Into Oblivion: What If the Earth Had No Moon?

AVAST gentle reader: mild SPOILER(S) and graphic depictions of shattered satellites ahead!

We recently had a chance to catch Oblivion, the first summer blockbuster of the season. The flick delivers on the fast-paced Sci-Fi action as Tom Cruise saves the planet from an invasion of Tom Cruise clones.

But the movie does pose an interesting astronomical question: what if the Earth had no large moon? In the movie, aliens destroy the Earth’s moon, presumably to throw our planet into chaos. You’d think we’d already be outclassed by the very definition of a species that could accomplish such a feat, but there you go.

Would the elimination of the Moon throw our planet into immediate chaos as depicted in the film? What if we never had a large moon in the first place? And what has our nearest natural neighbor in space done for us lately, anyway?

Earth is unique among rocky or terrestrial planets in that it has a relatively large moon. The Moon ranks 5th in diameter to other solar system satellites. It is 27% the diameter of our planet, but only just a little over 1/80th in terms of mass.

Clearly, the Moon has played a role in the evolution of life on Earth, although how necessary it was isn’t entirely clear. Periodic flooding via tides would have provided an initial impetus to natural selection, driving life to colonize the land. Many creatures such as sea turtles take advantage of the Full Moon as a signal to nest and breed, although life is certainly resilient enough to find alternative methods.

The 2000 book Rare Earth by Peter Ward and Donald Brownlee cites the presence of a large moon as just one of the key ingredients necessary in the story of the evolution of life on Earth. A Moon-less Earth is also just one of the alternative astronomical scenarios cited by Arthur Upgreen in his 2005 book Many Skies.

Save our satellite: A possible target for an alien attack? (Photo by author).
Save our satellite: A possible target for an alien attack? (Photo by author).

Contrary to its depiction on film, the loss of the Moon wouldn’t throw the Earth into immediate chaos, though the long term changes could be catastrophic. For example, no study has ever conclusively linked the Moon to the effective prediction of terrestrial volcanism and earthquakes, though many have tried. (Yes, we know about the 2003 Taiwanese study, which found a VERY weak statistical signal).

All of that angular momentum in the Earth-Moon system would still have to go somewhere. Our Moon is slowly “braking” the rotation of the Earth to the tune of about 1 second roughly every 67,000 years. We also know via bouncing laser beams off of retro-reflectors left by Apollo astronauts that the Moon is receding from us by about 3.8 cm a year. The fragments of the Moon would still retain its angular momentum, even partially shattered state as depicted in the film.

The most familiar effect the Moon has on Earth is its influence on oceanic tides. With the loss of our Moon, the Sun would become the dominant factor in producing tides, albeit a much weaker one.

But the biggest role the Moon plays is in the stabilization of the Earth’s spin axis over long scale periods of time.

Milankovitch cycles play a long term role in fluctuations in climate on the Earth. This is the result of changes in the eccentricity, obliquity and precession of the Earth’s axis and orbit. For example, perihelion, or our closest point to the Sun, currently falls in January in the middle of the northern hemisphere winter in the current epoch. The tilt of the Earth’s axis is the biggest driver of the seasons, and this varies from 22.1° to 24.5° and back (this is known as the change in obliquity) over a span of 41,000 years. We’re currently at a value of 23.4° and decreasing.

But without a large moon to dampen the change in obliquity, much wider and unpredictable swings would occur. For example, the rotational axis of Mars has varied over a span of 13 to 40 degrees over the last 10 to 20 million years. This long-term stability is a prime benefit that we enjoy in having a large moon .

Perhaps some astronomers would even welcome an alien invasion fleet intent on destroying the Moon. Its light polluting influence makes most deep sky imagers pack it in and visit the family on the week surrounding the Full Moon.

But I have but two words in defense of saving our natural satellite: No eclipses.

The diamond ring effect as seen during a 2008 total solar eclipse. (Credit: NASA/Exploratorium).
The diamond ring effect as seen during a 2008 total solar eclipse. (Credit: NASA/Exploratorium).

We currently occupy an envious position in time and space where total solar and lunar eclipses can occur.  In fact, Earth is currently the only planet in our solar system from which you can see the Moon snugly fit in front of the Sun during a total lunar eclipse. It’s 1/400th the size of the Sun, which is also very close to 400 times as distant as the Moon. This situation is almost certainly a rarity in our galaxy; perhaps if alien invaders did show up, we could win ‘em over not by sending a nuclear-armed Tom Cruise after ‘em, but selling them on eclipse tours…

And a receding Moon also means that in approximately 1.4 billion years, the final total solar eclipse as seen from the Earth will occur. Conversely, the Moon was closer and appeared larger earlier in Earth’s history. About just under a billion years ago, the first brief annular eclipse similar to the one occurring next week on May 10th would have occurred.  In the current epoch, annular eclipses constitute 33.2% of solar eclipses with total solar eclipses becoming ever rarer at 26.7%. (The remainder are hybrids and partials).

