How did radar observations reveal the rotation of Mercury?

Before the 1960s, astronomers incorrectly believed that Mercury’s rotation was synchronous, meaning it kept one face perpetually toward the Sun, matching its 88-day revolution period. This misconception arose because telescopic observers, limited by viewing conditions, repeatedly saw the same barely distinguishable surface features.1

Earth-based radar observations fundamentally corrected this view. By measuring the spectrum and time of flight of radio signals reflected from the planet's surface, scientists were able to deduce the actual rate of rotation. These radar studies revealed that Mercury rotates once every 58.6 Earth days—not 88 days—meaning it does not always show the same face to the Sun. This rotation period is exactly two-thirds of the planet's orbital period, a state maintained by tidal forces and the planet's orbital eccentricity.1234