THE STELLAR UNIVERSE. 



distance of the nearest of the fixed stars, but the minor limit of 

 that distance, that is to say, a distance within which the star 

 cannot lie. This limit, expressed in miles, is 



D = 206265 X 95,000000 = 19,595175,000000 miles, 



or nearly twenty billions of miles. 



16. In the contemplation of such numbers the imagination 

 is lost, and no other clear conception remains, except of the 

 mere arithmetical expression of the result of the computation. 

 Astronomers themselves, accustomed as they are to deal with 

 stupendous numbers, are compelled to seek for units of pro- 

 portionate magnitude to bring the arithmetical expression of the 

 quantities within moderate limits. The motion of light supplies 

 one of the most convenient moduli for this purpose, and has, 

 by common consent, been adopted as the unit in all compu- 

 tations whose object is to gauge the universe. It is known 

 that light moves at the rate of 192000 miles per second. If, then, 

 the distance D above computed be divided by 192000, the quotient 

 will be the time, expressed in seconds, which light takes to move 

 over that distance. But since even this will be an unwieldy num- 

 ber, it may be reduced to minutes, hours, days, or even to years. 



In this manner we find that, if any star have a parallax of 1", 

 it must be at such a distance from our system, that light would 

 take 3-234 years, or three years and eighty-five days, to come 

 from it to the earth. 



If the space through which light moves in a year be taken, 

 therefore, as the unit of stellar distance, and P be the parallax ex- 

 pressed in seconds, or decimals of a second, we shall have 



17. It will easily be imagined that astronomers have dili- 

 gently directed their observations to the discovery of some 

 change of apparent position, however small, produced upon the 

 stars by the earth's motion. As the stars most likely to be 

 affected by the motion of the earth are those which are nearest 

 to the system, and therefore probably those which are brightest 

 and largest, it has been to such chiefly that this kind of obser- 

 vation has been directed ; and since it was certain that, if any 

 observable effect be produced by the earth's motion at all, it must 

 be extremely small, the nicest and most delicate means of observa- 

 tion were those alone from which the discovery could be expected. 



One of the earlier expedients adopted for the solution of this 

 problem, was the erection of a telescope, of great length and 

 power, in a position permanently fixed, attached, for example, to 

 180 



