# How is Mercury tidally locked if the ratio is not 1:1?

The ratio for a planet to be tidally locked has to be 1:1, but the ratio for Mercury is 3:2. How is Mercury tidally locked if the ratio is not 1:1?

• Do you have a source saying Mercury is tidally locked? Mercury has a spin-orbit resonance of 3:2. Tidal locking is the specific case where that spin-orbit resonance is 1:1. By that definition, Mercury is not tidally locked. My guess is, whatever source you may have was being loose with the definition of tidal locking and referring to all bodies with a spin-orbit resonance as being tidally locked. – zephyr Mar 7 '17 at 14:38
• @zephyr the Wikipedia page on tial locking mentions Mercury in a rotation-orbit resonance section, and implies that spin-orbit locking occurs by similar mechanisms as tidal locking. Multiple other sources also make this connection. Depending on what definition you use, whether this counts as tidal locking may be fuzzy. – Cody Mar 7 '17 at 18:50
• @Cody That page makes the distinction that higher spin-orbit resonances are not tidal locking. They specifically state may end up in a so-called spin-orbit resonance, rather than being tidally locked [...] A well known case is the rotation of Mercury, which is locked to its own orbit around the Sun in a 3:2 resonance. They indicate these higher-order spin-orbit resonances (i.e., higher than 1:1) are separate from being tidally locked. – zephyr Mar 7 '17 at 18:54