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View Full Version : The Earth, Moon, Tides, Orbital Paths, Life....... heck, everything.


Cartooniverse
05-12-2005, 05:07 AM
Sure was a clear sky out the other night. I could see the moon. A sliver was lit but I could see it all, against the deeper inky blackness of space. I tell you, it gets a Doper wondering.

If the moon rotated around the earth in a perfect path, there would be no tidal force on the planet that would affect the oceans. Right? If there were no tidal forces on the oceans, would life still have evolved the same way it did? ( For the sake of this G.Q., since this ain't G.D., please allow the thread to stand on the supposition that life emerged from a primordial soup/ocean mass billions of years ago. ).

Is it possible that something as random as the path followed by the moon would have had enough affect on the earth's oceans as to alter the development of life? I am feeling that the tides literally had something to do with the emergence of life. I am wondering if ocean masses that more or less are standing water masses on other planets would develop or sustain life in the same way? Because of the tidal forces, materials from extremely deep out in the oceans eventually wind up being tossed up on the shorelines.

How would a lack of tidal forces contribute to a lack of development of life on earth? Were the tidal forces that existed billions of years ago ( and of course, still do now ) a key element in this process?

And, to plug in the Everything portion of my question, what the heck are the odds that the S.E.T.I. folks will ever find a combination of planetary masses, forces and raw materials anywhere else that have produced the exact results needed for not only life, but advanced life to develop?

Here, let me freshen up that cup of coffee. You're going to need it. :D

** For the sake of the factual portions of my O.P., let's see if this can survive in G.Q. before it gets shuffled off to G.D. **

Cartooniverse

mittu
05-12-2005, 05:25 AM
There is an equation to calculate the probability of intelligent life existing in the universe, it is as follows:

The Drake Equation, which attempts to estimate the probability of intelligent life elsewhere in our Galaxy is:

N = R* fp ne fl fi fc L

Where:

∑ N = the number of civilizations in MWG that could contact each other.

∑ R* = the rate at which stars form in the MWG.

∑ fp = the fraction of these stars which have planets.

∑ ne = the number of planets per solar system suitable for life (earth-like planets)

∑ fl = the fraction of these planets on which life actually arises.

∑ fi = the fraction of these life forms which develops intelligence.

∑ fc = the fraction of intelligent civilizations which choose to communicate and develop adequate technology.

∑ L = the lifetime of such a civilization.

Quoted from here (http://www.phyast.pitt.edu/Resources/Education/classes/astro88/0088u25.htm) though I don't believe they created the equation. The difficulty of such an equation is that it still involves a lot of guess work, especially on the factors L and fl. I have only ever seen one attempt to come up with an ultimate answer using this equation and it somewhat conveniently came up as 1, that 1 civilisation being us, all rather too convenient for me. IIRC they used 1,000 years as the approximate lifetime of a civilisation, that is before they either use up all of the available resources and destroying their planet or end up killing each other due to technology becoming so advanced that it becomes "easy" for just about anyone to kill everyone else.

I'm not 100% sure what MWG stands for in this context, a best guess would be "Milky Way Galaxy".

I'm not sure what the point of the SETI program is, even if we do recieve and decode a message from the depths of space saying "Hello, anybody out there?" we lack the technology to respond in a timely manner. I suppose it would solve the question of whether we are alone or not but we couldn't really communicate.

Malacandra
05-12-2005, 05:44 AM
Sure was a clear sky out the other night. I could see the moon. A sliver was lit but I could see it all, against the deeper inky blackness of space. I tell you, it gets a Doper wondering.

If the moon rotated around the earth in a perfect path, there would be no tidal force on the planet that would affect the oceans. Right?

Wrong.

We get tides because the side of the Earth that's furthest from the Moon "wants" to orbit the Moon slower than the side that's nearest, but is forced, because the Earth is one big lump, to orbit it at the same speed, or a little faster. But the oceans are able to deform to try to follow the path they "want" to.

(I know it sounds odd to talk of the Earth orbiting the Moon, but in simple terms that is what tides are down to.)

Whether the Moon orbits in a circle or ellipse doesn't greatly affect this. Mind you, it's damn nearly impossible for the Moon to settle into a circular path. An ellipse is self-correcting - as the Moon gets nearer, it speeds up, so it moves out, so it slows down, so it gets nearer...


If there were no tidal forces on the oceans, would life still have evolved the same way it did? ( For the sake of this G.Q., since this ain't G.D., please allow the thread to stand on the supposition that life emerged from a primordial soup/ocean mass billions of years ago. ).

