From Earth, the Moon consistently presents the same familiar face. This common observation, where craters and features appear unchanging, is due to a complex interplay of gravitational forces that have shaped the Moon’s behavior over vast stretches of time.
The Phenomenon of Tidal Locking
We always see the same lunar face due to tidal locking, also known as synchronous rotation. This occurs when a celestial body’s rotation period matches its orbital period around another object. For the Moon, it takes approximately 27.3 days to rotate once on its axis and to complete one orbit around Earth.
This alignment is the result of billions of years of gravitational interaction. Earth’s immense gravitational pull has exerted a braking effect on the Moon’s rotation. The Moon is not perfectly spherical; Earth’s gravity has subtly deformed it, creating slight bulges on its near and far sides.
As the Moon rotated faster in its early history, these bulges were constantly pulled back into alignment with Earth’s gravitational force. This continuous tugging created friction within the Moon, gradually slowing its spin. Over eons, this process continued until the Moon’s rotation rate decreased, aligning one of its bulges permanently with Earth, resulting in the current synchronous state.
Once this synchronized state was achieved, the gravitational forces found a stable equilibrium. If the Moon were to speed up or slow down slightly, Earth’s gravity would apply a corrective torque, pulling it back into the tidally locked configuration. This makes tidal locking a very stable and natural outcome for many moons and planets orbiting larger bodies throughout the universe.
Debunking the “Dark Side” Myth
A common misconception about the Moon is the idea of a “dark side.” This term is misleading, as all sides of the Moon experience both day and night, just like Earth.
The Moon continuously rotates on its axis while orbiting Earth and the Sun. During its approximately 27-day rotation period, every part of the lunar surface is illuminated by the Sun for about two weeks, then experiences two weeks of darkness.
Therefore, “far side” is a more accurate term. It refers to the hemisphere never visible from Earth due to tidal locking, not a perpetually unlit region. When the near side experiences night, the far side is bathed in sunlight, and vice versa.
Exploring the Moon’s Far Side
The Moon’s far side remained a mystery until October 1959, when the Soviet Luna 3 spacecraft captured its first images. Subsequent, more detailed missions revealed a lunar landscape distinctly different from the familiar near side.
The most striking difference is the relative absence of large, dark volcanic plains, known as maria, which are prominent features on the near side. The far side is instead heavily covered with impact craters of varying sizes and ages, giving it a much more rugged and ancient appearance. This heavily cratered terrain resembles other airless bodies in the solar system, such as Mercury.
Scientists propose several reasons for this stark difference. The far side’s crust is significantly thicker than the near side’s, by about 20 kilometers on average. This thicker crust may have prevented magma from easily reaching the surface to form large maria. Differences in the distribution of heat-producing radioactive elements within the Moon’s interior may also have contributed to the distinct geological evolution of its two hemispheres.
Can We Ever See More?
Despite the Moon being tidally locked, we actually get to see approximately 59% of its surface over time. This is thanks to lunar libration, which causes the Moon to appear to subtly wobble from our perspective on Earth.
There are two main types of libration. Optical libration is the most significant and arises from the Moon’s elliptical orbit around Earth and its axial tilt. Since the Moon’s orbital speed varies in its elliptical path, but its rotation rate is constant, we can sometimes peek around its eastern and western limbs. Because the Moon’s axis of rotation is slightly tilted, we can occasionally see a bit over its northern and southern poles.
Physical libration refers to minor wobbles in the Moon’s orientation caused by the uneven gravitational pull from Earth on its slightly irregular shape. These motions add to the observable surface area. While librations allow us to glimpse a larger percentage of the Moon’s surface, they do not negate the fundamental principle of tidal locking, which ensures the same general hemisphere always faces Earth.