章动是在行星或陀螺仪的自转运动中，轴在进动中的一种轻微不规则运动，使自转轴在方向的改变中出现如「点头」般的摇晃现象。

行星的章动来自于潮汐力所引起的进动，并使得岁差的速度不是常数，而会随着时间改变。这种现象是英国的天文学家詹姆斯·布拉德利在1728年发现的，但直到20年后才得到解释。

由于对行星的动力学已经有充分的了解，只有几个秒弧变化的章动值已经可以计算出数十年的周期变化。但另一种会干扰自转的极移（polar motion）仍只能估计几个月后的变化，因为它会受到风、洋流、以及地核的运动影响，使得他的变化迅速且难以预测。

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 behavior 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. A pure nutation is a movement of a rotational axis such that the first Euler angle is constant. In spacecraft dynamics, precession (a change in the first Euler angle) is sometimes referred to as nutation.