= Synchronous rotation
= Tidal synchronization
{synonym}
<Synchronous rotation> means one prograde turn about the body's own axis per <orbital period>, measured against fixed directions in an <inertial frame>. This $1:1$ <spin-orbit resonance> approximately describes the <Moon>'s <synchronous rotation> toward <Earth>. https://science.nasa.gov/moon/tidal-locking/[Tracy Vogel, NASA: synchronous rotation]{ref}.
\Image[/synchronous-rotation-facing-companion.png]
{title=Keeping the same face toward a companion requires one turn per orbit}
{description=Two circular-orbit cartoons viewed along the spin axis. The orange arrow and dot are fixed to the body. With synchronous rotation they turn once in space and always face the companion; without spin they keep a fixed direction in space and different faces point inward. Colors mark material orientation, not sunlight. Sizes are arbitrary; these are not observed lunar states or photographs.}
The striking geometric idea is that one turn in space can keep one face toward the companion: a fixed orientation in the frame turning with the orbit is a changing orientation in an <inertial frame>. This distinction helps us predict illumination: a synchronously rotating <exoplanet> orbiting its illuminating <star> can have persistent day and night hemispheres, whereas the <Moon>, synchronized to <Earth>, receives changing illumination from a different body, the <Sun>.
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