How did radar observations reveal Mercury’s true rotation period?

Prior to the 1960s, astronomers incorrectly believed that Mercury’s rotation was synchronous, meaning its rotation period was thought to be identical to its 88-day revolution period. This misconception arose because telescopic observers, limited by viewing conditions, consistently saw the same barely distinguishable surface features on each occasion, leading them to conclude the planet always kept one face toward the Sun.1

This belief was corrected through Earth-based radar observations, notably conducted at the Arecibo Observatory. By measuring the spectrum and time of flight of radio signals reflected from the planet's surface, scientists were able to accurately deduce Mercury's true rotation rate. These radar studies revealed that Mercury’s rotation period is actually 58.6 Earth days—exactly two-thirds of its 88-day orbital period—rather than being locked in a 1:1 synchronous rotation.123