Which term refers to the short-term periodic wobble in the axis orientation of a spinning astronomical body caused by gravitational forces of other bodies?

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

Answer: (A)

Orientation changes of a spinning astronomical body under external gravity involve two main motions: a slow drift (precession) and a short-term wobble (nutation). Nutation is the short-term periodic wobble in the axis orientation caused by gravitational forces from other bodies. For Earth, the Moon and the Sun tug on its equatorial bulge in varying ways as they move in their orbits, producing small oscillations of the axis that ride on top of the longer-term precession.

Precession, by contrast, is the gradual, conical motion of the axis over very long timescales (thousands of years) due to the same torques. Ecliptic tilt is simply the angle between the rotation axis and the plane of the ecliptic—it's a fixed geometric relation, not a wobble. Oblateness refers to the spinning body’s flattening at the poles, which is a property that influences torques but is not the wobble itself. So the short-term, periodic wobble caused by external gravitational forces is nutation.