Nutation (from Latin nūtātiō, "nodding, swaying") is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behaviour of a mechanism. In an appropriate reference frame it can be defined as a change in the second Euler angle. If it is not caused by forces external to the body, it is called free nutation or Euler nutation.[1] A pure nutation is a movement of a rotational axis such that the first Euler angle is constant.[citation needed] In spacecraft dynamics, precession (a change in the first Euler angle) is sometimes referred to as nutation.[2]