656 POPULAR SCIENCE MONTHLY. 



quent investigations have, however, made it probable that this result was 

 about double the true value of the parallax. 



The third successful attempt was made by Henderson, of England, 

 astronomer at the Cape of Good Hope. He found from meridian obser- 

 vations that the star Alpha Centauri had a parallax of about 1". This is 

 a double star of the first magnitude, which, being only 30° from the 

 south celestial pole, never rises in our latitudes. Its nearness to us 

 was indicated not only by its magnitude, but also by its considerable 

 proper motion. 



Although subsequent investigation has shown the parallax of this 

 body to be less than that found by Henderson, it is, up to the time 

 of writing, the nearest star whose distance has been ascertained. The 

 extreme difficulty of detecting movements so slight as those we have 

 described, when they take six months to go through their phases, will 

 be obvious to the reader. He would be still more impressed with it 

 when, looking through a powerful telescope at any star, he sees how 

 it flickers in consequence of the continual motions going on in the air 

 through which it is seen and how difficult it must be to fix any point 

 of reference from which to measure the change of direction. 



The latter is the capital difficulty in measuring the parallax. How 

 shall we know that a star has changed its direction by a fraction of a 

 second in the course of six months? There must be for this purpose 

 some standard direction from which we can measure. 



The most certain of these standard directions is that of the earth's 

 axis of rotation. It is true that this direction varies in the course 

 of the year, but the amount of the variation is known with great 

 precision, so that it can be properly allowed for in the reduction of the 

 observations. The angle between the direction of a star and that of the 

 earth's axis, the latter direction being represented by the celestial pole, 

 can be measured with our meridian instruments. It is, in fact, the 

 north polar distance of the star, or the complement of its declination. 

 If, therefore, the astronomer could measure the declination of a star 

 with great precision throughout the entire year, he would be able 

 to determine its parallax by a comparison of the measures. But it is 

 found impossible in practice to make measures of so long an arc with 

 the necessary precision. The uncertain and changing effect of the 

 varying seasons and different temperatures of day and night upon the 

 air and the instrument almost masks the parallax. After several 

 attempts with the finest instruments, handled with the utmost skill, 

 to determine stellar parallax from the declinations of the stars, the 

 method has been practically abandoned. 



The method now practiced is that of relative parallax. By this 

 method the standard direction is that of a small star apparently along- 

 side one whose parallax is to be measured, but, presumably, so much 



