> a much greater distance from the shipping ; a very slight change in our po- 

 sition will produce much less effect upon the relative position of the masts ; 

 perhaps it will require a very considerable change of position to produce a per- 

 ceivable change upon them. In fine, in proportion as our distance from the 

 masts is increased, so in proportion will it require a greater change in our own 

 position to produce the same apparent change in the position of the masts. 



Thus it is with all visible objects. When a multitude of stationary objects 

 are viewed at a distance, their relative position will depend upon the position of 

 the observer, arid if the station of the observer be changed, a change in the 

 relative position of the objects must be expected ; and if no perceptible change 

 is produced, it must be inferred that the distance of the object is incomparably 

 greater than the change of position of the observer. 



Let us now apply these reflections to the case of the earth and the stars. 

 The stars are analogous to the masts of the ships, and the earth is the station on 

 which the observer is placed, and which is changeable in its position by reason 

 of its annual motion. It would, doubtless, be expected that the magnitude of 

 the globe, being eight thousand miles in diameter, would produce a change of 

 position of the observer sufficient to cause a change in the relative position of 

 the stars, but we find that such is not the case. The stars, viewed from oppo- 

 site sides of the globe of the earth, present exactly the same appearance ; we 

 must, therefore, infer that the diameter of the globe of the earth is absolutely 

 nothing compared to their distance. 



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 first of January, and views them 

 again on the first of July, yet he finds, to his amazement, 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 accordingly resorts to 

 the use of instruments of observation capable of measuring the relative posi- 

 tions of the stars with the last conceivable precision, and he is more than ever 

 confounded by the fact, that still no discoverable change of position is found. 



For a long period of time this result seemed inexplicable, and, accordingly, 

 it formed the greatest difficulty with astronomers in admitting the annual mo- 

 tion 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 suppose that the immense change of position of the earth in the course of 

 half a year, which we have already mentioned, could produce no discoverable 

 change of appearance in the stars ; a fact which involves the inference that the 

 diameter of the earth's orbit, which measures two hundred millions of miles, 

 must be a mere point compared with the distance of the nearest stars. Such 

 an idea appeared so preposterous and inconceivable, that for a long period of 

 time many preferred to embrace the Ptolemaic hypothesis, beset as it wus 

 with difficulties and contradictions. 



Since, however, tlie annual motion of the earth must now be regarded as a 

 proved fac^, we are driven to the inference, deduced from the absence of all change 

 of relative apparent position in the stars, that the distances of these objects from 

 our system is, in the common popular sense of the word, infinitely great com- 

 pared with the dimensions of our system, and this inference is in perfect ac- 

 cordance with the other indications of the wide vacuity that surrounds the 

 system. 



