132 



KNOWLEDGE. 



April, 1911. 



The determination of the positions of the stars is 

 the basis of all that we know of the movements of 

 the sun. moon, planets, and, in fact, of the earth. 



To fix the position of the system of the sun and 

 our famil\- of planets among the stars it is necessary 

 to know, with everv exactitude, the positions of the 

 stars which ser\-e as landmarks or guide posts in our 

 journev. Not onl\' is it necessar\" to know the 

 positions now. in the \ear in which we li\e. hut to 

 compare them w ith the observations of former years 

 from the earliest times of exact astronomical research. 

 All the stars move: there is no exception according 

 to the laws of our science ; the difficuhy is to 

 distinguish between all these movements and the 

 proper motion of the solar system. It is necessarv' 

 to studv the movements in all parts of the sk\-. and 

 observe many stars in every part, in order to resolve 

 the problem of tlie movement of the solar system. 

 There is not a single place on the earth where all 

 stars ma\- not be observed with the necessary 

 accurac}'. At one observatory on the equator of our 

 earth one could observe nearly all the stars ; but here 

 difficulties would arise owing to the small altitude of 

 the poles, and one could not make exact obser\'a- 

 tions under such conditions. 



For a complete system of stars it is necessary to 

 have at least two places for observation, one in the 

 Northern hemisphere and one in the South. There 

 will be many stars which it will be possible to observe 

 in both places, and those observations will serve to 

 bind or connect the results, and for comparing the 

 conditions and results of a system which shall include 

 all the sky. Already man}- thousands of observations 

 of stars exist in the southern as well as in the northern 

 sky ; those observations were made in the earh' da\s 

 when the first obser\-atories were erected in the south. 

 But errors of various kinds appear in all the observa- 

 tions. Some are explained by errors of the instrument, 

 some by errors in the positions of the fundamental 

 stars, others by the methods used in the calculations, 

 others by the methods of observation. In general, 

 these are a class of s\'steniatic errors jieculiar to 

 an instrument, tf> a star catalogue, or to an epoch: 

 it is necessary to investigate all their sources. 



The observations of the northern sky include 

 similar kinds of systematic error as well : but the 

 observations are much older, and there has been 

 much more time for the study of the errors of method, 

 and for comparing the results w ith the fundamental 

 methods in use. 



Each time that one empln\s these methods, it is 

 possible to adjust or correct the observations to the 

 same epoch, and likewise we are able to improve the 

 results of the old observations. There is nothing 

 new in that process ; it has been used at various 

 times for connecting the results of all observatories 

 in the world, both in the north and south hemis- 

 pheres. But, in general, the zone of stars which 

 can be observed in both hemispheres has served to 

 adjust or connect the s\stem of the north w ith the 

 system of the south. It is difficult to extend the 

 range so as to embrace the positions from this zone. 



w ithout deviation, from one pole to the other. Other 

 difficulties exist and arise when \'arious instruments 

 are used, and when different methods of observation 

 and calculation are employed in obtaining the data 

 used in such a comparison. 



Now we have a more complete plan for adjusting 

 or connecting the observations of both hemisiiheres. 

 1)\- a fundamental method, and we hope that the 

 results of this plan will scrNe to impro\e the 

 positions of the stars in all the sk^■. It will be the 

 foundation from which to compute new positions 

 for our epoch, and the base for studying the old 

 positions. 



The Plan. 



The plan includes : — 



(i7) The obser\'ations, witli one iiis/riiniciit. ot 

 all the principal stars in the sk\- : 



(b) the use of a fundamental method to fix 



afresh the positions of those which ser\-e 

 for the basis of the calculations : 



(c) the investigation of the results of all obser- 



\ations by a uniform method ; 

 ((/) the comparison with the results of old 



observations by the use of these new 



positions ; 

 (c) and the binding together into one system 



all observations from pole to pole. 



One Instrument T(.) be Used. 



This is the plan which forms a part ot the work of 

 the San Luiz Observatory in .Argentina. It is the 

 first occasion on which one has used a single instru- 

 ment for observations of this class of work in both 

 hemispheres. The observations for this extensive 

 plan were commenced in the city of Albany, U.S.A.. 

 some years before taking the instrument to San 

 Luiz. and. after the conclusion of the obser\ation of 

 the more southern sky, the same instrument will be 

 mounted, at another time, in its old {>lace (at Albany) 

 in order to complete the rest of the observations. 

 The obser\-ations of the south will be made, therefore, 

 in the middle of those in the north, so far as relates 

 to the mean epochs. With the results of these 

 obser\'ations made upon this plan, it will be possible 

 to correct some of the systematic errors of the old 

 catalogues : as also to form a new basis for a 

 complete s\'stem of positions of the principal stars, 

 and a connecting link for other stars which one may 

 include in the same system. 



With this object, it is necessary to form a plan of 

 work which will be general and comprehensive, 

 avoiding, as far as possible, the effects of s\ stematic 

 errors in the observations and in the calculations. 



It is also necessary to study all the errors of the 

 instrument and apply the proper corrections. Of 

 these the most important are, the division errors or 

 graduation of the circles, errors or inequalities in the 

 form of the axes or pivots of the instrument, and the 

 flexure of the telescope. There are also the 

 corrections of the position of the instrument as in 

 other classes of meridian ojiservations. Besides the.se. 



