corotates

Corotates is the third-person singular present tense of the verb corotate. In general usage, corotate describes a situation in which two or more bodies rotate in synchrony, sharing the same angular velocity or maintaining a fixed orientation relative to one another. The term is common in astronomy, planetary science, and heliophysics.

In celestial mechanics, corotation often results from tidal locking or resonant spin-orbit coupling. When a body's rotation period matches its orbital period around a companion, it is said to corotate with that body. The most familiar example is Earth's Moon, which keeps the same face toward Earth and thus corotates with the planet. Other examples include the Pluto-Charon system, where both bodies are tidally locked to each other. In many exoplanetary systems, close-in planets are expected to be tidally locked, and therefore to corotate with their stars.

Corotation is also discussed in the context of the solar wind. Corotating interaction regions (CIRs) are large-scale structures that emerge from the interaction of fast and slow solar wind streams and rotate with the Sun, so that their longitudes remain connected to the same solar longitude over multiple solar rotations. Observations of CIRs rely on the corotating reference frame to track their evolution.

Corotation can be exact (tidal locking) or approximate (near-resonant rotation). It contrasts with other rotational regimes, such as non-synchronized rotation or higher-order spin-orbit resonances (for example Mercury's 3:2 resonance). See also tidal locking and spin-orbit resonance.