Some individuals perceive a visage on the lunar surface; others discern a hare or a amphibian. Nonetheless, irrespective of one's interpretation of the moon's features, we consistently observe only one side of our celestial companion. So why is the far side of the moon never visible to us?
From our vantage point on Earth, it seems as though the moon doesn't undergo any rotation, yet it does indeed revolve on its axis, akin to our planet. However, the moon is tidally locked to Earth, signifying that its rotational period matches its orbital period around Earth — roughly one lunar month.
Tidal locking arises from gravitational forces between celestial bodies. The gravitational pull between Earth and the moon distorts both entities, causing a slight elongation toward each other, resembling an American football, as explained by Robert Tyler, a physical oceanographer at NASA's Goddard Space Flight Center. "That would be the shape if all the fluids and solids could respond instantaneously," Tyler elucidated to Live Science.
Nonetheless, the materials composing both Earth and the moon do not respond instantaneously. When these bodies exert gravitational influence on one another, friction ensues, decelerating the rotation of both entities.
For instance, "the moon is exerting a pull on the ocean, causing part of it to attempt to form a bulge directly beneath the moon," Tyler elaborated. But "the tides are interacting with the ocean floor and endeavoring to maneuver around continents." It necessitates time and energy to shift the tidal bulge — the protruding end of the football — in reaction to the moon's orbit around Earth.
A similar phenomenon occurs as lunar rocks adjust in response to Earth's gravitational pull. "Rocks are not elastic. When they undergo flexion, energy dissipates," remarked Matija Ćuk, an orbital dynamicist at the SETI Institute. "Energy must be sourced from somewhere, hence it is derived from the body's rotation." Consequently, the moon's rotation with respect to Earth gradually decelerated until it ceased.
Moreover, the moon's gravitational pull is also impeding Earth's rotation. Around half a billion years in the past, Earth potentially had a 21-hour day, Tyler speculated. Given sufficient time, the moon could further decelerate Earth's rotation to the point of becoming tidally locked to the moon, permitting only one side of Earth to ever glimpse the moon. However, this scenario wouldn't unfold until another 50 billion years — long after the sun's demise roughly 5 billion years from now.
Despite the impossibility of directly observing the moon's far side from Earth, spacecraft have captured photographs of it. Soviet spacecraft Luna 3 immortalized the initial images of the far side in 1959. Subsequently, numerous other spacecraft, including NASA's Lunar Reconnaissance Orbiter and China's Chang'e 4, the inaugural spacecraft to land on the far side of the moon, have snapped pictures of the lunar far side.
These images depict a landscape adorned with craters and comparatively fewer prominent dark patches — known as maria — than the near side. With fewer maria, discerning shapes such as a face or a rabbit on the lunar far side is more challenging, yet there remains ample intrigue to behold.
