New model explains the moon's weird orbit

Simulations suggest a dramatic history for the Earth-moon duo


In the "giant impact" model of the moon's formation, the young moon began its orbit within Earth's equatorial plane. In the standard variant of this model (top panel), Earth's tilt began near today's value of 23.5 degrees. The moon would have moved outward smoothly along a path that slowly changed from the equatorial plane to the "ecliptic" plane, defined by Earth's orbit around the sun. If, however, Earth had a much larger tilt after the impact (~75 degrees, lower panel) then the transition between the equatorial and ecliptic planes would have been abrupt, resulting in large oscillations about the ecliptic. The second picture is consistent with the moon's current 5-degree orbital tilt away from the ecliptic. CREDIT Douglas Hamilton

In the “giant impact” model of the moon’s formation, the young moon began its orbit within Earth’s equatorial plane. In the standard variant of this model (top panel), Earth’s tilt began near today’s value of 23.5 degrees. The moon would have moved outward smoothly along a path that slowly changed from the equatorial plane to the “ecliptic” plane, defined by Earth’s orbit around the sun. If, however, Earth had a much larger tilt after the impact (~75 degrees, lower panel) then the transition between the equatorial and ecliptic planes would have been abrupt, resulting in large oscillations about the ecliptic. The second picture is consistent with the moon’s current 5-degree orbital tilt away from the ecliptic. CREDIT Douglas Hamilton

The moon, Earth’s closest neighbor, is among the strangest planetary bodies in the solar system. Its orbit lies unusually far away from Earth, with a surprisingly large orbital tilt. Planetary scientists have struggled to piece together a scenario that accounts for these and other related characteristics of the Earth-moon system.

A new research paper, based on numerical models of the moon’s explosive formation and the evolution of the Earth-moon system, comes closer to tying up all the loose ends than any other previous explanation. The work, published in the October 31, 2016 Advance Online edition of the journal Nature, suggests that the impact that formed the moon also caused calamitous changes to Earth’s rotation and the tilt of its spin axis.

The research suggests that the impact sent the Earth spinning much faster, and at a much steeper tilt, than it does today. In the several billion years since that impact, complex interactions between the Earth, moon and sun have smoothed out many of these changes, resulting in the Earth-moon system that we see today. In this scenario, the remaining anomalies in the moon’s orbit are relics of the Earth-moon system’s explosive past.

“Evidence suggests a giant impact blasted off a huge amount of material that formed the moon,” said Douglas Hamilton, professor of astronomy at the University of Maryland and a co-author of the Nature paper. “This material would have formed a ring of debris first, then the ring would have aggregated to form the moon. But this scenario does not quite work if the Earth’s spin axis was tilted at the 23.5 degree angle we see today.”

Collisional physics calls for this ring of debris–and thus the moon’s orbit immediately after formation–to lie in Earth’s equatorial plane. As tidal interactions between the Earth and the moon drove the moon further away from Earth, the moon should have shifted from Earth’s equatorial plane to the “ecliptic” plane, which corresponds to the Earth’s orbit around the sun.

But today, instead of being in line with the ecliptic plane, the moon’s orbit is tilted five degrees away from it.

“This large tilt is very unusual. Until now, there hasn’t been a good explanation,” Hamilton said. ” But we can understand it if the Earth had a more dramatic early history than we previously suspected.”

Hamilton, with lead author Matija Cuk of the SETI institute and their colleagues Simon Lock of Harvard University and Sarah Stewart of the University of California, Davis, tried many different scenarios. But the most successful ones involved a moon-forming impact that sent the Earth spinning extremely fast–as much as twice the rate predicted by other models. The impact also knocked the Earth’s tilt way off, to somewhere between 60 and 80 degrees.

“We already suspected that the Earth must have spun especially fast after the impact” Cuk said. “An early high tilt for Earth enables our planet to lose that excess spin more readily.”

The model also suggests that the newly-formed moon started off very close to Earth, but then drifted away–to nearly 15 times its initial distance. As it did so, the sun began to exert a more powerful influence over the moon’s orbit.

According to the researchers, both factors–a highly tilted, fast spinning Earth and an outwardly-migrating moon–contributed to establishing the moon’s current weird orbit. The newborn moon’s orbit most likely tracked the Earth’s equator, tilted at a steep 60-80 degree angle that matched Earth’s tilt.

A key finding of the new research is that, if the Earth was indeed tilted by more than 60 degrees after the moon formed, the moon could not transition smoothly from Earth’s equatorial plane to the ecliptic plane. Instead, the transition was abrupt and left the moon with a large tilt relative to the ecliptic– much larger than is observed today.

