THE VISIBLE STARS. 



Iff 



feelers are the COMETS, and in particular that called Halley's comet. Thia body 

 emerges from the system periodically, and makes an excursion into the sur- 

 rounding regions to a distance of little less than two thousand millions of miles 

 beyond the limits of our system, and returns at regular intervals to the sun. It 

 is a body of extreme levity and tenuity compared even with the smallest plan- 

 etary masses ; it is, therefore, eminently susceptible of the effects of gravitation 

 proceeding from a body external to it. 



We have shown, on another occasion, that when this body, once in seventy- 

 live years, departs from our system to make its vast excursion through distant 

 regions of space, the eye of science pursues it along its path, watches its move- 

 ments, and follows its course. That course is calculated upon the supposition 

 that it is subject to no attraction through the entire range of its orbit except 

 those of the sun and planets, and the calculations of its return are based upon 

 that supposition. The time and the place of each of its successive returns to 

 our system have been foretold on these suppositions ; and we have found that 

 its returns have corresponded faithfully with such predictions. It is certain, 

 then, that, in its range through space, this body has not passed in the neigh- 

 borhood of any mass of matter capable of exercising an observable attraction 

 upon it. In fact, it moves exactly as it would move if no material object exist- 

 ed in the creation save those of the solar system itself. It follows, therefore, 

 that all other objects must be too distant from our system to produce any dis- 

 coverable attraction even on so light a body as this. 



Yet when, on any clear night, we contemplate the firmament, and behold the 

 countless multitudes of objects that sparkle upon it, and remember what a com- 

 paratively small number are comprised among those of the solar system, and 

 even of these how few are visible at any one time, we are naturally impel- 

 led to the inquiry, Where in the universe are these vast numbers of objects 

 placed ? 



Very little reflection and reasoning, applied to the consideration of our own 

 position, and to the appearances of the heavens, will convince us that the ob- 

 jects that chiefly appear in the firmament must be at almost immeasurable dis- 

 tances from our system. The earth in its annual course round the sun moves 

 in a circle, the diameter of which is about two hundred millions of miles. We, 

 who observe the heavens, are transported upon the globe round that vast circle. 

 The station from which we observe the universe at one period of the year is, 

 then, two hundred millions of miles from the station to which we are transport- 

 ed at another period of the year. Thus, if we view the heavens on the night 

 of the 1st of January and note their aspect, and view them again on the night 

 of the 1st of July, we know that the two stations from which we take these 

 two surveys are separated by a space of two hundred millions of miles. 



Now it is a fact within the familiar experience of every one, that the relative 

 position of objects will depend upon the point from which they are viewed. If 

 we stand upon the bank of a river, along the margin of which a multitude of 

 ships are stationed, and view the masts of the vessels, they will have among 

 each other a certain relative arrangement. If we change our position, however, 

 through the space of a few hundred yards, the relative position of these masts 

 will not be the same as before. Two which before lay in line will now be seen 

 separate, and two which before were separated are now brought into line. Two, 

 one of which was to the right of the other, are now reversed ; that which was 

 to the right, is at the left, and vice versa ; nor are these changes produced by 

 any change of position of the ships themselves, for they are moored in station- 

 ary positions. The changes of appearance are the result of our own change of 

 position, and the greater that change of position is, the greater will be the rela- 

 tive change of these appearances. Let us suppose, however, that we are moved 