If the Moon was a necessary ingredient for life to take hold on Earth, then we may be a very rare occurrence in the universe indeed. The current theory for the formation of the Moon involves the Earth getting “wacked” by a Mars-sized body dubbed Theia early in its history. This would explain the relatively low density of our Moon compared to the Earth.

Oblivion isn’t the only science fiction to posit a moon-less Earth. Fans of 1970’s sci-fi will remember the TV series Space: 1999 which proposed an even more unlikely scenario of the Moon being “blown out of orbit” by a nuclear disaster. Of course, just how they managed to meet new alien civilizations every week was never explained, but hey, it was the 1970’s…

Oblivion did have one more glaring space science goof. Plutonium used for space travel and weaponized Plutonium are two different isotopes. It would not be possible (though it was a convenient plot device) to turn an nuclear-powered RTG such as one used on Mars to power the Curiosity rover into an explosive weapon.

But perhaps the greatest gift our Moon has to offer is its lessons to us as a species. The motion of the Moon provided early astronomers with a great lesson in Celestial Mechanics 101. Newton would have had a much tougher time deciphering the laws of motion and gravity were it not for the example provided by the Moon. Plus, it makes a great stepping stone for solar system exploration. Curse it or love it, the Moon is our celestial companion… let the sci-fi alien baddies be jealous!

36 Replies to “Into Oblivion: What If the Earth Had No Moon?”

  1. Putting a “spoiler alert” before a spoiler is customary…

  2. Sounds like Oblivion took “Moon” and fast forwarded time while also including: aliens, cataclysm, heroism etc to the mix. And no doubt shaken, not stirred.

  3. Isaac Asimov’s The Tragedy of the Moon nicely outlines the benefits and cons of having a moon, the main thrust being that is may have delayed scientific development because it reinforced earth-centricity. Worth a read, despite being a fairly old book.

      1. Because the movie could be good even if TC plays the main character?
        And although I don’t care about the story for the most of the time, but nevertheless I hate when someone ruins even the smallest twist in the movie.

  4. A planet similar to Earth without a moon might still be biologically active. However, the perturbation of the axis of rotation might require such life to evolve strategies for surviving such changes. This might then mean such life is less complex, though we can’t know this with certainty. An example might be Mars, which could have life in subsurface ecosystems where water exists.


    1. The projected changes will be in the millions and tens of millions of years. What you’d likely get is migratory behavior over very long time periods.

      A lot of this no-moon-no-life is politically/religiously motivated – “rare earth” is not an objective scientific hypothesis, it’s more of a rallying cry.

      1. I tend to think there is a small probability distribution for Earth-like planets with complex ecological systems. I did a study of chaos in the solar system and compared Lyapunov exponents with other identified solar systems. I did some Bayes analysis and estimated there may only be about 1000 planets with the sort of stable orbital configuration Earth has within our galaxy. This however does not restrict the number of bio-active planets to that number. There could will be many billions of bio-active planets in our galaxy. However, I question whether all but a rather few have the stability of conditions seen with Earth. It still must be pointed out that Earth has a measure of climate variation as is.

        The rare Earth hypothesis could of course be false. With probability you have of course a range of possible outcomes. An extremely rare Earth hypothesis might say Earth is the only planet in the universe with conditions necessary for complex life. A more intermediate range is probably more reasonable. I doubt there are billions or even millions of planets with complex life in our galaxy There may be that many planets with microbial level of life, which could include Mars or Enceladus or other bodies in our solar system.

        The idea of course has been picked up by the conservative and religious crowd. These people are always eager to squeeze God into some scientific question or uncertainty. They were early to pick up on the big bang, where this seemed commensurate with “let there be light.” They have gotten less interested in this as cosmology has pushed deeper into the physics behind the big bang, leaving less room for a god. These types will always do this.


  5. Aliens targeting the moon? Anything is possible I always say. But w/no moon there is no axis stability. No moon would be complete chaos on the surface & under the seas. Plus very weird tides w/mega tsunami’s galore. Plate tectonics effected. Good for you for expanding your thought of the impossible/improbable to the possible. …PEACE!

  6. NOBODY FORCED you to read this article yah know! Watch your mouth newbie.

  7. Interesting points, but you unfortunately missed one big flaw in your logic! If that massive of an explosion on the moon would occur, then large fragments due to close proximity of the moon’s orbit would hit the earth. There would be massive damage and here’s a recent research on the impact of one hit…

  8. Well, there was also Thundarr the Barbarian, one of my favorite cartoons when I was a kid. The moon looked about like it does in Oblivion but in this case led to some fun sword and sorcery adventures. 🙂

  9. “Contrary to its depiction on film, the loss of the Moon wouldn’t throw
    the Earth into immediate chaos, though the long term changes could be

    True. A lot of people continue to have superstitious feelings about the moon, and that’s why popular thought doesn’t have any problems with this depiction. Many religions go by the moon for their calendar, Astrology pays close attention to where it is. People who haven’t really put a lot of thought into the matter may still think it’s the Moon which drives their religion and not the system set up by people that goes by the movements of the moon. The loss of the moon would shake these people to their core, so its not much of a stretch they would think what happens to the Moon in oblivion and its effect on the earth is so far-fetched.

