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View a circular orbit along its perpendicular spin axis. Let $f$ be the orbital angle measured from a fixed direction, choosing an arbitrary zero for the circular limit of true anomaly, and let $\theta$ be the direction of an outward arrow painted on the body, measured from the same fixed direction. The direction from the body to its companion is $f+\pi$. Thus an inward-facing arrow satisfiesMore generally synchronous rotation keeps $\theta -f$ constant; its particular value selects which surface direction faces the companion. Over one orbital period $T$, both $f$ and the unwrapped $\theta$ increase by $2\pi$. This is the short geometric explanation for one turn per orbit. For a nonspinning body, $\dot{\theta }=0$ instead, and the companion direction sweeps through all surface longitudes. On an eccentric orbit, mean $1:1$ spin does not force $\theta -f$ to stay exactly constant: true anomaly advances nonuniformly, producing the longitudinal component of libration.

Libration is the apparent rocking of a body as seen by an observer. For the Moon, longitudinal libration arises because orbital eccentricity makes its orbital angular speed vary while its spin is much more uniform: the direction to Earth advances ahead of, then falls behind, the mean facing direction. The Moon's tilted geometry also causes latitudinal libration; small actual rotational oscillations provide further corrections. Thus lunar synchronous rotation holds in the mean, rather than requiring exact instantaneous alignment. ^[ref]. ^[ref].

The near side of the Moon is the hemisphere directed approximately toward Earth. Libration changes the exact visible boundary, so synchronous rotation does not mean that an observer always sees precisely the same hemisphere.