Fig. 5

Semiannual nutation and the Z-term. Here, we assume that, under the situation without nutation, Stars A and B are seen at the zenith from Mizusawa at midnight in June and December, respectively (a). With a significant amplitude of semiannual nutation (b), visible positions of Stars A and B from Mizusawa at midnight deviate in opposite directions from local zenith in June and December, causing annually varying bias (Z-term) to the astronomical latitude variation. Such biases appear as a common value for all observatories along the latitude 39° 08′ N (white dotted circle). For the sake of simplicity, I excluded astrometric effects of the nutation terms other than the semiannual term, precession, and polar motion in this figure