed at vast distances 

 outside the limits of the solar system, and in every imaginable 

 direction around it, the effects of annual parallax would be to 

 give to each of them an apparent annual motion in a circle or 

 ellipse, according to their direction in relation to the position of 

 the earth in its orbit, the ellipse varying in its eccentricity with 

 this position, and the diameter of the circle or major axis of the 

 ellipse being determined by the angle which the diameter EE" 

 (fig. 1) of the earth's orbit subtends at the star, which will be less 

 the greater the distance of the star, and vice versa. The apparent 

 position of the star in this circle or ellipse would be evidently 

 always in the plane passing through the star and the line joining 

 the sun and earth. 



13. Since then, with a few exceptions, which will be noticed 

 hereafter, no traces of the effects of annual parallax have been 

 discovered among the innumerable fixed stars by which the 

 solar system is surrounded ; and since, nevertheless, the annual 

 motion of the earth in its orbit rests upon a body of evidence, and 

 is supported by arguments which must be regarded as conclusive, 

 the absence of parallax can only be ascribed to the fact, that the 

 stars generally are placed at distances from the solar system, 

 compared with which the orbit of the earth shrinks into a point ; 

 and, therefore, that the motion of an observer round this orbit, 

 vast as it may seem compared with all our familiar standards of 

 magnitude, produces no more apparent displacement of a fixed 

 star, than the motion of an animalcule round a grain of mustard- 

 seed would produce upon the apparent direction of the moon or sun. 



The visual ray by which a star is seen, and which is its appa- 

 rent direction, is carried by the annual motion of the earth round 

 the surface of a cone, of which the earth's orbit is the base, and 

 of which the star is the apex. The line drawn from the centre of 

 the earth's orbit to the star, is the axis of this cone ; and, conse- 

 quently, the parallax of the star is the angle under the latter line, 

 and the visual ray by the motion of which the surface of the cone 

 is formed. 



The same optical effect would be produced by transferring the 

 orbital motion of the earth to the star, the observer being 

 supposed to be stationary, and placed at the centre of the 

 earth's orbit ; and this supposition will render all the parallactic 

 phenomena much more easily comprehended. Let the star, then, 

 be imagined to move in a circle equal and parallel to the 

 178 