Probably not. It's a fair guess that pools of water left stranded by the tide provided a niche where it was an advantage to be able to survive in shallow water, and eventually in no water at all.


Is it possible that something as random as the path followed by the moon would have had enough affect on the earth's oceans as to alter the development of life? I am feeling that the tides literally had something to do with the emergence of life. I am wondering if ocean masses that more or less are standing water masses on other planets would develop or sustain life in the same way? Because of the tidal forces, materials from extremely deep out in the oceans eventually wind up being tossed up on the shorelines.

How would a lack of tidal forces contribute to a lack of development of life on earth? Were the tidal forces that existed billions of years ago ( and of course, still do now ) a key element in this process?

And, to plug in the Everything portion of my question, what the heck are the odds that the S.E.T.I. folks will ever find a combination of planetary masses, forces and raw materials anywhere else that have produced the exact results needed for not only life, but advanced life to develop?

Here, let me freshen up that cup of coffee. You're going to need it. :D

** For the sake of the factual portions of my O.P., let's see if this can survive in G.Q. before it gets shuffled off to G.D. **

Cartooniverse

The odds? No data. We know of one world in the entire universe where it has happened. That's not enough to extrapolate from.

NillyWilly
05-12-2005, 06:54 AM
I thought tides were caused by the gravity of the moon "squeezing" Earth. Since water is free to move it rises in the areas where the Earth is squeezed lower. Or something like that.

Any physicists that can tell me where I'm mistaken, as I surely am.

Excalibre
05-12-2005, 06:59 AM
I thought tides were caused by the gravity of the moon "squeezing" Earth. Since water is free to move it rises in the areas where the Earth is squeezed lower. Or something like that.

Any physicists that can tell me where I'm mistaken, as I surely am.
It's more just that the oceans are freer to move under the influence of the moon. So they end up pulled into a sort of egg shape, which surrounds the more spherical rock beneath.

Malacandra
05-12-2005, 07:05 AM
The simple explanation is that the side of the Earth facing the Moon gets pulled more than the middle, which gets pulled more than the far side. The planet itself can't deform, but the oceans can. However, this simple answer is what's called a "lie to children", a bit like Newton's theory of gravity or the Bohr model of the atom. What I said above about orbits is closer to the truth.

Time to page the Bad Astronomer, methinks.

CurtC
05-12-2005, 08:30 AM
I thought tides were caused by the gravity of the moon "squeezing" Earth. Since water is free to move it rises in the areas where the Earth is squeezed lower. Or something like that. I think you're saying that the Earth's crust gets squashed somewhat, and the water rushes in to make the Earth spherical again, making the water level with respect to the land be higher.

That's not right.

Think of it this way. If the Earth always had the same side towards the Moon, the Earth would attain an elliptical shape, with the longer axis pointed towards the Moon. Since the Earth is spinning and the solid part (the crust) can't bend quickly, it stays pretty much circular, but the oceans, which can move much more easily, come in to fill out that elliptical shape. So the overall shape of the oceans is elliptical at any time, but the ellipse is always having to move with respect to the land mass because the land mass is spinning.

Mycroft H.
05-12-2005, 09:46 AM
Iíve thought about the effect of tides on evolution for a few decades now. (Well, not continuously or even that frequently.) I only recall two times where the possible impact of the moon was discussed, and I canít recall the sources or specifics. I do recall that one said that it was inevitable for swimming critters to eventually make use of the land niche, so the moon was not necessary. The other article mentioned that the moon was necessary, but sadly I did not keep a copy. Unfortunately my answer thus far isnít very good for factual GQ. Basically, from what I have come across, it is not thought of much and the opinions are split. Granted, I have not read any recent textbooks or scientific articles dealing the subject. The articles are what I came across in general reading. As this is a subject that interests me, I hope I will be forgiven for blabbing on about further thought better suited for GD.

My personal speculation is that our oversized moon assisted in the evolutionary process. Firstly, I wonder how common or unique it is for a rocky planet, as opposed to a larger Jupiter-type planet, to have a satellite that is a significant percentage of the size of the main planet. From my limited understanding of planetary formation, I believe that the earth having a satellite the size of the moon, big enough to affect tides even, is rather unexpected. I would expect that the action of swimming critters periodically being trapped in tidal pools would have created an evolutionary push towards critters that could scamper across land enough to get back to the comfortable ocean. It is my belief that the tide producing moon advanced the evolutionary process by a huge degree, perhaps by orders of magnitude. For my own belief, I manage to fit God into the equation, although one with a strange sense of humor and who would offend mainstream religious groups. (Consider eclipses. That has to be more than a grand Cosmic Coincidence. To me it is proof of a God with a quirky sense of humor. However, there is no requirement for anyone else to hold the same belief.)

