Tidal Locking

Synchronous Rotation & Permanent Hemispheres

Locked vs Unlocked Rotation

Near side (light hemisphere)Far side (dark hemisphere)Primary body (planet)

Key Parameters

Lock condition

Tspin = Torbit

Moon example

27.3 days

Lock timescale

Myr – Gyr

M-dwarf HZ

~0.1–0.4 AU

How It Happens

A rotating body in orbit develops a slight tidal bulge — the side facing the primary is pulled outward more than the far side. If the body rotates faster than it orbits, the bulge leads ahead of the planet-facing axis.

Gravity from the primary pulls that leading bulge back, creating a torque that continuously slows the rotation. This dissipates energy as internal heat until spin and orbit periods synchronise.

Once locked, one hemisphere receives permanent daylight, the other permanent night. The terminator — the boundary between them — is fixed relative to the body's surface.

Habitability Impact

Day side may be too hot for liquid water near the star

Night side can freeze atmospheric gases solid

Terminator zone may have a narrow habitable band

Atmospheric circulation could redistribute heat globally

Strong permanent winds driven by the temperature gradient

Known Examples

The Moon

To Earth

The most familiar example. The Moon's rotational period exactly equals its 27.3-day orbital period. The far side was completely unknown until 1959.

Pluto / Charon

Mutually locked

Both bodies are tidally locked to each other. Each always shows the same face to its partner — the only confirmed mutual lock in the solar system.

Mercury

3:2 resonance

Not fully locked, but in a 3:2 spin-orbit resonance — 3 rotations per 2 orbits. Once thought to be locked; the resonance is stabilised by its orbital eccentricity.

M-dwarf habitable zone

Likely locked

Planets in the habitable zone of red dwarf stars orbit so closely that tidal locking is expected within a few billion years, with permanent day and night hemispheres.