# What Is Nutation?

Jo Dunaway

Nutation is a word derived from the Latin nutare meaning to nod. It means a nodding, swaying, or rocking motion in an axis of rotation. The rotation may be of a symmetrical object, such as a bullet in flight or a gyroscope. Planets also have a nutation in their axis of rotation; the Earth's nutation was first discovered by a British astronomer in 1728. Years later, this nutation was described in greater detail and said to be due to tidal forces affecting the precession of winter and summer equinoxes that vary from time to time. Precession is a term for the elliptical orbit of the point of the Earth's axis as it spins; it sometimes points toward Polaris as the Pole Star, and sometimes toward Vega, in the Lyra constellation.

The Earth's equatorial mass follows this precessional orbit in seasons and in orientation, and, consequently, the Earth seems to wobble on its axis. Other planets also have such a nutation. Astronomers calculate the other planets' nutations by means of a set formula of calculations in order to locate exact coordinates and get a fix on an object in the sky. None of the objects in the sky are in a fixed position from day to day. Known nutation patterns are calculated when determining exact location on any given day.

A gyroscope is similar to a child's spinning top; as long as it spins quickly, it can remain poised well on its tiny point in contact with the ground. When the top begins to lose speed due to friction of air and loss of momentum, it will begin to wobble and eventually fall over. When the top is spinning quickly and standing straight, this is a depiction of precession which turns around a vertical axis. Gravity is in suspension with its horizontal pull, due to the torque of momentum speed in precession. As the top begins a slowing vibration with little nods out of a perfect vertical position, this is a depiction of the nodding of nutation.

The tidal forces of sun and moon on the Earth are the principal sources of its continuously changing location, causing a nutation of the Earth's axis. The moon has the largest pull, as Earth's nutation cycles coincide with the precession of the orbital nodes of the moon. The simple rigid-body mechanics of the gyroscope-top illustration, however, does not take into account the deformations of the Earth, which is not perfectly smooth, nor the pull from other planets, which also cause nutation variances.