The effect of lunar distance on the magnetic declination at Zi‐Ka‐Wei

The March 1916 issue of the Journal, pages 25–26, contains an interesting letter by Father de Moidrey, S. J., on the “lunar‐diurnal variation of the magnetic declination at Zi‐Ka‐Wei.” The fact which he has brought to light proves, on examination, to be somewhat remarkable and unexpected. It relates to the solar‐diurnal 1 amplitude of declination, defined as the difference between the elongations normally occurring towards sunrise and at mid‐day (occasional maxima or minima at other times, due to disturbances, are neglected). The values P, A of this diurnal amplitude at the times of lunar perigee and apogee, respectively, were determined from three cycles of 10 or 11 years each.