Sidereal Rotation Period of Mercury

For eighty years the textbooks were wrong about this number. Schiaparelli concluded in 1889 that Mercury keeps one face to the Sun, rotating once per 88-day orbit the way the Moon does to Earth, and the tidal-locking argument was so plausible that nobody checked until Pettengill and Dyce bounced radar off the planet from Arecibo in 1965. The Doppler spread of the echoes gave 59 days, not 88.

The true state is subtler than a simple lock: Mercury rotates exactly three times for every two orbits, the only 3:2 spin-orbit resonance known in the solar system. The high eccentricity makes it stable — tidal torques are strongest at perihelion, and at perihelion the 3:2 spin briefly matches the orbital angular rate, so the Sun's grip finds nothing to correct. Italian dynamicist Giuseppe Colombo worked out the mechanism within a year of the radar result.