Determination of the day of the week row C) to find the dominical letter that is E. The letter for September 18 is B, so the day of the week is Thursday. Use 16 (in the Julian century row, column E) and 76 (in row D) to find the dominical letter that is A. February 23 is a "D" day, so the day of the week is Wednesday. In a handwritten note in a collection of astronomical tables, Carl Friedrich Gauss described a method for calculating the day of the week for 1 January in any given year. He never published it. It was finally included in
IAU (1976) System of Astronomical Constants and in 1979 and 1980 by a theory of nutation . There were no reliable rotation elements for most planets, but a joint working group on Cartographic Coordinates and Rotational Elements was installed to compile recommended values The IAU(1976) system is based on the astronomical system of units: IAU commission 4: , IAU (1976) System of Astronomical Constants The International Astronomical Union at its XVIth General Assembly in Grenoble in 1976, accepted (Resolution No. 1 a whole new consistent set of astronomical constants recommended for reduction of astronomical observations, and for computation of ephemerides. It superseded the IAU's previous recommendations