DEPARTMENT OF MERIDIAN ASTROMETRY. 1 



Benjamin Boss, Director. 



The present report covers the activities of the Department of Meridian 

 Astrometry during the year September 1, 1921, to August 31, 1922. Special 

 attention has been devoted to a development of the treatment of differential 

 refraction as it applies to the work of the Department. While the preliminary 

 steps in the reduction of observations are not affected by the introduction of 

 corrections for differential refraction, the later treatment must be revised 

 if we are to gain the full value of the material with which we are dealing, and 

 if we are to develop the opportunity for a study of some of the basic principles 

 of positional astronomy. A large field of activity has been opened. 



DIFFERENTIAL REFRACTION. 



The history of meridian astrometry has been punctuated by successive 

 discoveries which, through the removal of sources of error, have lent refine- 

 ment to the observations. Sometimes we have been aware of the presence 

 of outstanding peculiarities in observations, but, through inability to fit the 

 causes, we have attributed them to accidental error or to some professedly 

 unknown cause, and have introduced arbitrary corrections in an attempt to 

 harmonize the results. Thus for a long time it has been recognized that a 

 distinct difference exists between daytime and night observations. It has 

 been the habit either to reject daytime observations or to correct them to fit 

 the night observations. 



Eventually it developed that where the corrections to the clock, as derived 

 from observations of the stars, were arranged according to the time of day, 

 a progressive change was taking place. This at first was attributed to varia- 

 tion in the clock-rate, but it appeared to us that the fact that clocks running 

 under entirely different conditions behaved in the same way discredited the 

 possibility of variation of rate. It was likewise difficult to attribute the 

 change of rate to any known cause. It seemed far more probable that a 

 diurnal effect in the observations was caused bjr the known diurnal changes 

 in the atmosphere. In addition, testimony has been .accumulating to show 

 that similar diurnal effects are inherent in observations in zenith-distance. 

 It therefore seemed reasonable to expect that the same cause was affecting 

 the observations in both coordinates. 



In the report of this Department, in the Year Book for 1921, the results 

 of some preliminary experiments by Mr. Varnum were given. These experi- 

 ments have been continued and amplified. 



In dealing with the reduction of meridian observations it has been assumed 

 that the strata of air he parallel with the earth's surface. The refraction 

 tables are computed on this basis. As a matter of fact, the strata generally 

 will be inclined toward the earth's surface, producing a prismatic effect which 

 tends to shift the positions of the stars bodily, and to affect both right-ascen- 

 sion and declination according to the tilt in the air strata. Heretofore the 

 material has not been available for a study of possible shifts due to varying 

 conditions of the air, but the observations taken at San Luis and at Albany 

 were planned in a manner to cover all probable contingencies, as far as pos- 



1 Address: Dudley Observatory, Albany, New York. 



191 



