What would Earth be like without the Moon?

The search for terrestrial extrasolar planets in the habitable zone of stars suggests that these planets may not be all that rare. However, examination of the solar system points to the fact that planets with a satellite similar to the Moon are probably a lot less common. It is massive compared to Earth, and while larger moons exist in the solar system, they also belong to much more massive planets, like Jupiter and Saturn. In fact, I’ve seen people refer to the Earth-Moon system as a binary planet.

While the sight of the Moon in the night sky looks serene now, current theory has it that the Moon resulted from a collision between early Earth and a Pluto-sized object.

What is the function of the Moon? Do we need it? Does life need it? Life on Earth evolved under the nightly glare of this huge satellite.

Aside its cultural value in mythology and human time-reckoning, the Moon performs two major functions:
– it is the main cause of the tides (not the waves, as people sometimes claim)
– it stabilises Earth rotation

I’ve often heard science fiction writers say that a planet without moon would have no tides. But is that true? Any fisherman or surfer knows that the height of highest tide in a day rises and falls over a period of roughly two weeks. For example, around Sydney spring tide is +2m, and neap tide only about +1.3m. Why the difference? Well, the tides are caused by the Moon’s gravity. During neap tide, which falls in between full moon and new moon, the force of gravity exerted on the ocean by the Moon is perpendicular to that exerted by the Sun. Yes, the Sun causes a tide that is approximately 45% of that caused by the Moon (This page has good information on calculations of tides). Hence, if we had no moon, we would still have the solar tides, except they would always coincide with a day-night cycle, which would have profound effects on communities of intertidal organisms.

Out of the two main functions of the Moon, the second is probably the most important, even though we do not notice the effect. A stable rotation is essential for evolution of life. If Earth were to wobble too much on its axis, we would have various parts of the Earth pointing to the Sun at irregular times. Thus, seasons would be irregular, and all living things have evolved with stable seasons. Imagine if one place on Earth would, over the course of twenty thousand years, go from a polar climate to a tropical climate. It would be a major impediment for the development of most species of plant or animal which are adapted to a fairly narrow temperature and water availability range. For this reason, people suggest that a planet with higher life needs a moon, but the stabilising function could probably be performed just as well by multiple moons or rings.



Patty writes hard Science Fiction, space opera and fantasy. Her latest book is Trader’s Honour, in the space opera series The Return of the Aghyrians. If you’d like to be kept up-to-date with new releases, remember to sign up for Patty’s new release newsletter.

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7 comments on “What would Earth be like without the Moon?

  1. Hmm. People who suggest that a planet with higher life needs a moon are making too many assumptions, I feel. Nothing to say that higher life cannot also be enormously adaptable if that was the way they evolved.

    But back to to the no moon thing – are you saying that both seasons and climate would be irregular, i.e. every year might be different from the next in unpredictable ways, as well as the climate changing substantially over what to us would be geological short periods?

    Lot’s of ideas there for a SF novel…

    • I think you are right, Glenda, that some people assume too many ‘magical’ properties to the Moon. Things like birds and other animals finding their way are related to Earth’s magnetic field and have nothing to do with the Moon.

      Life without a stabilising influence of a large moon would certainly be different. The problem with instability is that climatic changes could be dramatic and may bring species outside the temperature range they have evolved in. The other thing with instability would be its unpredictability. If there is a cycle, life can adapt, even if that cycle spans 20,000 years. When the climate is truly irregular, adaptation is much harder. That’s not to say life couldn’t exist, only that it would probably involve much more primitive organisms, because each time there is a dramatic change, the specialised organisms would die.

  2. But that’s to say that higher life forms need to be specialised, which I don’t think would necessarily need to be so. 20,000 years is nothing in evolutionary history. If things kept on changing right from the early history of organisms emerging from the primordial swamp or whatever, then those that adapted would be the ones that survived, and over a long evolution, some of these could also end up being higher life forms. Of course they would be vastly different from us.

    Perhaps it would be better to say that they would be specialised to many different situations, and which specialisation came to the fore at any one time would change and depend on which climate was prevalent at the time.

    Actually from my observation on birds, the ones that do best are the ones that specialise least…and they are often the most intelligent too, as they are able to take advantage of any situation. Think Indian mynas. They eat anything, they can be aggressive if need be, cooperative if need be, nest anywhere, survive in a variety of climates… In fact, humans are pretty good at NOT specialising too.

    Of course, that’s a bit simplistic when thinking about climate and weather changes on the scale we are talking about…but I can see higher life forms evolving which are able to cope. Migrating to different parts of the globe, hibernating, burrowing, building, becoming aquatic or terrestrial at will, eating different things, physical changes like growing fur coats or thick skins – all kinds of mechanisms could develop.

  3. On the other hand, even if intelligent life doesn’t have the opportunity to evolve without a moon, doesn’t mean the planet won’t be habitable for colonisation.

    You raise a valid point that I’ll have to think more about, though. I remember reading somewhere about a theory that chimps living on beaches was what led eventually to humans. Because of the changing conditions or something.

    • I raised that point in an earlier post. The fact that a planet is not inhabited by life doesn’t mean it’s not inhabitable by humans

  4. Pingback: Earth » Blog Archive » What if the Earth had no moon? Part 2 « Must Use Bigger Elephants

  5. If the Moon was cast into outer darkness and out of our Solar system life on Earth would cease to exist in a relatively short timeframe. The Moons main purpose is to give us a little light at night and to control the tides along with keeping the Earth in check. The Moon is just a giant reflector with the smooth side always facing the Earth while the far-side that we never see is very rough terrian and would not be an ideal relector.

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