Interpretation of Kimura's Annual Z-Term

The close relation between Kimura s annual z-term and the theoretical semi-annual solar nutation term based on the hypothesis of a liquid core of the Earth is suggested. Integers and n in the periodic term a sin( +na) can be derived from the values of z by three-group observation. The program of three-group observation in one night which was first adopted by the International Latitude Service at the beginning of 1955 made it possible to analyze an annual term in z. From the comparison among the observed amplitudes and the phase angles of the annual z-terms in each group, the integers of the principal annual term in z are concluded to be =2 and n= -1, that is, an argument of the annual z-term is (2 -a) and not of , where is the Sun's mean longitude and a is the mean right ascension of the stars observed. The following results were obtained as the intrinsic annual z-term for 1955-1966 from the analyses of data by two independent methods: (A) (B) o: 0137 sin(2 - a+2 2); o: 0203 sin(2 -a+4 3). +.0019 +7.8 +.0029 Recent theoretical investigations by JRFFRRYs and VICENTE (1957 a, b), and MoLoDENsxY (1961) have shown that the effect of a liquid core of the Earth may increase the coefficients of the semi-annual solar nutation term with respect to the figure axis fixed in space by o: 02. This correction is reasonable and may appear in the z-term with an argument of (2 -a). The observed Kimura's annual z-term is in good agreement with the theoretical results and the Earth tide observations (MELOHIoR and GEORIs 1968). When the initial latitude values are corrected for their declination errors by the conventional chain method, the amplitude of the (2 -a) term in z becomes twice as large as that of the intrinsic annual z-term, that is, about o: 03-0:04. Results for the International Latitude Service (1900-1966) and re'oent results for the independent stations give the values of this order. The constant term in z is explained by the errors of the catalog used, when we compare the present catalog with the MELOHIoR and DEJAIFFE (1969) Catalog for the ILS stars. Key words: Z-term; Nutation; Latitude variation; Catalog error.