As for life elsewhere, well that is, of course, still in the realm of speculation. The universe is so huge (I canít recall the proper HHGttG phrasing) that Iím sure that of the billons of stars in each galaxy and the billions of years galaxies have been around, in all probability there are at least one or two (or dozens?) of sentient life forms that develop in each galaxy. It is sadly unlikely that we will ever come across any of them though (IMHO). How does the quote go? As unlikely as it is that there is intelligent life somewhere else in the universe, it is even more unlikely that we are totally alone. Something like that.

Kevbo
05-12-2005, 10:11 AM
As stated above there are several ways to look at tides. The key is understanding that gravitational force decreases with distance, so the effect of lunar gravity depends on how far you are from the moon, which depends on which side of the earth you are swimming on.

The tidal forces ar reciprical, and since the earth is more massive, the moon is subjected to larger tidal forces than the earth. Exept that even were there seas on the moon, they would not have significant tides. ..because the the earth is almost stationary to a lunar observer. The tidal forces from the earth are credited with bringing the moons rotation into sync with it's orbit. As well as the evidence of heavy vulcanism in the moons early history.

The vulcanism angle may relate to evolution on earth. Certainly vulcanism is possible, even likely without the influance of a massive satillite, but it is very likely that the moon's influance greatly increases terrestrial vulcanism, plate movements, etc. Sulfer and other compounds which spew from volcanos is thought to have played a part in pre-life amino acid formation.

Also, there is overwhelming evidence that evolution took a huge turn when a massive metore struck the earth, causing massive extinctions, the rise of mammals, etc. It is likely that the moon provides some shielding from this type of catastrophy (though many think it was formed by just such an event). If that were true, it could give various life forms a longer time window to evolve before being "reset" by the next big smackaroo.

Stranger On A Train
05-12-2005, 10:27 AM
One significant effect the moon has had has been to slow the Earth's rotation (and it continues to do so, albeit at a diminishing rate, until the Earth and Luna are in tidal lock). This certainly has had an impact on photosynthetic organisms and meteorological conditions, though its individual contribution to the evolution of life is highly complex and not easily quantified.

It used to be thought that a large moon "skimmed" atmosphere from the Earth, and the lack of a large, close moon would result in a greenhouse world like Venus, but that theory has fallen from favor decades ago.

Although the human menstraul cycle is roughly equivilent (on average) to the lunar cycle, this appears to be a coincidence, and is not the case for other mammals who have eustrus cycles that do not mach lunar rhythms. The Master speaks (http://www.straightdope.com/classics/a990924.html) on this topic.

And of course, having a local moon with a high albedo has led to numerous romantic sonnets involving that phenomenon, without which it is sometimes doubtful that the female Homo sapiens sapiens could be compelled to cojoin with the male member of that benighted species and thus perpetuate the germ line. Think of it as a counter-influence to NASCAR and pee-splattered toliet seats. Og help us when a contingent of militant feminist genetic bioengineers figure out how produce children from two ova; men's only saving grace then will be our ability to read maps and open pickle jars. :p ::ducking for cover::

Stranger

Bytegeist
05-12-2005, 10:31 AM
I only recall two times where the possible impact of the moon was discussed, and I canít recall the sources or specifics. I do recall that one said that it was inevitable for swimming critters to eventually make use of the land niche, so the moon was not necessary. The other article mentioned that the moon was necessary, but sadly I did not keep a copy.

I'm a little vague on my citations too, but I recall that at one time there was another popular conjecture around: that the Moon was necessary for thinning the Earth's atmosphere enough to prevent the planet becoming like Venus. Larry Niven used this idea for the premise of at least one of his science-fiction short stories. Not that I'm citing Niven as an authoritative source here.

However, I believe that this conjecture has long ago fallen out of fashion among planetary astronomers. (On preview I see Stranger beat me to this.)


... I wonder how common or unique it is for a rocky planet, as opposed to a larger Jupiter-type planet, to have a satellite that is a significant percentage of the size of the main planet. From my limited understanding of planetary formation, I believe that the earth having a satellite the size of the moon, big enough to affect tides even, is rather unexpected.

It's now hypothesized that the Moon is probably the result of a massive collision that the Earth experienced, very early in its history, with a body approximately the size of Mars. Collisions with large meteors ó and damn if that ain't a large meteor ó would have been very frequent in the solar system's early period, although a huge collision like that would have been a singularly rare event. And so, the Moon could certainly be a fluke, astronomically speaking.

