September 16, 2011
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On the razor edge between reality and fiction, there is a realm in which worlds we have never seen could, indeed might, exist. I first became interested in these worlds in 1990, when my older son, James, was 5 and inundating my wife and me with “What if?” questions (for more details, see here). As an intellectual exercise, creating those worlds has been especially satisfying for me, second only to making actual scientific discoveries. Both require that I apply and expand the known models of nature, without violating them. The consequences of creating “what if?” worlds, as you can see in my books What if the Earth had Two Moons? and What if the Moon Didn’t Exist? are very different planets from Earth as we know it today. What was equally satisfying for me was that creating alternative worlds also gave fascinating insights into the properties of Earth as it is today.
For example, consider what we can learn about Earth by asking what our planet would be like if the Moon had never formed. To understand the consequences of changing the cosmos, I began by considering Earth as it has actually been transformed by the presence of the Moon over the past 4.5 billion years. As Sir George Darwin deduced back in 1897, the Moon is spiraling away from Earth under the influence of the ocean tides that the Moon creates here. The fact that the Moon is moving away from us has been proven over the decades since 1969 when Apollo 11 astronauts left a reflector array on the Moon. By sending laser beams from Earth to the Moon and timing their round trips, astronomers have determined that the Moon is spiraling away from Earth at a rate of about 3.82 cm (1.5 in) per year. This means, of course, that the Earth and Moon were closer together in the past.
The recession of the Moon from Earth implies that the Moon is gaining energy, just as a ball can rise in the air only if it is given energy by being thrown upward. That energy given to the Moon comes from Earth, namely from our planet’s rotation – we are spinning more slowly today than we were when Earth first formed. Earth loses energy because the Moon creates tides on our rapidly spinning planet. These tides are not in a straight line between the centers of the two worlds. Rather, the high tide closest to the Moon is pulled ahead of the Moon by Earth’s rapid rotation. In turn, the water in that high tide gravitationally pulls the Moon in the direction of the Moon’s orbit, giving the Moon energy (like you would give a ball hanging on a string energy if you held the string and spun around). As a result the Moon (and the ball on the string) spiral outward. Earth, in turn, slows down because of friction between the water in the high tides and the body of Earth.
How fast Earth was originally spinning depends on how close Earth and Moon were when the Moon formed, a distance that is not yet known. The first “aha” moment for me in going through this argument came when I took reasonable assumptions for the Moon’s original distance and, using the conservation of angular momentum, calculated plausible spin rates for the young Earth. The length of the day was originally between 5 and 10 hours. Work done by Jihad Touma and Jack Wisdom (Figure 1, in a paper published after What if the Moon Didn’t Exist?) suggest a 5-hour day.
As an example of how much care one must use in creating alternative, scientifically plausible, worlds, let’s continue on with the scenario of there never having been a Moon. If it had never existed, then the tidal force from the Moon that slowed our Earth would not have occurred and Earth’s rotation rate would not have slowed due to a lunar tidal effect. Are there any other sources of tides on Earth that would have slowed it down? The answer is yes: the Sun and the planets, especially Jupiter, create tides here. But would they alone slow Earth’s rotation significantly? The tides from all the bodies except the Sun are negligible for our purposes. The Sun creates about 1/3 of the tides on Earth today, so it might seem that Earth would have slowed down 1/3 as much as it did in real life with the Moon in play. This is not correct. The vast majority of the rotational energy lost by Earth came when the Moon was young and very close to Earth. This realization led to the second “aha” moment about Earth as it is that I felt while creating a Moonless Earth. If the Moon was originally 10 times closer than it is today, the equations reveal that tides on Earth back then were 1000 times higher than they are today! (Tides vary inversely as the third power of the distance between the worlds.) Therefore, when they were young, the Moon would have been spiraling away and Earth would have been slowing down much faster than they are today. Taking this into account, tides created by the Sun on Earth without the Moon over the past 4.6 billion years would cause Earth to now be rotating with somewhere between a 7 and 12 hour day.
Two takeaway insights for me about our world as it really is, gotten from asking “What if the Moon Didn’t Exist?“, are that Earth was initially rotating somewhere between 2½ to 4 times faster than it is today and that the tides were once between a few hundred and a thousand times higher than they are now. These facts then provide insight into the evolution of life on our Earth, as I discussed in What if the Moon Didn’t Exist.
As noted above, every new world I have created provides deeper understanding of our present world: I gained insights into the relative simplicity of our present tidal cycle from What if the Earth had Two Moons? and into the complexity of biological clocks from What if the Earth were a Moon? (both in the book What if the Earth had Two Moons?). “What if?” gives us important insights into “What is.”
About the Author: Neil F. Comins has published research on general relativity, optical and radio astronomy, computer simulations of galaxies, and astronomy education. He is the author of 15 trade and textbooks and, more impressively, is a cartoon character in Japan. The profile photo of Neil F. Comins was taken at Mitsubishi Pavillion, World Expo Aichi, Japan, 2005. The pavillion was based entirely on his book, "What if the Moon Didn't Exist?" Follow on Twitter @ncomins.
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There is one interesting aspect about tides: high tide appears twice a day (at least: in the Atlantic Ocean, I am not familiar with the Pacific) Which is strange as the moon only passes a point on the surface of the day once a day. Now I am not the physisist but have an electrical engineering background, and we learn quite a lot about resonance.
What happens is that the tidal way is lifted by the passing moon, travels along the Atlantic Ocean, bounces against the American coastline, bounces backwards to Europe and Africa, while the moon continues its travel.
There is time enough for the tidal wave to back and forth one more time between Europe/Africa and the Americas, before the moon arrives again to lift it up.
