ANNUAL PARALLAX. 



earth's orbit, the centre of the circle being the true place of the 

 star. The place of the star in this circle of parallax must always 

 be diametrically opposite to the corresponding place of the earth 

 in its orbit. The star so moving would suffer exactly the same 

 apparent displacement as it would appear to suffer if it were, as 

 it is, at rest in its true place, the earth moving in its proper orbit 

 round the sun. 



14. It might be supposed, that where the character and laws of 

 the phenomena are so clearly understood, the discovery of their 

 existence could present no great difficulty. Nevertheless, nothing 

 in the whole range of astronomical research has more baffled the 

 efforts of observers than this question of the parallax. This has 

 arisen altogether from the extreme minuteness of its magnitude. 

 It is quite certain that the parallax does not amount to so much as 

 1" in the case of any of the numerous stars which have been as 

 yet submitted to the course of observation which is necessary to 

 discover the parallax. Now, since in the determination of the 

 exact uranographical position of a star there are a multitude of 

 disturbing effects to be taken into account and eliminated, such as 

 precession, nutation, aberration, refraction, and others, besides the 

 proper motion of the star, which will be explained hereafter ; and 

 since, besides the errors of observation, the quantities of these are 

 subject to more or less uncertainty, it will astonish no one to be 

 told that they may entail, upon the final result of the calculation, 

 an error of 1" ; and, if they do, it is vain to expect to discover 

 such a residual phenomenon as parallax, the entire amount of which 

 is less than 1". 



15. If in any case the parallax could be determined, the distance 

 of the stars could be immediately inferred. For, if this value 

 of the parallax be expressed in seconds, or in decimals of a second, 

 and if E denote the semidiameter of the earth's orbit, D the 

 distance of the star, and P the parallax, we shall have 



206265 



D = E X - 



P 



If, therefore, P = 1", the distance of the star would be 206265 

 times the distance of the sun, and since it may be considered 

 satisfactorily proved, that no star which has ever yet been 

 brought under observation has a parallax greater than this, it may 

 be affirmed that the nearest star in the universe to the solar system 

 is at a distance, at least, 206265 times greater than that of the sun. 

 Let us consider more attentively the import of this conclusion. 

 The distance of the sun, expressed in round numbers (which are 

 sufficient for our present purpose), is 95 millions of miles. If 

 this be multiplied by 206265, we shall obtain, not indeed the 

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