[Earthlit Moon: Dorst/Druckmüller]
One of the biggest mysteries — and wonders — in the known Universe flies over our heads every day. Or night, to be more precise. Alas, few human beings ever come to realize these questions even exist, much less ponder about them.
Although often quoted polls suggest some people are still in the Middle Ages, most do know that planets are more or less spherical and revolve around the Sun. Or at least that they are not just points of light on a celestial bowl over a Flat Earth. Everything is moving, evolving and revolving.
Now, then why does we always see the same Moon? That is, the same old side, with a Man, a rabbit or even a woman carrying an infant? Our natural satellite is not flat, and even if it was, fact is we never get to see its other side. Because there is one. Popularly called the “dark side”, it’s in fact as often illuminated by the Sun as our familiar side, and that’s exactly because the Moon evolve and revolve. So how come it cannot be seen from Earth, ever? No human being had ever seen the far side of the Moon until 1959, when soviet probe Luna 3 captured the first glimpse of it.
One of the first images of the far side of the Moon, captured on 1959
The simple answer is that the Moon’s rotation period coincides with its orbital one, which in turn means that the time it takes for the Moon to turn around itself is the same as it takes for it to fly around Earth. The slightly more tricky question is why this happens to be so. It’s certainly no coincidence, one must assume. Given all the possibilities, it would be insane to think such a match would happen fortuitously.
Science does have an answer for it, and it involves the most traditional of the forces. Initially, the Moon — however it was formed — didn’t have equal rotational and orbital periods, but good old gravity took care of synchronizing them. Called the tidal locking effect, it explains this synchronization not only on our Moon, but on many satellites of the other planets that have also synchronized their rotation and orbital periods. It also means that this synchronization will continue for eons to come, even as these periods change. Long Live to the Rabbit on the Moon.
The mechanism of the tidal locking is fascinating by itself. While the rotational and orbital periods of a satellite are not synchronized, it will have tides. Here on Earth we associated tides with the sea water, but the effect also applies to solid matter, which doesn’t flow as easily. The result is friction, which generates heat, much the same way you can heat a paper clip by bending it repeatedly. This exact mechanism may be generating enough heat in one of Jupiter’s satellites, Europa, to sustain life under miles of ice!
All this heat doesn’t come from nothing, and it effectively decelerates the rotation period, exactly the one that determines the frequency of the tides. And it does that until the rotation period equals the orbital, when tides cease to be significant. It’s as if there were giant cogs linking the orbiting objects. It’s not surprising such intricate and precise celestial mechanisms were compared to clockworks.
And this mechanism (click for a longer and nicer explanation) works both ways: the tides we have here on Earth, due to the Moon, also generate friction, which decelerate the motions responsible for them. Meaning that in a few billion years, if our Moon is still in orbit, Earth would also synchronize its rotation period with that of Moon’s orbit around it. Meaning that the Moon would not only show one side, but that it would only be visible from one hemisphere of Earth, “fixed” in the sky above one point of the planet. Our Earth would also have a “far side” to anyone who happens to be on the Moon.
Neat, huh? A very nice example of the amazing success science has had on explaining things as nicely as clockworks mechanisms that we just mentioned. But there’s one more thing to be explained about the Moon, something just as casual but amazingly more intriguing, because cogs are nowhere to be seen. Not even the metaphorical ones.
The Great Cosmic Coincidence
This “mystery” can also be noted every day by looking at the sky. Every night and day, to be more precise. But it’s more explicit on total solar eclipses, when the Moon covers the solar disk almost perfectly. And it’s exactly that that is our mystery: the apparent sizes of the Moon and the Sun are practically the same. On a surprising proportion, the Sun is around 400 times bigger than the Moon, but it’s also around 400 times more distant.
It may seem absurd, preposterous, downright crazy, but as far as we know, this is a coincidence. There’s no gravitational, electromagnetic or any kind of mechanism to answer for this coincidence. Not just a coincidence, actually, but a mind-boggling unbelievable coincidence. To correctly understand and appreciate it, let’s give some thought to it.
First, things could certainly be very different. There’s no apparent reason why our only Moon and our only Sun should have the same apparent size in the sky. This coincidence of apparent sizes, unlike the tidal locking, doesn’t occur anywhere else in the solar system, in none of the other 64 moons of the many planets.
