Since I'm creating a "new world" for my story, I wanted to know if the following conditions would work for an Earth-like planet:

  • Its moon (more or less same size as our moon) being much closer (but in a stable way).
  • Higher tides than Earth.
  • A twenty-six (26) hour day/night cycle (full rotation of the planet with respect to its Sun).
  • A twenty (20) day lunar cycle.

I just want to know if all the conditions I mentioned above are realistic, and if they could lead to a planet that has complex life- just like Earth.

Thanks in advance!

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    $\begingroup$ It is often best to avoid planets with non-24 hour day cycles due to it either having to come up all the time which would be unlikely in a story with characters who live on that planet for much of their lives or really even after a year or so. Secondly there are some details that might need extrapolating. In so far as how closer is really important on the moon for stablity but also so that the moon doesn't get ripped appart by the planet. $\endgroup$ – Adrienne Jun 9 '17 at 2:24
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    $\begingroup$ @Adrienne I don't understand your 24 hour argument? I think you are making a valid point here from which I want to learn (not that I would ever specify such a thing). As I said, it seems incoherent what you wrote, but you could be going into a direction that makes no sense or into a direction that makes a lot of sense and I want to know which of these it is. $\endgroup$ – Raditz_35 Jun 9 '17 at 9:03
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    $\begingroup$ I think the point about a 26 hour rotation is to ask what is actually achieved by it? Does it just add complexity to the worldbuilding for no gain or is there an actual real impact from it? Why do you want the longer day? It's not necessarily a bad thing (and doesn't make the planet not valid) but it might be an irrelevant detail. $\endgroup$ – Tim B Jun 9 '17 at 10:33
  • $\begingroup$ @TimB There are a number of arguments for and against. And indeed, it would have little story impact and only a fool would go ahead and write a story about a world where clocks go to 13 because of a longer day. But there are also arguments for it - often a day is too short for stuff to get done, an extra hour might help and a more alien feel to a world could be good if executed right. I'm curious what Adrienne is talking about still. $\endgroup$ – Raditz_35 Jun 9 '17 at 10:48
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    $\begingroup$ I was purely talking from a story telling point of view. Worlds with weird day cycles are common in even our solar system so not having 24 hrs is totally possible. But it is definitely one of those things where unless the story is about how alien the world is it isn't worth having to explain to the audience (who are likely to forget and still think of it as a 24 hr day in their heads). So really I am just speaking from the perspective of there is a limited amount of new stuff you can add to your world before your audience gets confused. Day cycle doesn't add enough to pick. $\endgroup$ – Adrienne Jun 9 '17 at 14:26

Yes that sounds reasonable. Your moon has to stay out of the Roche limit in order not to get ripped apart.

The Moon is ~ 400,000 km away, the Roche limit is only ~10,000km so you can move it closer. Having the moon closer to the planet will reduce the time it takes to orbit, the Moon orbits in 27 days, so 20 day orbital period sounds reasonable.

Slightly larger planet will give you 26 hour day/night cycle.

This sounds like very Earth like planet.

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    $\begingroup$ Good answer. The planet, as described, is fairly Earthlike. Mars is a smaller planet and has a twenty-five hour diurnal (day/night) cycle. A closer moon will, most likely, slow the rotation of the planet to a greater degree than that of our Earth. The planet doesn't necessarily need to be larger for a longer diurnal cycle. Although that can be a possible factor, it's not the only one. Plus one from me. $\endgroup$ – a4android Jun 9 '17 at 2:59
  • $\begingroup$ I thought keeping the rotational speed the same as Earth's will make it more likely for the planet to keep its magnetic field. Also, large planet means higher gravity and thus better chance for the planet to have atmosphere. $\endgroup$ – ventsyv Jun 9 '17 at 3:05
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    $\begingroup$ you don't need to make the planet bigger, the length of the day on earth has changed, it was 22 hours 600 million years ago and gets shorter as you go back. the earth's rotation has been slowly slowing since it formed. A 26 hour day would still produce an earth like planet, with a normal magnetic field. $\endgroup$ – John Jun 9 '17 at 4:06
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    $\begingroup$ A larger planet won't change the day/night cycle. In order to do that you need to change the speed of rotation and nothing else. $\endgroup$ – Tim B Jun 9 '17 at 10:34
  • $\begingroup$ @TimB sure it will. t = s/v, s=C=2*pi*r => t = (2*pi*r)/v ; if the v is kept constant you need to increase the radius. Some back of the envelope calculations show that increasing Earth radius by ~ 8% will be enough to give us 26h cycle $\endgroup$ – ventsyv Jun 9 '17 at 15:28

All your settings are completely plausible.