  10. I liked Oblivion. Science issues, yes – but as we say over on TV Tropes: Tropes are Tools. I did wonder why after 60 years the pieces of the moon weren’t orbiting each other or hadn’t coalesced into a single unit again. I’m going to guess that the bad guys used “screws with gravity” kind of bomb that has the after-effect of preventing it from coming back together.

  11. If aliens wanted the earth for themselves, they probably would want the earth with the moon intact because of the special stability of earth that the moon provides.

    If they wanted to merely eradicate mankind from the planet and take over the earth then why would aliens want to blow up the moon and remove the stabilizing influence? All it would take to destroy mankind would be to guide one or two good sized asteroids or comets, say 20-40 miles in diameter, into a collision with the earth and “poof” civilization (and likely humans) essentially becomes extinct but if not, then certainly mankind would no longer represent any threat to anyone occupying the earth. A few nukes might survive but would have little to no effect because the collisions would have already created a “nuclear winter” and exploding the few remaining nukes would just irradiate small portions of our planetary surface. After about 2 years and no longer than 10 years after such impacts, the surface of the earth would be ripe for conquest and there is no possibility that whatever traces of our civilization that managed to survive would get any chance to recover in that short period of time. It would take decades to centuries before humanity could make any meaningful recovery after such a catastrophic set of events.

    If they just wanted to destroy life on earth, and they had the ability to destroy the moon, then why wouldn’t they just blow up the earth too … or just leave the moon alone and blow up the earth.

    It is a MOVIE PLOT and doesn’t need any reasonable story … it has Tom Cruise who is always as erratic as ever.

    1. True. Additionally our planet may well be the garden spot in the galaxy. It has been reasonably habitable for over 500 million years. A lot of time for any exploring aliens to find it and set up camp. They certainly wouldn’t have waited until the first sentient species developed here if conquest were their aims and the desire to acquire some prime real estate in the galaxy were in the itinary. If they are out there, and I believe they are, they have been looking after and tending our garden for many millions, possibly billions of years…to some unknown and enigmatic goal.

  12. I can’t believe this article didn’t cover that the moon was STILL THERE in the movie. Just broken up into many pieces. The gravity would be different, but not nearly the same as having no moon at all.

    1. Exactly. The moon’s center mass is still roughly the same sans the material that might have been ejected into space. The article is dumb.

  13. Is there something about being close to the sun that makes it less likely to have a moon? Seems like there must be if Mercury and Venus have no moons and Earth only has one by accident, as well as Mars, whose moons are incredibly tiny, possibly asteroid captures. If it is true that it is rare for planets closer to the sun having moons, man are we lucky!

    Our moon is beautiful, awe inspiring and one of the biggest in the solar system. We need it for the long-term survival of our species. I, for one, vow to defend it, with my life if necessary, against the alien aggressors!!! Who’s with me?

  14. “What If the Earth Had No Moon?”

    Woohoo! Deep sky observing & imaging EVERY clear night!

  15. Did I just hear the Moon is 1/80th the Earth’s mass? I thought the Moon is 1/6 Earth’s mass because there is 1/6th Earth’s gravity on the moon. Last I heard, gravity was directly related to mass. What is this 1/80th business?

    1. The moon is only 1.2% of the Earth’s mass… its surface gravity is a product of not only its mass but its radius & density, which is less than that of the Earth. But you’re right, its gravity is 1/6th that of Earth’s at its surface. Surface gravity can be worked out as g=GM/r^2. Thus, if you condensed an object like the Earth or Moon down in size, its density and surface gravity goes up.

  16. It seems very strange that apart from a throw away mention, there was absolutely no mention of the fact that the Moon causes the rise and fall of the tides. This has to be, scientifically, the most important aspect of it’s very presence aboive us.

    Life in all it’s wonderful variety has become utterly dependent on the lunar cycle, from the very depths of them ocean to the menstrual cycles of mammalian and other biological life.

    Whole cultures, beyond the biological element, have risen and passed into historical oblivion, whilst practicing some sort of Lunar worship, or calender periodical observance.

    The third major religion, Islam, is totally based in the Lunar cycle for virtually all of it’s religious observances.

    I agree that scientifically the Moon can be discussed as in the above article, but as we are mere temporary tenants of this Earth of ours, I suggest what we get involved in, culturally, and socially, has an equal importance to the basic scientific factor.

    If not a greater importance to the vast majority.

  17. Did I miss the part of the article that discusses that without the
    gravitational effects of the moon then the magnetic core of the Earth
    would reduce and so diminish the Van Allen Belts. More Solar radiation
    would reach the surface of the Earth, thus wiping us out ??? Pretty
    major point, I think…

  18. It makes me wonder how different Earth would have been if we had two moons instead of one-going to the opposite side of the moon spectrum. Or, how different would Earth be if we had one moon, and rings like Saturn? It is neat to think about:-)

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