I am considering writing a fantasy novel. This novel would be set on an planet, but certain story details require that the planet have either a much larger or much closer moon than Earth does. I'll be honest and say that I know virtually nothing about planet mechanics, and don't know what the differences would be between having the moon be the same size but closer, or the same distance away but bigger, than our Moon. Some insight into those differences, if they are significant, would be appreciated.

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This is about the size of moon I want. Note that I am speaking of the smaller, ringed planet/moon, not the second, much larger, planet.

What I'm after are all different effects (different from our own Moon) of having such a moon on water bodies, but also climate, weather, geology, and anything else which might be changed by having such a large/close moon.

For all purposes, assume the moon is of the exact composition of our Moon, just larger/closer. A comparison (if there are significant differences) between having the moon be larger versus having it be closer than our Moon would be appreciated.

NOTE: If additional details are required, I am happy to provide them. Please let me know what you need via a comment.

  • $\begingroup$ Not enough for an answer, but one immediate difference I can think of: with the "larger moon" approach, the system's centre of mass may end up outside the planet's radius, creating a semi-binary system similar to Pluto and Charon. This, I think, would cause the planet to appear to "wobble" on its orbital path, in time with the moon's orbit, but I don't think it would be drastic enough to affect the climate. $\endgroup$ – F1Krazy Feb 27 '18 at 21:45
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    $\begingroup$ "What I'm after are all effects of having such a moon on water bodies, but also climate, weather, geology, and anything else which might be changed by having such a large/close moon." WAY TOO BROAD $\endgroup$ – Aify Feb 27 '18 at 21:50
  • $\begingroup$ @Aify The definition of too broad, from the help page: "There are either too many possible answers, or good answers would be too long for this format." There are obviously not too many possible answers, so you must be suggesting that good answers would simply be too long. Considering that I have changed one thing, and am asking for differences, I can't imagine that it would seriously take as long as you propose to answer. That being said, I obviously don't know the answer, and could be wrong. As it stands though, I don't see this as too broad. $\endgroup$ – Thomas Myron Feb 27 '18 at 22:00
  • $\begingroup$ If the"moon" gets closer to the size of the "planet" then you have a binary planet system. The smaller of the two bodies will always be the "moon". In other recent questions someone did the math and determined that the largest that a moon/planet could appear from the surface of any world is 63 degrees plus change. $\endgroup$ – pojo-guy Feb 28 '18 at 2:12
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    $\begingroup$ A "Good" answer would have to detail ALL the effects (as per your question) on all of climate, weather, geology, and everything else that might be changed (all of it!) which would be a huge list, and therefore, much too broad. $\endgroup$ – Aify Feb 28 '18 at 2:35

I'm afraid there is a vast of tiny details, way too long to be counted, that would be affected by such a large moon.

It is believed that our moon, Luna, was formed between 20,000 and 30,000 kilometers away from Earth, and it drifted away to its present position. When it formed, its apparent size would have been about 8º of arc, so pretty much like the ringed planet of your picture or even a little larger. If that's the case, let's see some things that the Moon is suspected to have done to Earth:

  • Slowed down Earth's rotation to a third of its former speed - part of this braking was caused by the Moon drifting away from Earth, but part of it it's due to tidal locking.
  • Big tides, increased tectonics and volcanism.
  • Probably, increased amount of heavy elements in the upper crust which would had sank to the Earth's inner layers. As a consequence, increased surface radiactivity too.
  • Effects on luminosity, due to being incredibly bright at nighttime, and frequent solar eclipses during daytime.

A single one of these things is enough to cause huge changes in the Earth as we know it. Increased vulcanism could put so much CO2 in the atmosphere to make a new Venus, for example. The Moon's shadow could have been the proverbial camel's straw who sealed the fate of snowball Earth.

This is a highly complex, chaotic system. Tiny differences in the origin can produce mind-blowing changes in the long run, and you are talking about introducing a huge difference so, how much different to Earth you want your planet to be? Because anything could happen. Had the Moon stayed in its original orbit the Earth could be now a soaring hellhole like Venus or a frozen planet like Mars or absolutely anything in between.


Well, tides would be a lot stronger. Also more of an effect on the planet's crust. It should have more volcanoes, and probably smaller more fractured crustal plates which might lead to smaller continents.

I think you'd get more full eclipses. It might have an effect on local temperature and weather. However, that might depend on if the moon's orbit is exactly lined up with the star system's orbital plane. As an extreme example if it orbited at 90 degrees to the star you'd only get eclipses twice a year.

If it's really large and close I believe (no evidence on this) that it would tend to pull off the planet's atmosphere. There might be a shared atmosphere around both the planet and moon. Which would cause drag and over enough time might lead to the moon crashing into the planet.

Oh, and I can't remember what the experts call it (Roche Limit?) but at certain distances which depend on planet size, moons can't form, they become rings instead. The tidal forces on the moon are too high.

  • $\begingroup$ Yes, it is definitely the Roche Limit. $\endgroup$ – Muuski Feb 28 '18 at 13:58

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