For many decades, scientists thought the same side of Mercury always faced the Sun. But in 1965, astronomers discovered that Mercury completes three full turns on its axis for every two orbits around the Sun. Because of this motion, we see the same side of Mercury each time the planet comes closest to Earth. It is the only planet in our solar system with an orbital:rotational resonance ratio of 2:3. This resonance ratio is caused by Mercury's mass not being evenly distributed throughout the planet. One side is more massive than the other. Because of this, the Sun exerts a torque (a turning force) on the planet, especially at perihelion. After countless revolutions this has resulted in the rotational period of the planet being coupled with its revolution period.