The Moon’s Fixed Face
The Moon; a beautiful white ball in the night sky that many ancient civilizations that came to worship it as a God. The Greeks called this orb Selene, or Artemis, but we have gotten a habit of naming our celestial bodies after Roman gods and goddesses so we have come to also refer to the Moon as Luna.
The moon has been known to humans since millions of years before the human race existed. Even the dinosaurs have had the pleasure of staring up at the Moon and have the same kind of deep thoughts that we have, such as “Rawr…” The Moon has existed almost as long as the Earth, the most widely accepted theory is the Double Impact theory. But that’s not why we’re here today.
I’m sure some of you have looked up at the Moon at night and noticed that you only ever see the same side of it? That the familiar view of the Moon is the only view you will ever see without the help of the Lunar Reconnaissance Orbiter taking pictures and sending them back down to Earth. How come? Thankfully, in this Universe, sometimes the answer to one question gives you the answer to many other questions, and even some questions you couldn’t even hope to think of. Sir Isaac Newton gave us one of those answers.
Newton’s famous equation allows for the computation of the force of gravity between any two objects in the Universe. With a basic application of Newton's equation you can see that the force of gravity is greatest where an object is nearest another object and least at the point where it is farthest from the object. As you stand here on Earth, for example, Earth's gravity is slightly stronger at your feet than at your head. The differential is small, so don't blame your light-headedness on this phenomenon. The Earth pulls on your feet with a force that is only one ten thousandths of one percent stronger than that at your head. Not enough to where you could hope to ever notice the difference, but it is there.
This slight difference in gravity is known as tidal force, and is felt by all objects as they are pulled by all other objects in the Universe. Tidal forces are the direct causes of lots of natural phenomena; the rise and fall of the ocean tides, the gradual slowing of the Earth rotation which makes the days longer and longer, The Moon showing only one face to us here on Earth at all times, the Moon’s slow spiral away from the Earth.
The gravitational forces between the Earth and Moon cause some interesting effects; the most obvious to us is the tides. However, the Moon’s gravitational attraction to the Earth is strongest on the near side to the Moon and weaker on the far side. This gives the Earth’s oceans two bulges, one stretching towards the Moon, the other relaxing away from it on the opposite side to the world. But the Earth is not completely fluid; some parts like the continents are much more ridged. The Earth’s rotation carries the bulges slightly ahead of the Moon to produce a torque on it, an accelerating force on the Moon which causes it to fly out farther away from the Earth. This requires a transfer of energy from the Earth to the Moon, slowing our rotation by 1/500 of a second every 100 years, and the Moon to spiral outward by about 2 inches per year.
The asymmetric nature of this gravitational interaction has also slowed the Moon down to rotate synchronously with the earth, so that one side of the Moon always faces the Earth, becoming tidally locked to the Earth. But how did this part of the Moon come to face the Earth? By chance? Interestingly, no.
As you look up at the Moon, you will notice white areas and darker grey areas. The white areas are mountainous regions while the grey areas are lava mare (Latin for “sea”) made out of heavy basalt rock. If you see the far side of the Moon, you will notice a very white side of the Moon. The far side has virtually no lava beds, which raises the question as to why. As the Moon became tidally locked to the much bigger Earth, the entire core of the Moon shifted slightly closer to the Earth, and consequently closer to the near side’s crust. This heated the inside of the crust to very high temperatures and started to melt the inside of the crust until the overall thickness of the near side’s crust was much thinner than the far side’s. This made heavier impacts from meteors, comets, asteroids, etc. much more damaging, fracturing the crust and causing lava to bubble up onto the surface, creating a much heavier side. This was the final nail in the coffin, as the Earth is stabilizing the Moon in its orbit, trying to create the most energy efficient orbit by tidally locking the Moon, the Moon’s surface became heavier on the near side, allowing the Earth to gain a tighter grip on that face of the Moon, preventing it from rotating away.
In all honesty, I’m quite happy with the side of the Moon that faces us. It is very inspiring to look at, and many different cultures see different shapes on the face of the Moon. Some people can see a woman; others see a face, while others 856an see the famed “Man on the Moon.” It is quite a beautiful sight to behold. However, have any of you seen pictures of the far side? It is almost all completely white, except for one single dark spot near the middle. To me, it looks like a giant eye. How creepy would that be in the sky? A giant eye that follows you everywhere you went at night, staring down at you. I’m sure some religions would pick that up as being the “Eye of God” looking down at us from the heavens, keeping a watchful eye on us. But I still think it would be very creepy.
Sources and Additional Reading:
“The Extraordinary Back of the Moon”
“The Tidal Force”
“Explanation for Lunar Maria”
Image courtesy of: