From Earth, humans seem to always see the same “face” of the Moon . Regardless of the time of year, the phases of the Moon may change from crescent, half-moon to full moon, but the characteristic craters, bright areas, and dark patches on its surface remain almost constant.
For thousands of years, the familiar pattern of bright highlands interspersed with vast, dark plains, known as the lunar "sea" or Maria, has always been oriented toward Earth. It is a sight that almost every generation of people who have lived on this planet has been able to observe with the naked eye.
This raises an interesting question: The Earth rotates on its axis, and the Moon also orbits the Earth, so why don't we see the entire surface of this natural satellite in turn? The answer lies in a crucial phenomenon of celestial mechanics called tidal locking.
The Moon still rotates on its axis, but it takes exactly the same amount of time to complete one orbit around the Earth.
Many people mistakenly believe that the Moon does not rotate on its own axis. In fact, the Moon rotates on its own axis. Remarkably, it completes one rotation in approximately 27.3 days, almost exactly the same amount of time it takes to complete one orbit around the Earth.
It is this coincidence that causes the Moon to always keep almost the same hemisphere facing us.
To put it simply: if the Moon did not rotate on its axis as it orbited the Earth, after completing one orbit, we would observe its entire surface. Conversely, if the Moon rotated faster or slower than its current speed, the portion of its surface facing Earth would constantly change.
But because the Moon's rotation speed and orbital speed are synchronized, every time it completes one revolution around the Earth, it also completes exactly one revolution around its axis. As a result, it always faces almost the same direction toward our planet.
In astronomy, this state is called tidal locking.
However, saying that we only see exactly 50% of the Moon's surface isn't entirely accurate. Due to a slight oscillation effect called libration, or apparent lunar oscillation, observers on Earth can see a small additional portion near the edge. In total, over time, humans can observe approximately 59% of the Moon's surface.
The Earth's gravitational pull has slowed the Moon's rotation.
To understand why the Moon is in a tidal lock state, we need to go back to when it first formed, about 4.5 billion years ago.
When it was young, the Moon rotated much faster than it does today. But Earth's extremely strong gravitational pull constantly exerted pressure on this satellite. Similar to how the gravitational forces of the Moon and the Sun cause tides in Earth's oceans, Earth's gravity also created very small but significant deformations in the Moon's structure.
Simply put, the Moon is not a perfect sphere. The difference in gravitational force causes it to appear slightly "bulging" along the junction between the Earth and the Moon.
During the Moon's rapid rotation, the position of this deformed section is not fixed but constantly shifts across its surface. This shifting creates internal friction within the Moon's material, gradually dissipating rotational energy.
Over time, this effect acts like a giant natural brake, causing the Moon's rotation speed to slow down progressively. This process, spanning billions of years, eventually brings it to a state of equilibrium: its rotation time equals its orbital time around the Earth.
When this state is reached, the hemisphere facing Earth is held almost fixed. And that is why, to this day, people on Earth still observe the same familiar side of the Moon.
Tidal locking is not uncommon in the universe. Many other natural satellites are also tidally locked to their host planets. Even Earth is subject to tidal effects from the Moon, causing our planet's rotation speed to change extremely slowly across geological timescales.
Why does the Moon change shape every night?
Although always facing the Earth, the Moon clearly doesn't look the same every night. Sometimes it's just a thin crescent, then gradually becomes a half-moon, then a full moon, and finally shrinks again.
This is not due to the Moon changing its direction toward Earth, but rather due to the phases of the Moon.
The lunar phase is simply the proportion of the surface illuminated by the Sun that we can see from Earth. At any given time, about 50% of the Moon's surface is always illuminated by sunlight. But from our observation point on Earth, we don't always see that illuminated portion.
For example, during a new moon, the illuminated hemisphere faces primarily towards the Sun and is on the opposite side of the Earth, making the Moon almost invisible. As the Moon continues to move through its orbit, the angle between the Sun, Earth, and Moon changes, causing the visible illuminated area to gradually increase.
During a full moon, the Earth is positioned near the center between the Sun and the Moon, so almost the entire hemisphere facing us is illuminated.
This is also why astronomers don't call the side of the Moon that doesn't face Earth the "dark side." This term is misleading, because that side also experiences day and night and is illuminated by the sun just like the side we see. A more accurate term is the far side of the Moon.
A complete cycle of lunar phases, from new moon to full moon and back to new moon, lasts approximately 29.5 days. This cycle is known as a synodic month.
(According to BBC Skyatnightmagazine, Astronomy, Earthsky)