Whether the Moon, as a generator of ocean tides, was essential to life's development on Earth is of course a different question. But in any case, it could be that the increase in mass that the Earth underwent, in this ancient collision, was essential to the development of life later on. So Earth anyway is probably a fluke ó but that we already knew.


For my own belief, I manage to fit God into the equation, although one with a strange sense of humor and who would offend mainstream religious groups. (Consider eclipses. That has to be more than a grand Cosmic Coincidence. ...)

It's only a coincidence in "recent" times ó recent on geological time scales that is. In the solar system's early history, the Moon was much closer to Earth, and would have appeared noticeably larger than the Sun when the two were close together in the sky. Eclipses of both kinds would have been more frequent, and solar eclipses would never have been annular. (Only total, or partial.)

I'll admit though that it's a startling coincidence that the time when the Moon and Sun's angular diameters match each other happens to be about the same time that a species of hairy chimpanzee achieved sapience, built cities, and started making telescopes and space probes. You wouldn't have predicted that to happen, four billion years ago.

Perderabo
05-12-2005, 10:39 AM
Chronos gave us Why does the same side of the moon always face the earth? (http://www.straightdope.com/mailbag/mlunarrotate.html). After reading that, I finally felt that I understood tides. Please read it. It is a classic.

Malacandra
05-12-2005, 01:35 PM
I'm a little vague on my citations too, but I recall that at one time there was another popular conjecture around: that the Moon was necessary for thinning the Earth's atmosphere enough to prevent the planet becoming like Venus. Larry Niven used this idea for the premise of at least one of his science-fiction short stories. Not that I'm citing Niven as an authoritative source here.


Wrong Way Street. And you could cite worse authorities than Niven: he likes his science pretty durn hard (even if he had the Earth rotating backwards in the first edition of Ringworld, and the Ringworld itself was unstable).

I'll admit though that it's a startling coincidence that the time when the Moon and Sun's angular diameters match each other happens to be about the same time that a species of hairy chimpanzee achieved sapience, built cities, and started making telescopes and space probes. You wouldn't have predicted that to happen, four billion years ago.

Ah well, we can probably invoke the anthropic principle again...

RM Mentock
05-12-2005, 01:50 PM
Chronos gave us Why does the same side of the moon always face the earth? (http://www.straightdope.com/mailbag/mlunarrotate.html). After reading that, I finally felt that I understood tides. Please read it. It is a classic.
The second link in that column to a Cecil column is busted, currently it is at Why are there high tides twice a day when the earth rotates beneath the moon only once a day? (http://www.straightdope.com/classics/a5_199.html)

Cecil disagrees with Chronos. We discussed this in a previous thread (http://boards.straightdope.com/sdmb/showthread.php?t=159592)
The simple explanation is that the side of the Earth facing the Moon gets pulled more than the middle, which gets pulled more than the far side. The planet itself can't deform, but the oceans can. However, this simple answer is what's called a "lie to children", a bit like Newton's theory of gravity or the Bohr model of the atom. What I said above about orbits is closer to the truth.

Time to page the Bad Astronomer, methinks.
I would say that your "lie to children" is closer to the truth.

The Bad Astronomer's explanation (http://www.badastronomy.com/bad/misc/tides.html) agrees, and agrees with Cecil's.

Satchmo
05-12-2005, 02:31 PM
Wow, I can't believe I'm the first to mention this. But even if the Earth had no large moon, there would still be tides caused by the sun, albeit smaller. Whether or not the difference in size would change the probability of land-based life developing or not is left as an exercise for the reader.

I remember Asimov writing about this, I think he more or less concluded that possibly life is fairly common in the universe, but land-based life is very rare. Hard to work metals in the ocean.

RM Mentock
05-12-2005, 02:41 PM
I remember Asimov writing about this
Triple Triumph of the Moon (http://www.mountainman.com.au/i_asimov.html), by Isaac Asimov:
The moon made it possible for man to evolve and so exist.
The moon made it possible for him to develop mathematics and science.
The moon made it possible for him to transcend Earth and conquer space.

Stranger On A Train
05-12-2005, 02:52 PM
I remember Asimov writing about this, I think he more or less concluded that possibly life is fairly common in the universe, but land-based life is very rare. Hard to work metals in the ocean.Not really. Metals precipitate readily in the ocean; see manganese nodules. It is, however, hard to generate rapid oxidation required for high energy reactions, as we do with fire, which creates some limits on tool-making ability. One imagines that a sentient sea-borne life would have to build technology on electrochemistry rather than fire-based metallurgy.

Sealife has no problem developing tool-using capability, though. Witness the octopus.

