370 ASTRONOMY 



Herschel, systematically measuring the relative direction of one star 

 from another very near it for the purpose of detecting, if possible, 

 any appreciable effect of annual parallax due to the differing dis- 

 tances of the two stars, discovered that many of the so-called double 

 stars really constitute physical systems, since known as binary 

 stars. 



From incidents like these has grown a dictum that every recorded 

 measurement of the position of a moving celestial body is important 

 irrespective of any immediate utility of it that may be apparent at the 

 time it is made. It is an inspiring thought that every such measure- 

 ment records a fact unique in the history of nature. If the oppor- 

 tunity to measure be neglected the omission can never be supplied. 

 Can it be said that any measurement of the position of a moving body 

 is superfluous? Should response to a special need constitute the only 

 recognized motive in making such measurements? The forces at the 

 disposal of astronomical research are finite. The demands of the hour 

 are always great. Masses of measurements for the uses of posterity 

 must usually contain an element of uncertainty as to exactly what 

 may really be needed. Uncertainty as to future improvements in the 

 art of measurement may be such as to raise the question whether 

 present standards are not liable to become obsolete. Therefore, the 

 accumulation of measurements upon the celestial bodies chiefly for 

 the use of posterity is not to be commended without limitations. It 

 would seem that a large element of comparatively immediate utility 

 should inhere in every such undertaking; or it should become very 

 clear that posterity will certainly need, and will be able to profit by, 

 the observations we now make for them. 



These limitations do not seem to operate against the extensive 

 scheme of accurate observation upon all stars to the eleventh magni- 

 tude, known as the Astrographic Chart. This is an undertaking which, 

 while it has its present uses, will probably offer a much larger measure 

 of utility to astronomers of future generations engaged in stellar 

 research. The first and most important requirement in the solution 

 of this problem is that astronomy shall be in possession of measure- 

 ments upon as many stars as possible, repeated at various epochs 

 over the longest possible interval. In this way the unavoidable sys- 

 tematic errors of the measurements may be rendered relatively less 

 obnoxious to the total of observed stellar motion, especially where 

 that motion is very small. It seems probable that the measurements 

 from the Astrographic photographs mark very nearly a practical 

 limit of accuracy for operations on a large scale. Accordingly we may 

 be quite certain that no astronomical work of the present generation 

 will be better appreciated by astronomers of later ages than that upon 

 the Astrographic Chart, which will convey to them the first accu- 

 rately observed positions of the vast majority of stars that it contains. 