“As the moon moved outward, the Earth’s steep tilt made for a more chaotic transition as the sun became a bigger influence,” Cuk said. “Subsequently, and over billions of years, the moon’s tilt slowly decayed down to the five degrees we see today. So today’s five degree tilt is a relic and a signature of a much steeper tilt in the past.”

Hamilton acknowledges that the model doesn’t answer all the remaining questions about the moon’s orbit. But the model’s strength, he says, is that it offers a framework for answering new questions in the future.

“There are many potential paths from the moon’s formation to the Earth-moon system we see today. We’ve identified a few of them, but there are sure to be other possibilities,” Hamilton said. “What we have now is a model that is more probable and works more cleanly than previous attempts. We think this is a significant improvement that gets us closer to what actually happened.”


The research paper, “Tidal Evolution of the Moon from a High-Obliquity High-Angular-Momentum Earth,” Matija Cuk, Douglas Hamilton, Simon Lock, and Sarah Stewart, appears in the October 31, 2016 Advance Online edition of the journal Nature.

80 thoughts on “New model explains the moon's weird orbit

      • “””””….. Its orbit lies unusually far away from Earth, …..”””””
        No; I looked at it just last night and it was still in its same old usual place.
        Earth has only one moon, and it is still in its usual place.

  1. What other moon in the solar system always has one one side facing the planet it is circling? You have a model, , Why doesn’t the moon rotate independently as the earth does, so that we see most sides of the moon from earth? We see generally just one side from earth…There are many other moons in the solar system. How many of them behave in the same way?

    • Most of them, sir. The few which are not rotationally “locked” are quite distant from their primaries, or have other peculiarities that suggest collision events in the recent (planetologically speaking) past.
      However, making comparisons between the different moon systems and the Earth / Moon system is fraught with danger. There are just too many differences in all of the other factors. The mass ratio being the biggest one, followed either by the orbital to primary radius ratio, or the (theorized) method of formation.

      • It’s not “current thinking” it is tidally locked. The current state of E-M system is known to a very high precision. That is nothing to do with what is being discussed here.

      • Yes, the theory is pretty cut and dried here. We use the same effect in communications satellites to keep their antennas facing the Earth all the time as they orbit.

      • Greg says

        It’s not “current thinking” it is tidally locked.

        “Current thinking” doesn’t imply future thinking necessarily be different. You would do better to consider everything that is “known to be true” today to simply be “Current thinking” because that’s the way science works.

      • @Bartemis
        Satellites don’t share a common barycenter with the earth, the moon does.
        Satellites are falling down with a certain speed forward, following same curve as earth’s surface (aka “falling beyond horizon”).
        The moon is not rotating on it’s own axis.
        Earth does, probably because it’s axis is tilted a bit, like a spinning tilted flywheel doing it’s own thing while inside a larger wheel (the motion of Earth and Moon circling each other with) that goes different way. Don’t know why it started to spin on it’s own axis, but i guess this tilted position makes it possible. Maybe the moon was rotating too once, but without tilted axis it wouldn’t work out.
        If someone can verify this with something interesting i would love to hear…

      • Joe, if the moon wasn’t spinning on it’s axis, then we would be seeing a different part of the moon’s surface everyday, and see all of the moon’s surface over a month.
        All orbits are merely falling down while moving forward.

    • I asked an astro professor that question and he gave a convincing gravitational answer that I can’t recall now! 😉 The other thing he didn’t have an answer for was the angular size of the mooning being exactly such* that it is able to eclipse the sun completely, allowing us to view the corona from Earth easily.
      *The Moon just happens to be 400 times smaller than the Sun and 400 times closer!

      • The moon is drifting away from the Earth. That has been measured.
        There had to come a time when that ratio was in effect. It just happened that the time is now.

      • If it was “exactly” the right size it would be a great conincidence but the angular ( apparent ) size of the moon changes by about 5% between perigee and apogee. That is why sometimes we get full eclipses and other times we get annular eclipses.

      • @Scott
        I’ve always figured this coincidence deserves a footnote in “rare Earth” theories because of its ancillary scientific benefits. Of course there are other ways to learn about solar physics or to confirm relativity which don’t require a solar eclipse, but this lucky coincidence makes it easier by offering information to observers which may not be available otherwise.

      • And @MarkW . . . Yes, but it is still a peculiar coincidence that “the time is now” just when we can observe it. In fact as coincidences go it is rather a biggy wouldn’t you say?
        I am not ascribing any supernatural cause of this eclipse effect but rather that it is indicative of some relationship that we are simply not equipped to understand, yet.

