Did you ever realize, Why we see only one side of Moon always? Because the other side’s a bit shy? Actually, No!
Rate of The Moon’s Rotation and Revolution Around the Earth
Around the Earth, the Moon takes almost 27.3 days to revolve. About the same time, it takes 27 days to rotate around its own axis. This concludes that the Moon orbits the Earth at a similar speed as the moon spins on its axis. Which ceases us to the primary reason why we see only one side of the Moon always? Thus, the same side of the Moon constantly faces the Earth, causing us to see the same side of the Moon.
1. Tidal Locking
Tidal locking means that a body spins around its own axis once for each time it orbits around another specific body in space. In other words, tidal locking occurs when an orbiting body in space always has the same face as the object it is orbiting. This is also called synchronous rotation; the duration of spinning and revolving around the partner of a tidally locked body is almost the same.
For instance, the same side of the Moon always faces the Earth, although there is a minor variation since the Moon’s orbit is not perfectly circular; it’s oblong or egg shape. Mostly, only the satellite is tidally locked to the larger bodies, with a circular orbit. Hence tidal forces locking of moon during the orbital period is another reason why we see only one side of the moon always. Apart from this, we should not overlook that the Earth is spinning around its axis.
But initially, to simplify our discussion, let’s assume for a while that the Earth stays still. Now, what if the Moon merely rotated on its axis, just revolved around the Earth? Then, maybe we could see the other side of the Moon (the dark side) as well. What if the Moon revolved around the Earth with half the speed with which it rotates around its own axis, keeping the Earth still? Woah! We here on Earth would be able to look up and see all the sides of the Moon. And, What if the Moon didn’t rotate, and it only revolved around us? We still might be able to see all the sides of the Moon, right? Yes.
Unfortunately, these are not the cases. Moreover, what if the Moon rotated just as fast as it’s revolving around the Earth? Spinning at a rate that matches the revolution so that only one side of the Moon ever faces the Earth is precisely the situation we find ourselves in today. Besides this, did you know we can see almost 59% of the surface of the Moon? You must not. Due to the Moon’s elliptical orbit, we can view 59% of the Moon’s surface from the Earth.
From the Earth, the only side we ever see, we call the near-side; and the side we never see, we call the far-side. The first time we ever saw the first side was from photographs sent by a Russian lunar spacecraft in 1959, but no human ever saw the far side directly until the Appolo 8 mission in 1968. Why can’t we here on Earth ever see the far side? Why does the Moon revolve around the Earth approximately as fast as it rotates? A good question, rather an excellent question.
2. Why Are the Rate of Rotation and Revolution of Moon Around the Earth Same
2.1. Center of Gravity on Near-Side of Moon
For starter, let me start with the basics about why we see only one side of Moon always viewed from Earth. We know Earth’s gravity gets weaker with increasing distance, but over the distance of a few meters or even kilometers, the same difference is not that much! The Earth’s pull of gravity on your head for all intents and purposes is the same as it is on your feet. The Moon, though, is thousands of kilometers broad and thousands of kilometers far. So the Moon’s near-side encounters a significantly greater gravitational field from Earth than does the far-side; what this means is that the Moon’s center of gravity, relative to the Earth, is not the same as its center of mass!
For your knowledge, the Moon’s center of gravity is the point upon which all of Earth’s gravity appears to act. You could crush the Moon to a single point, i.e., at the point of the center of gravity; gravitationally speaking, there would be no difference. On the other hand, the center of mass is the average point of all the atoms within the Moon! If the Earth’s gravity played evenly across the Moon, the Moon’s center of gravity would be the same as the Moon’s center of mass! But because the Earth’s gravity is stronger on the near-side of the Moon, the Moon’s center of gravity is shifted towards the Earth, i.e., on the near-side.
Now, the Moon’s center of gravity can be considered a point upon which the Earth pulls. If the Moon rotated a bit faster or slower, gravity’s center might be lifted away from the Earth. And you are well aware of what happens to the things lifted away from the Earth; that’s right, they pull back down! The Earth would apply what we call a torque, which would align the Moon right back to a place where the center of gravity is directly below the center of mass. This is what wards off the Moon from rotating any faster or slower than it actually does.
We can conclude as the Moon can only rotate so fast because the Earth won’t let it do otherwise. As a result, the Moon rotates only once for every revolution. Therefore, do we see only one side of Moon always visible from Earth! The difference is in Earth’s gravity; between the near and far side of the Moon has other effects. For example, it has the effect of stretching a moon into a somewhat oblong shape. The Moon is prolonged to an oval shape because the force of gravity on the Moon isn’t applied uniformly. Earth’s uneven gravity acting on the Moon causes an oblongish deformation.
Remarkably, the Moon has the same effect on Earth in the solar system. This is most recognizable with the Earth’s oceans since they are comparatively easier to stretch. As the Earth rotates below this oblong shape, the sea level goes up and down while we pass under each bulge. Because of this, we experience high tide twice a day because there are two bulges.
When we pass under the narrow regions of this oblong distortion, the two low tides form. This oblong distortion endures the oceans still relative to the Moon. Therefore, there is a bit of friction between the ocean floor in the water as the Earth rotates. The conflict on the ocean floor, combined with a small torque on us by Moon, reduces Earth’s rotation rate. As I mentioned earlier, back in the dinosaurs’ time, the day was about 19 hours long. In a billion years, a day’s duration will gradually slow down to about 46 hours. Solely one face of the Earth will face the Moon!! Much like only one face of the Moon now faces the Earth, just the opposite of today, which means no night for the other side of the Earth, exciting.
Due to the conservation of momentum, the Moon will be much farther away, till only one face of the Earth faces the Moon. The Moon slowing down Earth can only do so by moving outward, like a fast spinning ice-skater slowing down would need to throw her arms outward. Then the Earth and the Moon will be completely locked in, and they would dance around each other. This can also be termed as tidal locking since Earth’s same side will face the Moon. Contrary to this, the Earth is shy and always turning away from the Moon’s constant gaze.
But when Earth finally returns that gaze, when we’re finally wholly locked together, the Moon appears at a single location in our sky day and night. People on the other side of Earth won’t ever get to see the Moon, except for when they go on vacation to the moon side facing our planet, in that decade. I’m sure real-estate prices will be higher along with the sea level in a billion years from now; we will still be around!
Let’s conclude all the amazed reasons why we see only one side of moon always. First, Moon’s speed revolving around the Earth is almost the same as that of the Moon spinning on its axis. Moon’s speed revolving around the Earth and spinning on its axis is similar because Moon’s center of gravity is shifted towards the near side. If the Moon rotated faster or slower, gravity’s center might be lifted away from the Earth. Hence concluding the reason the Moon’s revolution speed is like the moon rotating speed.
-Edited by Steffy|20/7/22