J30 NEWCOMB 



The nineteenth century was well on its way before the 

 instruments of the astronomer were brought to such perfec- 

 tion as to admit of the measurement. From the time of 

 Copernicus to that of Bessel many attempts had been made 

 to measure the parallax of the stars, and more than once 

 had some eager astronomer thought himself successful. But 

 subsequent investigation always showed that he had been 

 mistaken, and that what he thought was the effect of paral- 

 lax was due to some other cause, perhaps the imperfections 

 of his instrument, perhaps the effect of heat and cold upon 

 it or upon the atmosphere through which he was obliged to 

 observe the star, or upon the going of his clock. Thus 

 things went on until 1837, when Bessel announced that 

 measures with a heliometer the most refined instrument 

 that has ever been used in measurement showed that a 

 certain star in the constellation Cygnus had a parallax of 

 one-third of a second. It may be interesting to give an idea 

 of this quantity. Suppose one's self in a house on top of a 

 mountain looking out of a window one foot square, at a 

 house on another mountain one hundred miles away. One 

 is allowed to look at that distant house through one edge of 

 the pane of glass and then through the opposite edge; and 

 he has to determine the change in the direction of the dis- 

 tant house produced by this change of one foot in his own 

 position. From this he is to estimate how far off the other 

 mountain is. To do this, one would have to measure just 

 about the amount of parallax that Bessel found in his 

 star. And yet this star is among the few nearest to our 

 system. The nearest star of all, Alpha Centauri, visible only 

 in latitudes south of our middle ones, is perhaps half as far 

 as Bessel's star, while Sirius and one or two others are 

 nearly at the same distance. About 100 stars, all told, have 

 had their parallax measured with a greater or less degree 

 of probability. The work is going on from year to year, 

 each successive astronomer who takes it up being able, as a 

 general rule, to avail himself of better instruments or to 

 use a better method. But, after all, the distances of even 

 some of the 100 stars carefully measured must still remain 

 quite doubtful. 



Let us now return to the idea of dividing the space in 