Stranger

rjung
05-12-2005, 03:12 PM
A book related to the OP:

What If the Moon Didn't Exist?: Voyages to Earths That Might Have Been (http://www.amazon.com/exec/obidos/tg/detail/-/0060168641/qid=1115929123/sr=8-1/ref=sr_8_xs_ap_i1_xgl14/002-0207196-0544017?v=glance&s=books&n=507846)

I used to have a copy; pretty good read.

Chronos
05-12-2005, 03:38 PM
Mind you, it's damn nearly impossible for the Moon to settle into a circular path. An ellipse is self-correcting - as the Moon gets nearer, it speeds up, so it moves out, so it slows down, so it gets nearer...I think that what you mean is that an elliptical orbit is stable: That is to say, that if you have two point masses in the Universe, and they start off in an elliptical orbit, they'll stay in that elliptical orbit forever. Unfortunately, our Universe is a bit more complicated than that, with considerably more than two objects, most of which aren't points. As a result, there are many forces which can change orbits, and if an orbit is changed by some means, there's no "self-correction" which would cause it to change back. As it happens, most of the factors which change orbits tend to cause them to become more circular: This is why the orbits of all of the planets and most moons and asteroids are so close to circular. In the case of the Moon, the primary circularizing effect is tidal forces, but friction with an atmosphere or the interplanetary medium, collisions with other orbiting bodies, and relativistic effects, where they're relevant, all tend to cause orbits to become more circular.

On the question of the likelihood of life/intelligence/civilization, etc., we have very little data, having only one world known to have life, and only a handful more where we can even attempt to detect it (at least with current technology). We're still not convinced that Mars never had life (though we're pretty sure it doesn't now), and we don't even have a clue yet about Europa (the next-best prospect known, after Earth and Mars). Outside of our Solar System, we can't even detect an Earthlike planet yet, much less any hypothetical life it might bear.

However, that one data point does tell us a few things. Most obviously, it tells us that life is at least possible. Perhaps more significantly, life on Earth arose very early, in the Earth's history, showing up only a few hundred million years after the crust solidified. I believe it was Carl Sagan who argued that this rapid appearance of life on Earth suggests that life is very likely to arise, given the opportunity.

As for the question of communication with alien intelligences, the first message would tell us little more than the fact that we're not alone (which itself would, I think, be a significant enough finding to justify SETI). But that would not be the extent of it. While the first message would probably just be a list of prime numbers or digits of pi or some other such proof of sentience, more messages would follow. Any civilization which has developed radio is bound to have learned a thing or two in the process, and they could share that information with us. Even if we assume that their technology is on average at the same level as ours, it won't be the same as ours. Some things that they've discovered would have been overlooked by us, and vice-versa (of course, there's no way of knowing just what those discoveries might be). Both civilizations could only benefit from exchanging notes in this manner.

Stranger on a Train, I, too, had heard the hypothesis that the Moon was partly responsible for the Earth's thin atmosphere, but I hadn't heard that it had passed out of vogue. What is the present dominant explanation for why our air is so thin?

Smeghead
05-12-2005, 04:51 PM
The Earth, Moon, Tides, Orbital Paths, Life....... heck, everything.


42, of course!


Seriously, though, I seem to recall that one of Steven Baxter's "Manifold" books included descriptions of alternate Earths with different sized moons and how they affected human evolution, if you're interested.

sciguy
05-13-2005, 01:30 PM
Related to the OP and evolution, there is a situation where the Earth-Moon system wouldn't cause tides. If the moon was in geosynchonous orbit, there'd be no cyclical tides, just a stationary bulge of water.

Of course, geosynch orbit is pretty darn close for an object that big. Geosynch is roughly 36,000 km above the surface of the earth, the moon's orbit is about 10x that. But the moon's orbit has been expanding slowly due to tidal effects (Earth slows down the Moon's velocity, causing the orbital radius to expand). Do we know if the moon was ever that close, and if so when? I suppose there's no way to know for sure (since there's still debate on where the Moon came from), but does taking the current orbital change rate and extrapolating back to geosynch give an obviously wrong answer (before the formation of the solar system, for example)? Or is it too hard to figure out, since the Moon's tidal effects on the Earth has also changed the length of a day?