Then, the thing that tops it all is that this coincidence is… temporary! Just 150 million years ago the Moon must have been visibly bigger than the Sun. In another 150 million years, it will look clearly smaller, mainly on solar eclipses, which will no longer be total.
It just so happens our satellite is distancing itself from us, and it couldn’t care less if its apparent size viewed from Earth is the same as the Sun. Which means that the coincidence is not just in relation to the Earth-Moon system, it’s also temporal and just by pure chance, as it seems, it occurs when we evolved as an intelligent species capable of building civilizations capable of generating scientists capable of finally appreciating this coincidence.
And, let the Moon be praised. Without it, we may have not even been here. It effectively stabilizes the axis of Earth’s rotation, sans Moon we would have polar shifts all the time.
Some people would want for god to leave a clear message that he exists to us. He may just have.
Let there be coincidences
If you enjoy all this crazy talk, you may probably have already thought about a “god” arranging for such a convenient, and coincident, Moon. Placing a huge satellite, unique in many aspects, to stabilize our planet’s rotation and then, just as a nice touch, having it appear the same size as the star that gives us life. Very nice indeed.
But even if one would assume that our Moon was somehow an artificial gift, having it appear the same size as the Sun would mean that whoever was behind this would have a hell of a job. Because, as we have seen, this coincidence is temporary. The guy (or girl, or it) may just have had to exterminate the dinosaurs because they would have become intelligent before the right time. Or he may have never have seen an intelligent species arise on Earth. And if he had to control everything, including our own “evolution” to happen as it did, as some believe, why not just simply appear and tell us what he wants? Having the Moon the same apparent size as the Sun is a clever “message” for an absent god. For a god that is constantly meddling with us, that’s not a very clever way of communicating.
Which brings us to review the wonderfulness of this coincidence. Remember once again that it’s temporary, that our Moon looked bigger millions of years ago, and it will look smaller in some million years? That means this coincidence was bound to happen, and it’s in fact due to the fact that our Moon started from an orbit sufficiently close to our planet, an orbit that is gradually increasing in size. Given these conditions, none of which is so mind-bogglingly extraordinary, a coincidence in apparent sizes was bound to happen some day.
The only extraordinary thing here is that this coincidence is happening while we are here to watch it. But even this coincidence must not be seen as a divine message per se, because it could be the same fallacy as thinking how an amazing coincidence it is that I know that you are reading this sentence right now. This is no coincidence, if you are reading this, suddenly all the possibilities for what you could be doing are not endless. I know that you are reading this.
Much the same way, we can pick this particular coincidence in apparent sizes of the Moon and the Sun and marvel at it, but if it’s there, it’s there. There could have been an endless number of other coincidences, like having the word “Coca-Cola” written on the Moon by meteorite impacts, or the average apparent size of the human belly button being the same as the apparent size of the Moon. And the Sun. As far as I know, these coincidences don’t happen. Picking the coincidences that do happen and considering them miraculous is a typical human folly.
A particular version of this reasoning is also useful to consider, in a weak version of the Anthropic Principle. It “states that humans should take into account the constraints that human existence as observers imposes on the sort of universe that could be observed. In other words, the only universe we can see is one that supports life. If it were a different type of universe, we would not exist to see it”. Sounds familiar?
If it was a very different Moon, not big enough, incapable of stabilizing our planet’s poles, we would not exist to see it. Maybe some other species. But not we.
Did I debunk this silly coincidence? I hope not.
We do have a Great Cosmic Coincidence flying over our heads every day. And night. All the previous arguments hopefully discarded the more naive thoughts about it, replacing them with deeper reflections. Because the Moon is indeed one of the elements that made our existence possible. Because our time over this planet has been just an astronomical blink of an eye, and yet we can see with our own eyes, by not so pure chance, beautiful total solar eclipses. Which we can appreciate as coincidences. But not mere coincidences, but due to a Great Cosmic Coincidence.
Even Einstein must have been in great debt to this Great Cosmic Coincidence. Solar eclipses are not signs of death and destruction, as the ignorance of the past fueled our fears. They are signs of life, of the elements to which we owe our existence, as much as the fortuitous aspects of the Universe.
And all of this, as the scientific knowledge may finally allow us to realize. And prove. And appreciate.
The silver light, which, hallowing tree and tower, Sheds beauty and deep softness o’er the whole, Breathes also to the heart, and o’er it throws A loving languor which is not repose. — Lord Byron
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