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Mercury Rotation

In earlier times it was thought that Mercury was tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth. But, radar annotations in 1965 proved that in fact, the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable.

The original reason astronomers thought it was tidally locked since whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it was totally locked. Because of Mercury's 3:2 spin-orbit resonance, although a sidereal day (the period of rotation) lasts about 58.7 Earth days, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days.

At certain points on Mercury's surface, an observer would be able to see the Sun rise about halfway, then reverse and set, then rise again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury's orbital velocity exactly equals its rotational velocity, so that the Sun's apparent motion ceases; at perihelion, Mercury's orbital velocity then exceeds the rotational velocity; thus, the Sun appears to be retrograde. Four days after perihelion, the Sun's normal apparent motion resumes.Mercury's axial gradient is only 0.01 degrees, which is over 300 times less significant than that of Jupiter, which is the second smallest axial tilt of all planets at 3.1 degrees. This means an observer at Mercury's equator never sees the sun more than 1/100 of one degree north or south of the zenith.