Stranger On A Train
05-13-2005, 03:58 PM
Stranger on a Train, I, too, had heard the hypothesis that the Moon was partly responsible for the Earth's thin atmosphere, but I hadn't heard that it had passed out of vogue. What is the present dominant explanation for why our air is so thin?Well, my primary source is Niven, too. :D Seriously, while I am not a planetary scientist nor do I play one in any distributed medium of fictionaly entertainment, my knowledge on the topic, based upon a cursory reading, is that the moon skimming hypothesis was falsified by modeling of the tidal effect of the moon on the escape velocity of atmospheric molecules in the Earth's primordal reducing atmosphere. Even a faster moving, closer orbiting moon would not provide a sufficient impetus for outgassing to explain the difference between Venus' runaway greenhouse effect and Earth's oxidizing atmosphere. The moon does cause a certain amount of outgassing, of course, but we're in a more-or-less state of equilibrium, and the effect on the atmosphere beneath the stratopause is insignificant.

The current family of theories that are most widely accepted are based upon the emergence of the carbon cycle dominated by aerobic life, which absorbed most of the organic constituants and precursors and bound them nonvolitile forms. (One estimate I've read is that 200 times the original carbon content of the atmosphere is now bound up in plant life and other non-atmospheric forms.) Here's (http://www.astronomynotes.com/solarsys/s11.htm#A2.3.3) an astronomynotes.com cite on the effect of the carbon cycle on the Earth's atmosphere (though it is pretty sparse on detail information.) Because we have essentially no fossil history on the anaerobic life that first developed on the primordial Earth we have a number of uncertainties about how the atmosphere developed and indeed what the original constituants and their proportions in the Earth's reducing atmosphere we can only guess at the incidence and order of mechanisms and processes, and so there are many competing theories but I think the general principle is accepted by the planetary science community. Perhaps someone with experience in paleometeorology or planetology will be able to provide more details.

Interestingly, although we consider the greenhouse effect to be a dangerous, runaway process, one might imagine that anaerobic based life would think the same about the modern oxidizing atmosphere with it's hideously caustic proportion of molecular oxygen. Niven has a short story--one of the Draco Tavern tales, though the title escapes me at the moment--about an ancient trader who had previously visited Earth over a billion years previous, and lamented the extinction of the then dominant species to the "atmospheric pollution" of the waste products created by photosynthetic blue-green alge--oxygen! One speculates that such a race might have a complementary fear of the "bluesky effect" eliminating their protective blanket of heat and moisture-protecting clouds.

Stranger

Stranger On A Train
05-13-2005, 05:03 PM
Related to the OP and evolution, there is a situation where the Earth-Moon system wouldn't cause tides. If the moon was in geosynchonous orbit, there'd be no cyclical tides, just a stationary bulge of water.Well, even disposing of other considerations (gravitational influence of sun and other major planets, effect of solar wind and albedo, et cetera) this still wouldn't be the case. One issue is that such a large object in geosynchronos orbit would exert its own significant gravitational pull back on the Earth; therefore, one could not dismiss the mass of the moon as being insignificant as compared with the Earth. Such a system would have both objects tidally locked to each other and have to orbit about a common center of mass. (The earth and moon system do, in fact, orbit about a common COM, but it is well within the surface of the planet.) At the Earth's current rotational speed, the system would not be stable; the centrifugal effects would exceed the gravitational attraction and pull both bodies apart because the kinetic energy would always be higher than the gravitational potential energy (though I admit I haven't run the calculation and am just basing this from the guestimate that they wouldn't coincidentally be in balance).

Also, while the tidal forces might be constant the geologic stresses would not be; both worlds would experience significant shocks and shear effects that would alter their nominal shape and potentially tear them apart. And any irregularity in their orbit about one another--nutation due to out of plane periodic movements, or side components due to precession--would make such a system unstable even over relatively short timeframes. Theoretically, you could have any number of objects orbiting a common center of mass; in reality, we almost always have large masses as the focus (or nearly so) for a much less massive object, Pluto and Charon excepted.

Stranger

RM Mentock
05-13-2005, 07:25 PM
Well, even disposing of other considerations (gravitational influence of sun and other major planets, effect of solar wind and albedo, et cetera) this still wouldn't be the case. One issue is that such a large object in geosynchronos orbit would exert its own significant gravitational pull back on the Earth; therefore, one could not dismiss the mass of the moon as being insignificant as compared with the Earth. Such a system would have both objects tidally locked to each other and have to orbit about a common center of mass. (The earth and moon system do, in fact, orbit about a common COM, but it is well within the surface of the planet.)
??
At the Earth's current rotational speed, the system would not be stable; the centrifugal effects would exceed the gravitational attraction and pull both bodies apart because the kinetic energy would always be higher than the gravitational potential energy (though I admit I haven't run the calculation and am just basing this from the guestimate that they wouldn't coincidentally be in balance).
The centifugal effects would not be any greater then, for the earth, than they are now--if you're assuming that the earth was rotating at the same speed as now.

