The precession of the equinoxes

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Abstract
Lemmas I, II, and III, culminating in Proposition XXXIX ‘to find the precession of the equinoxes’ is one of those sections, interspersed throughout the Principia, in which new ideas are formulated in the context of new problems that are constantly arising. In Lemmas I, II, and III, Newton introduces, for the first time, notions that are relevant to the dynamics of rigid bodies: the moment of inertia (which determines the ‘efficacy to wheel [a body] with circular motion about a centre’); the moment of momentum (which measures the ‘motions of the whole [body] about its axis of rotation’) and the torque-’the power to wheel about’--exerted by an external force on a non-spherical body; and, indeed, the principles underlying gyroscopic motion. All these notions are defined in the context of a dynamical theory of the precession of the equinoxes which derives from the oblateness of the Earth and the inclination of its axis of rotation to the plane of the ecliptic.

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