Nut (of Anchor).Key piece, on stock, that fits in slot in shank and prevents stock from turning.
Nutation. A 'nodding' of Earth's axis during a period of 18 years 220 days. Caused by variation in the Lunisolar gravitational pull on Earth's equatorial bulge - the variation being due to Moon's maximum declination varying between 18° 18' and 28° 36' in the period. Pole of Equinoctial describes a small ellipse about its mean place. The value of nutation is I8 ½ of arc, that is 9 ¼ above or below its mean value.