The Roche limit (http://www.daviddarling.info/encyclopedia/R/Rochelimit.html) for the earth is a lot lower than the geosynchronous orbit (http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/geo-high.html), so the moon would not be torn apart.

Also, while the tidal forces might be constant the geologic stresses would not be; both worlds would experience significant shocks and shear effects that would alter their nominal shape and potentially tear them apart.

Of what sort? Do you mean from the irregularities in the orbit?

And any irregularity in their orbit about one another--nutation due to out of plane periodic movements, or side components due to precession--would make such a system unstable even over relatively short timeframes. Theoretically, you could have any number of objects orbiting a common center of mass; in reality, we almost always have large masses as the focus (or nearly so) for a much less massive object, Pluto and Charon excepted.
Three or more objects typically do not orbit a common center of mass--for instance, earth satellites do not orbit the common center of mass of the earth/moon/satellite system.

Jimpy
05-14-2005, 05:54 AM
Also, while the tidal forces might be constant the geologic stresses would not be; both worlds would experience significant shocks and shear effects that would alter their nominal shape and potentially tear them apart. And any irregularity in their orbit about one another--nutation due to out of plane periodic movements, or side components due to precession--would make such a system unstable even over relatively short timeframes.
Stranger

The geological stress of the Moon being in geosynchronous orbit wouldn't be enough to tear either of them apart., although there would be serious volcano and earthquake activity.

You have a point on the second part though. The Earth and the Moon don't exist in isolation. And a geosynchronous Moon's orbit would be unstable. In fact you can make the argument that the Moon's current orbit is unstable, when you take into account the sun, and to a lesser extent Jupiter (and to a lesser extent Venus and Saturn and to a lesser extent, everything else in the solar system) . In fact, you can make the argument that the Moon doesn't orbit the Earth (in the sense that every other natural satellite orbits it primary), but rather the Moon and Earth orbit the sun in a complicated manner rather like the leapfrog moons of Jupiter orbit Jupiter.

Isaac Newton supposedly said that the orbit of the Moon was the only problem that gave him a headache. The complex interaction of the Earth/Moon/Sun system doesn't lend well to simplistic Moon orbit scenarios like a geosynchronous Moon.

RM Mentock
05-14-2005, 06:56 AM
The complex interaction of the Earth/Moon/Sun system doesn't lend well to simplistic Moon orbit scenarios like a geosynchronous Moon.
I don't see why not. The moon is moving away from the earth because of the tidal braking of the earth--at some point, the rotation of the earth could be more than a (current) month, and match the revolution of the moon. That would take a long time, but it's possible in theory.

Stranger On A Train
05-14-2005, 02:07 PM
The centifugal effects would not be any greater then, for the earth, than they are now--if you're assuming that the earth was rotating at the same speed as now.sciguy was positing a geosynchronous orbit, and I was assuming that he meant that orbit at the Earth's current rotational velocity. At that speed (assuming for the moment that the moon is rotating about the Earth) the Δv and ω, the rotational velocity of Luna, is enormous due to the curvature of the orbit. Of course, the two bodies are actually rotating about their common center, but for such an orbit to be stable their gravitational attraction has to equal the outward reaction, or an easier calculation is that their kinetic energy has to equal the gravitational potential energy. Such an orbit has one stable rotational speed for a given distance (hence, a single altitude for geosynchronous and geostationary orbits), and finding that stable value would require both speeding up the moon and slowing the rotation of Earth.

Point of note: sciguy used the term geosynchonous (and I unthinkingly copied it) but I believe he actually means a geostationary orbit above the equator.

The Roche limit (http://www.daviddarling.info/encyclopedia/R/Rochelimit.html) for the earth is a lot lower than the geosynchronous orbit (http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/geo-high.html), so the moon would not be torn apart.I'm surprised that the Roche limit for a body the size of the Moon is so low but I'll take your source's answer at face value. Even if the bodies are not close enough to tear each other in half and fling the bits into bizarre orbits, I suspect the Moon, being volcanically inert and inflexible, would suffer some severe geological effects, perhaps even to the point of reducing it to disconnected chunks. The Earth, with its molten core, would remain more or less intact but would bulge significantly about its equator.


Three or more objects typically do not orbit a common center of mass--for instance, earth satellites do not orbit the common center of mass of the earth/moon/satellite system.Well, no; that's exactly my point. Theoretically, three or more objects of identical mass could form a stable orbit about their common center (assuming they're all in plane and with the same angular velocity) but in reality this never happens; even if the conditions were just so to permit this, small perturbations in the system cause it to become rapidly unstable. This is noteworthy only for being (I think) the only degenerate, closed-form solution to the otherwise onerous three-body (or more generally, n-body) problem.