Now what is the relation with the above article? Well, you can imagine that a 2 bounce behaviour like the Atlantic has different energy characteristics than a tidal wave that only travels once a day. Maybe the tidal way would cancel out completely if the sea has certain characteristics like having a width of exactly half the globe.
But yo! That has long been the case: Once there was a giant single continent (Pangaea) and one giant sea. Would your calculations not be a little bit different (understatement) if you take the (im)possibility to resonate into account?
Only after the relatively recent splitup between the Americas and the Eurazian/African continents a resonance of waves could occur.
Nice hypothesis, isný it?
Link to thisI think the jury is out on how the Moon may have affected abiogenesis, heavy bombardment, climate and evolution all. In some scenarios tides are useful for abiogenesis, in others (most) it doesn’t matter much. In some scenarios tides regulate climate by counteracting tilt, in others it doesn’t matter. Et cetera.
Interesting as it is, the idea that the Moon has regulated orbital period goes both ways. The impactor that created the Earth-Moon system should have had enough energy, unless I am mistaken, to impart the angular momentum all by itself.
Since I was in a hurry I only checked energy. Assuming the putative Mars massed planetoid impactor colliding with a typical speed of Earth orbital speed, it imparts 10^3 times (~ 3*10^31 J) the energy of Earth rotation (solid sphere momentum; ~ 3*10^28 J). So with a modicum of efficiency it should have been able to spin up Earth from stand still.
Now Earth should have had an angular momentum comparable to what it got as already orbiting and spinning planetoids aggregated to form it. But if the aggregation was with planetoids as it is believed, the result should then be akin to my quick check above: the collisions erased any original momentum imparted from the protoplanetary disk formation.
I think it is impossible to sort out what the angular momentum of the Earth-Moon system would have looked as before last large impact. As a reference, the 0.8 Earth massed (IIRC) Venus spins slowly retrograde.
Link to thisOops. I am of course assuming some form of glancing impact. But a dead center impact is unlikely.
Link to thisWithout a moon we would never have such wonderful poetic lyrics like the “moon in June” etc. What a tragedy that would be.
Link to thismeebi – I’m not a physicist either, but I think the elliptical tidal distortion of the Earth’s generally spherical form is more a function of surface tension than a resonating wave traveling around the Earth’s surface. I’m just guessing, though, and do not understand the dynamics involved.
Link to thismeebi
Nice theory but the reason for the two tides is to do with the fact that that centre of gravity is not a static point in the centre of the planet. As the moon moves around the Earth the two bodies pull on their respective centres of gravity.
So, when the moon is on the far side of the Earth from the Pacific, for example, gravity’s hold on that body of water is weaker. That, and the spin of the Earth cause the Pacific to rise. It later gets pulled back towards Earth and causes a high tide.
Phil Plait explains it in much more detail better than I can in his book, Bad Astronomy.
Link to this“The Tragedy of the Moon” by Isaac Asimov
Link to thisAND….a whole lotta music with moon, june and spoon wouldn’t have anything with which to rhyme, as well as nursery rhymes that wouldn’t have their muse, horror stories without their full moon, bare butts hanging out a car window. the list just goes on and on.
Link to thisAnother interesting question to consider. The moon rotates once every time it make one revolution around the Earth. Hence it appears on Earth that the moon does not rotate at all because the same side of the moon always faces the Earth. What if the moon had a faster rate of rotation? Would that have any effect on scientific development, mythology, psychology, poetry, and geophysics? There would be no stillness of the moon or of the night except during the new moon phase. There really is no dark side of the moon.
Link to this#9 The moon did have a faster rate of rotation when it first formed. The earth has de-spun the moon just as the moon to a lesser degree has de-spun the earth.
Link to thisI’m not sure if this is the right crowd but, I’ll give it a whirl:
In a world where mankind is ruled by a giant, intelligent beaver what food is no longer consumed?
…
Here’s a hint: It’s a European pastry.
Link to thisNo, no, no. What happens is that the moon pulls at the water on its side of the Earth, at the Earth itself, and on the water on the far side of the Earth – in order of decreasing strength. This means that all three of these things are pulled away from each other. The water closest to the moon gets pulled away from the Earth, creating a high tide on that side, and the Earth gets pulled away from the water on the other side, leaving a high tide on that side.
Link to thisI have always wondered if the moon had an effect on plate tectonics and continent formation. After all, the moon’s tidal forces must pull on the ground as well as the earth’s water, and cause some types of daily undulations in the earth’s crust (however minor). And as you mentioned, the tidal forces would have been stronger early in the earth’s history.
Have the moon’s tidal forces “greased” or helped the gradual formation and motion of the contentents over time. Would be even have continents without the moon?
Link to thisAnd also, “How Would the Absence of the Moon Affect Werewolves: The Fun of Counterfactuals in Science Fiction”
Link to this“Earth, in turn, slows down because of friction between the water in the high tides and the body of Earth.”
Not the entire explanation. Tidal friction works on rocks too. Otherwise these same tidal effects would not have slowed the moons rotation to coincide with its orbital period.
Link to thisI came upon a question at vark.com as to why vampires are able to venture out into the moonlight when moonlight is actually reflected sunlight.
Link to thisI at first skipped the question, then on second thought came back and opined that it is only in the paradigm of science that moonlight is reflected sunlight. In the paradigm of the vampire, the moon shines from its own internal source as the soul of the night.
“Earth, in turn, slows down because of friction between the water in the high tides and the body of Earth.”
No. The moon has slowed (despun) by the earths tidal effects on it, yet it has no oceans. No fluidity is required, just elasticity.
Link to thisInteresting information. I hadn’t ever heard that high tide was connected with the moon. Life would definitely be so different without the moon. Now you can even buy property on the moon? It will be interesting to see where the times will take us.
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