Tidal locking is a gravitational phenomenon in which one astronomical body always shows the same face to another. This happens because the body’s rotation period becomes equal to its orbital period around its partner. A familiar example is the Moon, which is tidally locked to Earth—meaning we always see the same lunar hemisphere from our planet.
This effect occurs due to tidal forces: as a body orbits, its gravitational interaction with its partner creates tidal bulges. Over time, energy dissipation from internal friction within the rotating body slows its rotation until it synchronizes with its orbit. Once locked, the torque from the tidal bulges is no longer changing the rotation significantly.
Tidal locking is common in planetary systems, especially for:
- Moons orbiting planets (e.g., many of Jupiter’s and Saturn’s moons)
- Planets orbiting very close to their stars (e.g., exoplanets around red dwarfs)
This phenomenon has important implications for climate and habitability, especially for exoplanets, as one side may always face the star while the other remains in darkness.