Relating back to the OP (and continuing the string of Niven-related asides) I recall that he had a race, the Glick-something-or-anothers, who evolved on a tidally locked worldlets orbiting a red dwarf star. The creatures evolved to expect an environment of high winds, dim light, and freezing temperatures. It seems an unlikely environment for abiogenesis, but then again we have but one datapoint (and our knowledge of that one massively incomplete) to reason from. Cartooniverse asks:

And, to plug in the Everything portion of my question, what the heck are the odds that the S.E.T.I. folks will ever find a combination of planetary masses, forces and raw materials anywhere else that have produced the exact results needed for not only life, but advanced life to develop?We really don't know what conditions are required for "advanced life", and indeed, I find it likely that we'll probably overlook it when and if we ever find it, especially if it isn't mobile, carbon-based, water soluble life that we're accustomed to. We're hard pressed to acknowledge even the capacity for intellect among creatures that look almost exactly like us, like other primates or ursines; people boggle at me when I describe some of the intelligent and cognitive behaviors we've seen in cephalopods.

Stranger

Jimpy
05-14-2005, 04:41 PM
I don't see why not. The moon is moving away from the earth because of the tidal braking of the earth--at some point, the rotation of the earth could be more than a (current) month, and match the revolution of the moon. That would take a long time, but it's possible in theory.

The problems are, the Earth isn't a perfect oblate spheroid (neither is the Moon, for that matter), neither of them orbit in perfect circles, the Moons orbit about the Earth is in a different plane than the Earths about the sun and the Earth, Venus and Jupiter's orbits are not co-planar. If the Moon were in geosynchronous orbit, it would quickly move out. The orbit would most likely be quasi-stable and it would follow a chaotic path with an attractor that would be centered on the geosynchronous orbit, which is to say, it would average a geosynchronous orbit but would vary.

This is like the Trojan asteroids. They are rarely actually at the Lagrange spots but move around them.

RM Mentock
05-14-2005, 08:38 PM
Originally Posted by RM Mentock
The centifugal effects would not be any greater then, for the earth, than they are now--if you're assuming that the earth was rotating at the same speed as now.
sciguy was positing a geosynchronous orbit, and I was assuming that he meant that orbit at the Earth's current rotational velocity.
I accepted that assumption.
At that speed (assuming for the moment that the moon is rotating about the Earth) the Δv and ω, the rotational velocity of Luna, is enormous due to the curvature of the orbit. Of course, the two bodies are actually rotating about their common center, but for such an orbit to be stable their gravitational attraction has to equal the outward reaction, or an easier calculation is that their kinetic energy has to equal the gravitational potential energy. Such an orbit has one stable rotational speed for a given distance (hence, a single altitude for geosynchronous and geostationary orbits), and finding that stable value would require both speeding up the moon and slowing the rotation of Earth.
Not necessarily. See the example I gave above, where both the moon and earth slow down. That's a natural progression that is more or less happening now.
I'm surprised that the Roche limit for a body the size of the Moon is so low but I'll take your source's answer at face value. Even if the bodies are not close enough to tear each other in half and fling the bits into bizarre orbits, I suspect the Moon, being volcanically inert and inflexible, would suffer some severe geological effects, perhaps even to the point of reducing it to disconnected chunks.The more synchronous the rotation and revolution, the less the effect. And the ability to break it into disconnected chunks is kinda the definition of the Roche limit.
The Earth, with its molten core, would remain more or less intact but would bulge significantly about its equator.It would be a permanent "tidal" bulge, like exists on the moon now. The current centrifugal bulge of the earth is twenty kilometers, the tidal bulge would be a lot smaller than that.
The problems are, the Earth isn't a perfect oblate spheroid (neither is the Moon, for that matter), neither of them orbit in perfect circles, the Moons orbit about the Earth is in a different plane than the Earths about the sun and the Earth, Venus and Jupiter's orbits are not co-planar. If the Moon were in geosynchronous orbit, it would quickly move out. The orbit would most likely be quasi-stable and it would follow a chaotic path with an attractor that would be centered on the geosynchronous orbit, which is to say, it would average a geosynchronous orbit but would vary.You mean, like the moon is not geosynchronous today? With libration?

If not, then I'm pretty sure that the situation is fairly stable. The other effects that you mention would cause the orbit to change, but it would remain geosynchronous, since that response is faster than the others.

Planet X could always attack...