__________ is a phenomenon which causes the orientation of the axis of rotation of a spinning astronomical object to vary over time. It is caused by the gravitational forces of other nearby bodies acting upon the spinning object.

• A. Precession
• B. Nutation
• C. Oblateness
• D. Ecliptic Tilt

Answer: (B)

Nutation is the periodic wobble in the orientation of a spinning object's axis caused by external gravitational torques from nearby bodies acting on its equatorial bulge. For Earth, the gravitational pull of the Moon and Sun on the equator drives a small, regular oscillation in tilt and direction that sits on top of the longer, slower drift called precession. This nutation produces the short-term variations you observe in the axis orientation, with the Moon’s changing orbit giving rise to a noticeable cycle. The axis does indeed vary over time due to these gravitational forces, and nutation specifically describes that periodic wobble. Oblateness refers to the body's shape, not a motion, and ecliptic tilt is just the angle between planes, not a time-varying wobble.