THE STELLAR UNIVERSE. 



But the astronomer has still a much larger modulus to fall 

 back upon. He reflects, as has been already observed, that he is 

 enabled to view the stars from two stations separated from each 

 other, not by eight thousand miles, the diameter of the earth, but 

 by two hundred millions of miles, that of the earth's orbit. He, 

 therefore, views the heavens on the 1st of January, and views 

 them again on the 1st of July, the earth having in the meanwhile 

 passed to the opposite side of its orbit, yet he finds, to his amaze- 

 ment, that the aspect is the same. He thinks that this cannot be 

 that so great a change of position in himself cannot fail to make 

 some change in the apparent position of the stars ; that, although 

 their general aspect is the same, yet when submitted to exact 

 examination a change must assuredly be detected. He accord- 

 ingly resorts to the use of instruments of observation capable of 

 measuring the relative positions of the stars with the last con- 

 ceivable precision, and he is more than ever confounded by the 

 fact that still no discoverable change of position is found. 



9. For a long period of time this result seemed inexplicable, 

 and accordingly it formed the greatest difficulty with astronomers, 

 in admitting the annual motion of the earth. The alternative 

 offered was this ; it was necessary, either to fall back upon the 

 Ptolemaic system, in which the earth was stationary, or to sup- 

 pose that the immense change of position of the earth in the course 

 of half a year, could produce no discoverable change of appear- 

 ance in the stars ; a fact which involves the inference, that the 

 diameter of the earth's orbit must be a mere point compared with 

 the distance of the nearest stars. Such an idea appeared so inad- 

 missible that for a long period of time many preferred to embrace 

 the Ptolemaic hypothesis, beset as it was with difficulties and 

 contradictions. 



Improved means of instrumental observation and mierome- 

 trical measurement, united with the zeal and skill of observers, 

 have at length surmounted these difficulties; and the parallax, 

 small indeed but still capable of measurement, of several stars 

 has been ascertained. 



10. To render these results, and the processes by which they have 

 been attained, intelligible, we shall here explain the general 

 effects of annual parallax. 



Since the earth moves annually round the sun, as a stationary 

 centre in a circle whose diameter must have the vast magnitude 

 of two hundred millions of miles, all observers placed upon the 

 earth, seeing distant objects from points of view so extremely 

 distant one from the other as are opposite extremities of the same 

 diameter of such a circle, must necessarily, as might be supposed, 

 see these objects in very different directions. 

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