1. Catalogue of Primary Reference Stars. Two lists of primary reference stars containing, in all, 821 stars were prepared by Prof. Kopff and published in Ast. Nach.
224, No. 5375 and 226, No. 5403. These stars were widely observed at the following Observatories: Babelsberg, Cincinnati, Cape, Greenwich, Bergedorf, Heidelberg, La Plata, Leiden, Lick, Padova, Uccie and Washington. The stars were selected to give as uniform a distribution as possible to a distance of I° on either side of the path of Eros according to the preliminary ephemeris of Prof. Gustav Witt (Ast. Nach.
224, No. 5375; M.N.R.A.S.
85, 997, 1925). The early observations of Eros in the autumn of 1930 indicated that the actual path of Eros deviated considerably from the path indicated by this ephemeris and that the deviation near the time of opposition, when the motion of Eros was almost entirely in declination, would amount in R.A. to about 15 minutes of arc. The primary comparison stars as selected would thus extend on one side of the path, near opposition, to a distance of only 45’. This necessitated either (1) centring the photographic plates on the computed path of Eros, or (2) centring the photographic plates on Eros itself and selecting and observing sufficient additional primary comparison stars to cover the blank strip. It was finally decided to adopt the second alternative and a third list of reference stars, containing eighty-seven stars, was prepared by Prof. Kopff and published in Ast. Nach.
240, No. 5756, with a recommendation that sufficient stars of the second list should be re-observed along with the stars of the new list to enable the new observations to be reduced to the system which Prof. Kopff had meanwhile derived from the observations of the two main lists.