ADDRESS BY WILLIAM A. ROGERS, VICE PRESIDENT, SECTION A. BODI VVI [From the PROCEEDINGS OF THE AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE, Vol. XXXII, Minneapolis Meeting, August, 1883.] PRINTED AT THE SALEM PRESS, SALEM, MASS. ADDRESS BY WILLIAM A. ROGERS, VICE PRESIDENT, SECTION A. THE GERMAN SURVEY OF THE NORTHERN HEAVENS. THE illustrious Argelander was accustomed to say, in the quaint form of speech which he often employed, "The attainable is often not attained if the range of inquiry is extended too far." In no undertaking is there greater need of a judicious application of this sound maxim than in the systematic determination of the exact positions of all the stars in the visible heavens which fall within the reach of telescopes of moderate power. The first subject which engaged the attention of the Astronomische Gesellschaft, at its formation in 1865, was the proposition to determine accurately the coördinates of all the stars in the northern heavens down to the ninth magnitude. To this association of astronomers (at first national, but since become largely international, in its character and organization) belongs the credit of arranging a scheme of observations by which, through the coöperation of astronomers in different parts of the world, it has been possible to accomplish the most important piece of astronomical work of modern times. With a feasible plan of operations, undertaken with entire unity of purpose on the part of the observers to whom the several divisions of the labor were assigned, this great work is now approaching completion. While it is yet too early to speak with confidence concerning the definite results which the discussions of all the observations are expected to show, we may with profit consider the object sought in the undertaking, the gen eral plan of the work, the difficulties which have been encountered, and the probable bearing which the execution of the present work will have upon the solution of a problem concerning which we now know absolutely nothing with certainty; a problem of which what we call universal gravitation is only one element, if, indeed, it be an element; a problem which reaches farther than all others into the mysteries of the universe, the motion of the solar and the sidereal systems in space. Our first inquiry will be with respect to the condition of the question of stellar positions at the time when this proposal was made by the Gesellschaft in 1865. All the observations which had been made up to this time possess one of two distinct characteristics. A portion of them were made without direct reference to any assumed system of stellar coördinates as a base, but by far the larger part are differential in their character. This remark holds more especially with reference to right ascensions. Nearly all of the observations of the brighter stars made previous to about 1818 were referred to the origin from which stellar coördinates are reckoned, by corresponding observations of the sun; but since that date it has been the custom to select a sufficient number of reference stars, symmetrically distributed both in right ascension and declination, and whose coördinates were supposed to be well known. The unequalled Pulkowa observations for the epoch 1845 form, I believe, the only exception to this statement. From the assumed system of primary stars are derived the clock errors and instrumental constants which are employed in the reduction of all the other stars observed. The positions of these secondary stars, therefore, partake of the errors of the assumed fundamental system, in addition to the direct errors of observation. The following list comprises the most important of the catalogues which have been independently formed; viz., Bradley for 1755, the various catalogues of Maskelyne between 1766 and 1805, D'Agelet for 1783, Piazzi for 1805, Auwers' Cacciatore for 1805, Bessel for 1815, a few of the earlier catalogues of Pond, Brinkley for 1824, Bessel for 1825, Struve for 1824, Bessel for 1827, Argelander for 1830, and Pulkowa for 1845. An analysis of the important catalogues of secondary stars published previous to 1865 reveals four important facts :— 1. That nearly all of the observations relate to bright stars, at least to stars brighter than the eighth magnitude. |