Abstract
Spherical harmonic analyses of the earth's magnetic field are made for 1600, 1650, 1700 and 1770, based mostly on the data of the declination and partly on the total intensity and the inclination. The declination used for the analysis has its basis on the instrumental observations, while the total intensity is interpolated between archeomagnetic data and instrumental observations. As for the inclination some of the data are interpolated and the other are the direct measurements. The results are similar to the analyses by Fritsche except for the absolute magnitude of several terms. The present analysis indicates that there was a maximum in the absolute intensity of g10 around 1800 and that the g20 has varied in a way similar to g10, while they had been constant with time in Fritsche's analyses.
From the time variations in the Gauss-Schmidt coefficients obtained in this analysis, harmonic coefficients for the geomagnetic secular variation are computed.