      • Keitho, what other than the supernatural would you propose to have humans develop intelligence at that one point in the 12 billion year period between the earth forming and it being destroyed by the sun going red giant?
        You’re the one claiming that it must have some kind of meaning.

        • Not at all Mark. I am actually hoping that someone will bring forward an explanation. The last one I got was from the electric universe guys which was unconvincing.
          I am a long standing atheist so I don’t think much of supernatural explanations for anything but this conundrum has long been a matter of interest to me. Rather like the actuality of life which we are told is just an outcome of simple chemistry and physics yet we are so far incapable of replicating so the eclipse geometry is just an odd coincidence.
          Now we are told the actual orbit of the moon is unique. There is something we do not understand at all and this is relatively simple stuff compared with the climate.

    • Jupiter’s moons revolve and rotate at the same speed. I believe that Saturn’s moons do as well. It is called tidal locking.

      • Tidal locking I understand — but therefore by the same mechanisms and logic all the planets should be tidally locked with/by the Sun. Yet they are not. Seems information is lacking.

      • The forces between the sun and planets is too small.
        You need a substantial gravitational difference between the near side and far side.

  2. Moon orbital oscillations (if existed) should clearly show in the past continental plates movements. The Atlantic ridges could be a good proxy.
    I have looked into this and there are indications that plate movements were periodic jerks rather than smooth glide.
    Kind of a ‘Grand’ Milankovic cycle well above 100 ky.

  3. No explanation how the 75 deg tilt got decreased to 23.5 degrees? That’s a big question because the story is incomplete, as given here, at least.

    • The new gravitational effects would cause oscillations in both the axis of the Earth and the moon, I suspect. Which means we may have very small oscillations still occurring… which could also explain ice-ages, at least to a small extent.

      • I suspect higly7 is wrestling with the same beast I find in the article. The parameters for the models output seem to be driven by the required output. Earth’s initial tilt had to be between 60 and 80 degrees. The statements in the article are statements of belief not of fact, therefore need to be explained as to how they work. Is the 75 degree what makes the model work? We see output driven “data” in AGW models, fortunately we have the real world conditions to check the model.

    • It didn’t get to exactly 23. The Earth’s obliquity varies from about 21 to 24 degrees over a ~40 thousand year cycle. It is now at about 23.5 and decreasing. How does that figure in to this.

    • It didn’t get to a steady 23.5 The Earth’s obliquity varies from about 21 to 24 degrees over a ~40 thousand year cycle. It is now at about 23.5 and decreasing. How does that figure in to this.

    • The earth is not a perfect sphere, it bulges at the equator. When the moon shifted to an ecliptic orbit, it would pull on this equatorial bulge causing the earth’s axis of rotation to shift.

      • Nothing real is a perfect sphere.
        A sphere is a fictitious mathematical object. You can even have four or five dimensional spheres or as many as you want. I dunno if their are fractal spheres.

    • Well as the moon orbited the Earth at a 75 deg inclination to the ecliptic plane, the sun would pull the Moon towards it more when it is closest. The Moon wouldn’t necessarily be closest when it is in the line between the Earth and the Sun either, but each tug the Sun made on the Moon would slightly move the moon’s orbit around the Earth toward the plane of the Earth’s orbit around the Sun. The Sun’s and the Moon’s tidal forces on the Earth would tend to upright the Earth’s rotational axis toward 0 degrees. Like a pendulum the Moon’s orbital inclination would shoot past the equilibrium point to an unstable extreme and back many times over the billions of years, but eventually settle into a tidally locked orbit of 0 deg and the Earth’s day will eventually match the Moon’s orbit; assuming the Sun hasn’t gone Red Giant and engulf both the Earth and Moon.

      • But if it goes Red Giant, it is obvious that man made CO2 did it.
        It must be nice to be naive enough to have a single all purpose answer to everything.

  4. Could the oscillations caused by the new gravitational mix have played any role in bringing on ice ages?

    • Probably not. Neither the moon nor the earth’s axis of rotation have changed significantly since the ice ages started.

  5. I would have liked to see a list from the authors of questions NOT answered by their hypothesis.

  6. Too many times I have seen a claim of “Finally, a possible solution to the fatal flaw in theory X!” when no previous acknowledgement of any flaw had been made.

    • Unless you have studied the subject, it’s unlikely that you would be aware of such controversies. It’s not like they get regular press.

    • My gripe is that known fatal flaws don’t get any press by the proponents. This is the same issue we see with CAGW proponents. Real scientists address shortcomings in their hypotheses.

      • Well Stevan, one would wish that to be so “real scientists address…etc” but when a man mounts a woman he doesn’t warn her “this may not be as fun for you as it will be for me”. People who are passionately attached to an idea will tend to lose sight of shortcomings as they explain it. The real problem is denying those shortcomings when they are legitimately pointed out.