I upvoted the @ventsyv answer; but I need to correct one thing: Planets can rotate at any speed; in any direction; thanks to collisions during their formation. It is not mass dependent. Asteroids can spin like tops!

See the NASA Planetary Fact Sheet, or for a more detailed explanation, see this more technical explanation.

Venus, Uranus and Pluto all have opposite directions of rotation. Also look at the Length of Day in the NASA table; correction if anything bigger planets have shorter days. Earth is 24hrs, but Mars has a slightly longer day with less mass; in fact Mars is about 1/10th the mass of earth, yet rotates slightly faster.

Venus is lighter than Earth, it's day is 100x longer. Jupiter is the heaviest, with the shortest Day: 9.9 hours.

Day length can be anything you want it to be.

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    $\begingroup$ Venus and Uranus are really the odd-balls in our solar system. Venus because of its retrograde rotation and to some extent its extremely slow rotation; Uranus because of its axial tilt. But even our own solar system goes to show that you can have pretty much whatever rotation you like in a planet. $\endgroup$ – a CVn Jun 9 '17 at 12:54

If your planet and your moon have roughly the same masses as our Earth and Moon, respectively, then your moon cannot be "much closer" as you requested, if its orbital period must be 20 days, because the orbital period is not a free parameter. From Kepler's third law (with some help from WolframAlpha) you have:

$$a = \sqrt[3]{{T² \times G(m_1 + m_2)} \over {4 \pi²}}$$


  • $a$ = semimajor axis (maximum radius) of the moon's orbit
  • $T$ = orbital period
  • $G$ = gravitational constant
  • $m_1, m_2$ = masses of planet and moon

If you plug in your numbers, that gives a semimajor axis of 312500 km.

If you want your moon to be closer and still revolve around the planet every 20 days, you need to lower the combined mass of the two bodies. The thing is, as you see, the orbital period is proportional to the cube root of the masses, so anything but a large decrease will make almost no difference. For example, if your planet had 50% of Earth's mass, the orbital radius of the moon would be 249030 km.

If you want to have a large moon close to a large planet, you have to have a fast moon. If you achieve that, more forceful tides will follow as a natural consequence. For example, with a period of 3.5 days your moon will be at about a quarter of the Earth-Moon distance from the planet. That would mean extremely strong tides, though (check out this answer for the calculations).

Your planet can rotate around its axis at pretty much any reasonable rate (26 hours is perfectly reasonable). The mass and distance of the moon are irrelevant in principle. Of course, a large moon on a close orbit will suffer a lot of tidal acceleration and as a result both the moon and the planet will tend to rotate more slowly and to get away from each other over geological timescales.

  • $\begingroup$ Hi! Thanks for your answer. I was wondering, then, what would be a more correct cycle for the moon then? If it's a large planet, with a large moon, then what could be considered a "fast moon"? I'm trying to make it as realistic as possible, even the tiny details! $\endgroup$ – C. Marshall Jun 9 '17 at 21:21
  • $\begingroup$ @C.Marshall Well, you specified a 20-day period. If you allow for a much shorter period, the moon could be closer to the planet. I've edited my answer to add an example. The important thing is that you cannot choose the distance of the moon and its orbital period independently. Play around with the equations (WolframAlpha is great for that) and see what comes out. $\endgroup$ – pablodf76 Jun 9 '17 at 21:56

One thing I feel needs pointing out, our Moon is already in an unstable Orbit (some time millions of years from now it's likely going to impact drift away from Earth), so how exactly did you stabilize the Lunar Orbit?

