At аny gіven tіme, one ѕіde of the Moon аlwаyѕ fасes Eаrth, whіle the other ѕіde thаt we саnnot ѕee іѕ referred to аѕ the “dаrk ѕіde” of the Moon. So, why саn’t we ѕee the entіre Moon from Eаrth?
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The reason for this phenomenon is that the Moon is in synchronous rotation with Earth, meaning its rotation speed matches its orbit around Earth. As a result, no matter where the Moon moves in relation to Earth, only one side of it faces Earth.
Initially, when it formed, the Moon had a different rotation speed and orbit. Over time, Earth’s gravitational field caused the Moon’s rotation speed to gradually slow down until it stabilized and synchronized with its orbit around Earth. But what led the Moon to stabilize its rotation speed and orbit? The answer lies in the effects of friction and tidal deformation.
As we know, tides on Earth are caused by the Moon’s gravitational pull. Similarly, Earth also exerts a gravitational force on the Moon, although much stronger (about 81,000 times). So, while the Moon is trying to move in a straight line, Earth pulls it toward itself, making the Moon become a satellite orbiting Earth.
The gravitational force between these two closely positioned celestial bodies results in a phenomenon called tidal bulge, where the surfaces of both bodies are deformed by gravitational force, forming a bulge toward the other body. Initially, when the Moon was rotating rapidly, its tidal bulge advanced ahead of the Earth-Moon connecting line, as it couldn’t flatten the bulge fast enough to keep it on that line. The rotational force caused the bulge to always surpass this line. This phenomenon generated torque, slowing down the Moon’s rotation speed, akin to twisting a tightened screw. When the Moon’s rotation speed decreased enough to match its orbital speed, the bulge always faced Earth, aligning with the Earth-Moon line, and the torque disappeared. This explains why the Moon rotates at the same rate as its orbital speed, and we always see only one side of it.
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On the left is the side always facing Earth, and on the right is the dark side of the Moon.
Another intriguing fact is that we don’t actually see just 50% of the Moon’s surface; in reality, we can see 59% of the Moon’s surface. This is due to the Moon’s elliptical orbit around Earth, not a perfect circle. As the distance between Earth and the Moon changes, it alters the angular velocity (the rate of rotation around Earth), while the Moon’s rotation speed remains constant. As a result, we can see an additional 9% of the Moon’s surface. If the Moon’s orbital path were a perfect circle, we would only see 50% of its surface.
And if we were to live for another 10 or 20 billion years, we might always see the Moon, or we might never see it again. Why is that? Similar to what Earth did to the Moon, the gravitational force and tidal bulge caused by the Moon on Earth also affect Earth’s rotation. Although this friction effect is small due to the Moon’s weak gravitational pull, we don’t currently notice any difference. However, in a few billion years, only one side of Earth will always face the Moon. This also means that those living on the opposite side of Earth (if humans still exist) will never see the Moon again.