Indeed, the fact that Mercury is not tidally-locked with the Sun was discovered using the Arecibo radar telescope. If I remember correctly, they used the Doppl

Indeed, the fact that Mercury is not tidally-locked with the Sun was discovered using the Arecibo radar telescope. If I remember correctly, they used the Doppler shift imparted to the radio wave --which expresses differently across the width of the rotating planet -- to learn this. "The rotation of Mercury was not discovered until 1965. Until then the most widely accepted theory had Mercury tidal locked to the Sun. Soviet scientists bounced radar signals off the planet’s surface in 1962 verifying that the planet rotated, but it wasn’t until scientists using the Arecibo Observatory verified the planet’s sidereal rotational period of 58.647 day." (From: https://www.universetoday.com/14008/rotation-of-mercury/ https://www.universetoday.com/14008/rotation-of-mercury/)