There's a few different options and I'm curious which you decide to use, the one that comes to my mind is to speed up it's rotation around the Planet, but when combined with the limit of the 27~28 Day Cycle into a 20 Day Cycle, I'm curious just how much closer you moved it if you did increase velocity.

Another facet I don't see covered is the Axis and Rotation, our Moon for example is on equal Axis with us and it's rotation keeps pace with us, this is known as Tidal Lock, one side always faces us and that side never changes unless something else comes along to alter that, does your Moon still have Tidal Lock?

If not that's going to affect Tidal Pull, which affects Weather, Ocean Activity (High/Low Tide, Hurricanes, etc.), Erosion Patterns and Erosion Speed, and also Earthquakes due to the shifting pull on the crusts (in your case, more everything due to increased Activity, it becomes a variable if not Tidal Locked, Constant if it is)

If the rotation is changed, you need to consider how it's changes ​are going to reflect on the Planet, if it's going with the spin, you have plenty of room to accelerate the Lunar Speeds, as the Planet will have very little Pull on the Moon, where if you are going opposite direction from Planetary Rotation, it's like running your hand across Sandpaper, it's going to grab a lot of velocity in very little time, not as much wiggle room, and generally destabilizes the orbit faster.

You're also going to need to factor in Mass, Gravity is directly proportional to Mass, if you have a huge chunk of Rock, and you move a smaller chunk of Rock next to it, those Rocks want to Smash together, but all the Gravity already affecting them (Earth) is disrupting the Pull between the two and drowning it out (making it heavy, this is how we Weigh it's Mass, which tells us it's gravitational pull), Bigger Earth, closer Moon, just bigger Rocks, it's up to you to decide their Mass though.

If your Earth is raining Diamonds, I'd give it about three years before it's raining a Moon, that 10km Radius in the Roche Limit is with 1.0 Gravity, which is Earth (our Baseline), more Mass, larger Radius, Jupiter for example, it actually does rain Diamonds on the Surface, the Surface it's self is likely smaller than our Moon though, combined with the fact that we've never measured the Surface Depth beneath that Cloud layer (not for a lack of trying), you can imagine the Roche Limit on it.

Also, the Magnetic Field is generated by our Planet's Core, one currently active theory is there's a gigantic electromagnetic Storm on the Core's surface because the Iron is under so much intense pressure that it's crystalized into Iron Crystals, which in turn gives off a piezo-electric Discharge (proportional to it's size), which reacts with the various Cosmic effects in our localized band of orbit (Cosmic Radiation and whatnot) to generate our Massive magnetic field which keeps us safe and allows Compasses to function.

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    $\begingroup$ Earth's moon is gradually drifting away from Earth, not moving towards it - and at a rate of centimetres per year, which on an astronomical scale is comparatively stable. There's currently no danger of it ever impacting Earth. $\endgroup$ – F1Krazy Jun 9 '17 at 12:51
  • $\begingroup$ Odd, I'd heard the opposite. Perhaps they're simply not sure lol, as you said, it's centimeters in a global theater, either way, if he moved the Moon closer, AND made the Planet larger, the Moon won't be moving away anymore unless he reduced Mass. $\endgroup$ – Blue64 Jun 9 '17 at 12:58
  • $\begingroup$ Corrected the initial Drift Direction in my post. $\endgroup$ – Blue64 Jun 9 '17 at 13:04
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    $\begingroup$ Thanks to the Lunar Laser Ranging experiment mirrors placed on the Moon by Apollo 11, 14 and 15, we can measure the distance to the Moon with very high levels of accuracy. See en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment and eclipse.gsfc.nasa.gov/SEhelp/ApolloLaser.html; particularly, right at the bottom of the second linked page is the data that the Moon is receding (moving away from) Earth at a rate of about 3.8 cm/year. Regardless of one's feelings toward a US administration, I consider NASA reliable in space matters. $\endgroup$ – a CVn Jun 9 '17 at 13:27
  • $\begingroup$ Why the second downvote? I already corrected it, complete with strike through. $\endgroup$ – Blue64 Jun 9 '17 at 14:31

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