      • There’s only so much press time available. Listening to controversies regarding the orbit of the moon is not one of those things that get a young man’s blood to boil.
        None of us would have ever heard of this model had Anthony not found it and presented it to us.
        You are straining over gnats here.

  7. The graphic accompaning the post is strange. The continental form is present time, but the moons proximity to the Earth is not.

  8. Any impact which is going to put the spin axis at 75 degrees would have had to misshapen the first Earth by so much that like a third of it would have been flung into orbit. Basically exploding the whole conglomeration into a scattered jumble two or three times the size of the present Earth. How does the moon form from that.

    • That all depends on the speed and angle at which the debris left the earth. For a tangential hit that transfers most of it’s momentum too the earth, the velocity of the exploded debris would have been small enough that it could be recaptured by the earth in a few hundred thousand years.

    • I question how the collision hypothesis manages to get sufficient material blasted into Earth orbit to coalesce into such a large moon compared to the Earth. Analysis of moon rocks did not detect any indication that the moon contains material from other regions of the Solar system. Thus, whatever supposedly struck the Earth would have had to have been in an orbit similar to the Earth’s, with very similar orbital speed around the Sun. Relative velocities would have been too low to put that much mass into Earth Orbit.
      Contrarily, if a hypothesis does manage to codger up a scenario with sufficient energy invested in flying debris for it to attain orbital velocity, the problem becomes how to keep the debris near the Earth. The difference between orbital velocity and escape velocity is small.

      • I’ve had this question, before. In musing (without serious research) I’ve wondered off and on over the years if we didn’t have a low-speed very-close-encounter with another planetoid in the Earth orbit and both came out relatively (when compared to the large impact scenario) unscathed.

      • First off, the matter from the earth and the crustal matter from the colliding object would have been thoroughly mixed together by the collision, the denser matter for both would have sunk to the center of the earth, and once again, mixed thoroughly. So it’s hardly surprising that the two bodies as they are found today, have virtually the same mix of materials.
        Beyond that, even a low velocity collision when you consider masses the size of the earth and the proposed collider, involve HUGE amounts of energy. Even if the two bodies were in nearly identical orbits, as they approached each other, their combined gravity would have accelerated both of them tremendously.
        Finally, a low speed impact (relatively speaking) would be just the thing to make sure that most of the ejecta would either be recaptured by the earth, or go into orbit around the earth. High speed impacts result in ejecta being scattered across the solar system. (Witness rocks found on earth that came from Mars.)

      • My first assumption was that in order to give the earth’s axis of rotation that big a whack, the collider couldn’t have been coming in from any where close to the same orbital plane as the rest of the planets.
        But then it occurred to me, that if the collider came in from the same plane, but whacked the earth close to one of the poles, it could still have the same affect on the Earth’s axis of rotation.

    • They’ve got these computer models, BIll, that claim it all comes together and works. In fairness, the models are presumably grounded in well understood Newtonian physics without too many “simplifying” assumptions. Personally, I find this whole collision hypothesis to be entirely too Velikovskyish for my taste. But I’m not about to spend a decade of my life playing with computer models to try see if their claims are actually plausible. If they have it wrong, that’ll come out and be fixed eventually

  9. Scientist trying to explain everything and failing hard like all Main stream Scientists! The Moon is to big to be in that orbit, the Moon is to important to the Earth and life cycle. The Moon (Diana) is Esoteric and to important to the Tides and Birth of Life, and Mystery School teachings.
    This new Paper trying to explain the Moon is like Historians trying to explain Rome built all it’s Megaliths. In other words it’s complete and total fantasy by Humans trying to have an answer for everything. Yes, obviously intelligence is slowly slipping away…

  10. Because the moon’s orbit around the earth is so relatively slow and the speed of the earth’s orbit around the sun quite fast, the moon’s orbit around the SUN is a crenulated circle, like the decorative edge of an apple pie crust!

  11. I gotta take a look at some journals because I have a hard time figuring out how stable orbital mechanics allows for a ‘slowing drifting” lunar orbit.

  12. I agree with Asimov that the “Moon” and the Earth are a binary planet system. The moon is not a moon, but a twin planet.

  13. I wonder if their paper addresses the significant problem of the age of the earth/moon system. Simple tidal mechanics of the two-body, earth/moon system yields a maximum age of less than two billion years. Do the gravitation effects of the sun and major planets allow this to be stretched out to many “billions” of years?

    • Considering the type of event needed to create an Earth/Moon system, I would think they would be relatively rare.

