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.