Is Mercury tidally locked 1:1 like the Moon?

No—it is in a 3:2 resonance, so a solar day on Mercury is long and the Sun can appear to move oddly in the sky.

Does this integrate tidal torques?

No—it is qualitative kinematics for classroom intuition.

Spin–Orbit Resonance

Spin–orbit coupling can lock a satellite's rotation to its orbit (1:1 tidal locking, as for the Moon showing one face to Earth) or trap higher-order resonances. Mercury occupies a 3:2 resonance with the Sun: three sidereal rotations per two heliocentric orbits, a consequence of solar tides and orbital eccentricity rather than perfect tidal circularization. The animations here are schematic—Mercury also librates and feels general relativistic perihelion precession not drawn.

Who it's for: Planetary science after Kepler; complements tidal Moon page and Mercury GR precession in gravity.

Key terms

Tidal locking
Spin–orbit resonance
Mercury 3:2
Synchronous rotation
Moon
Tides

How it works

Tidal locking drives many satellites toward synchronous rotation: the Moon keeps one face toward Earth (1:1 spin–orbit). Mercury is special: solar tides helped trap a 3:2 resonance—three rotations per two heliocentric orbits—so a Mercury solar day is long and the subsolar point drifts oddly. This page is a schematic animation, not a full N-body/torque integration.

Key equations

Moon: ω_spin ≈ ω_orbit · Mercury: 3 T_spin = 2 T_orbit (trapped)

Frequently asked questions

Is Mercury tidally locked 1:1 like the Moon?: No—it is in a 3:2 resonance, so a solar day on Mercury is long and the Sun can appear to move oddly in the sky.
Does this integrate tidal torques?: No—it is qualitative kinematics for classroom intuition.