      • The type of event needed would be a Mars-sized planet colliding with an Earth-sized planet. I would think that combination exists in numerous solar systems.
        We need bigger telescopes. 🙂

      • Not just a collision, but a collision within fairly narrow parameters in terms of collision speed and angle.

    • According to R Daneel Olivaw it is the only such pairing in the Galaxy. The consensus was that It was only a myth.

  14. I wonder if the yet undiscovered planet 9 orbit through the inner solar system as any effect on the Earth/Moon system, or maybe the earth’s climate as it passes through.

  15. Er… if something hit the Earth hard enough to chip off the moon, the really important questions are “Is it coming back for another go?” and “If that one isn’t coming back, is another one like it coming along?” Because if the answer to either is “Yes”, then we’re doomed.

    • My guess is small groups of humanity will be living off the Earth permanently within 50 years or so. Anything that’s going to knock out the human race better do it before then. Otherwise, we are off to the races! Look out Universe, here we come!

  16. Thanks again Anthony.
    Good science begets good hypotheses that evolve into good theories that stimulate good research and finally useful knowledge.
    Bad science wastes money, time and effort. And makes some folks rich and others very poor.

  17. That same intelligent Mind that coded our most intricate sophisticated body’s DNA and RNA cutter instructions for our innumerable proteins which also form our body’s micro nano machines, – That very same extratemporal Mind also fashioned our Moon and its amazing orbit which always faces Earth. That was some infallible balancing act! – No random result of some arbitrary shattering collision. Merely one of many fine-tuned settings of this small planet in infinite space, which makes it so very special.
    “Macro evolution” and its grotesque “cosmology” was a wildly successful consent engineering campaign by Edward Bernays sponsored by Cecil Rhodes’ Round Table, reinforced by its scholars, and also the greatest deception ever in history.
    The “social engineers” are laughing all the way to the bank, as we obediently abolished our critical powers & abdicated from our intellectual independence for their manic manufactured consent, so we would willingly vote for Killary, World War 3, and slavishly submit to One World Government. Mission accomplished.
    PS: Far sooner than later, you’ll also get a free implant to buy and sell, as extra bonus slipped in, so you’ll be a real cool trans-human without soul. Happy cashless shopping!

  18. I recall a couple of years ago a lively discussion on this site regarding the behaviour of the moon. A few of us pointed out that the behaviour of the moon was rather unusual and how it behaved depended on whether you were taking a view from a terrestrial perspective or or one from outside the moon/earth situation.
    The discussion was picked up by an odd climate commentator from Australia ( no names mentioned) who used our discussion as an example of how stupid sceptics must be. It’s very rewarding to see this paper which suggests many of the same things.

  19. Oh, no, here’s yet another example of a brilliant, but speculative theory out of touch with real constraining data from the Earth system. If Earth’s axial inclination had been of the order suggested, then around the solstices, the polar regions would have pointed toward Sun 24/7, causing constant heating and consequent evaporation and photodissociation of Earth’s water, precluding the development of life and an oxygen-rich atmosphere, which would have prevented the development of an ozone layer and a stratosphere. Exactly this, in fact, happened on Mars and Venus, both of which have little or no water and they therefore have deuterium-to-hydrogen ratios way higher than Earth’s, which indicate severe water loss (the heavier deuterium remained on the planet while the lighter hydrogen escaped).
    Earth’s mild, almost constant axial tilt is the reason why Earth escaped such a photodissociative fate as was endured by its planetary neighbors. Why did it do so? Moon’s gravitational influence on Earth’s equatorial bulge, of course. Mars and Venus didn’t have a large satellite to perform this function, so their axial tilts were free to swing through large angles. This stabilizing effect couldn’t have happened with the oscillatory motion postulated by this scenario.The gravitational interplay between Earth and Moon, incidentally, handily accounts for the lunar orbit’s 5 degree deviation from the ecliptic.
    A collisional origin for Moon is highly unlikely, despite its general acceptance. It would require that the impactor had a nearly identical mineralogical composition to Earth’s, which is a very improbable circumstance. It’s more likely that Earth and Moon is a twin-planet system. A scenario more consistent with the distribution of angular momentum in the Solar System (less than 1 percent of it is in Sun) is that all the planets were spun out sequentially by the rapidly rotating proto-Sun during dust cloud collapse BEFORE Sun began thermonuclear burning. Earth and Moon would then have been spun out from Sun together as a twin-planet system, accounting for their nearly identical mineralogical compositions.
    The above model is more fully explained in my 1991 text, “The Earth System,” but the most important consideration here is that when new theories are advanced, those who advance them really should test them first against real constraining data rather than simply throwing out new ideas willy-nilly.

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