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In light of recent reading into Pluto, I noticed that it has the unusual quality of being reciprocally tidally locked to its moon Charon. I have taken this idea and am going to use it in a fantasy campaign wherein an artifact triggers the planet's rotation to slow down to match that of its moon's orbit. I plan on using the earth/moon for rotation speeds until eventually the day on the planet is 28 normal days, and the planet is tidally locked to the moon. In this magical world, this process will take several weeks.

Edit: Through some perusing I have found that there are a multitude of questions asking how climates/weather/etc. will be affected by an earth-like planet becoming tidally locked or having a slowed or stopped rotation on the order of the one I am proposing. In light of this I will narrow the question:

Assume that effects on the natural, physical world are somehow made negligible (including effects on plants and animals, ocean and air currents, etc). What effects could this "slow down" have on medieval level cultures? For example, half of the planet will no longer be able to see the moon, while it will always be visible in the sky for other peoples.

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  • $\begingroup$ The questions you ask about this scenario are topics far too big to cover well in an answer. I suggest you have one question (you can put the others into separate posts if you want to keep them), and narrow down what you're looking at - "natural effects" can cover pretty much anything. Do you want effects on the climate, the animals, the people, the societies..? $\endgroup$
    – ArtOfCode
    Commented Jun 17, 2015 at 15:48
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    $\begingroup$ Someone could clarify or correct me, but if the planet spins while the moon orbits it, it's rotation is simply in synch. To have a planet tidally locked with a moon wouldn't it need to be tidally locked to a center of gravity outside of it's body? $\endgroup$
    – Spacemonkey
    Commented Jun 17, 2015 at 15:54
  • $\begingroup$ @Spacemonkey Tidal locking means the planets spin and the moons orbit are synchronized. It takes 1 full day for the moon to do one full orbit. This means the moon ends up always over the same part of the planet's surface. It's the same orbit as geostationary satellites are in. $\endgroup$
    – Tim B
    Commented Jun 17, 2015 at 17:56
  • $\begingroup$ There are other questions here about slowing Earth, including links to a hour-long show on the subject. Slowing to 30 days is essentially the same as stopped, in terms of the loss of centripetal force. $\endgroup$
    – JDługosz
    Commented Jun 23, 2015 at 6:31
  • $\begingroup$ See worldbuilding.stackexchange.com/questions/8408/… , worldbuilding.stackexchange.com/questions/16842/… , worldbuilding.stackexchange.com/questions/10581/… and my Answer worldbuilding.stackexchange.com/questions/15498/… (and others). $\endgroup$
    – JDługosz
    Commented Jun 23, 2015 at 6:45

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Well start with a note, that the Earth is slowing down and might eventually be tidally locked with the moon, and even more eventually tidally locked with the sun. However, the time calculated for that to happen is would be some time after the sun burns out.

I think there are other questions about slowing down the Earths rotation. But ultimately what you would have are mass extinctions. Plants (and many animals) are used to a ~24 hour cycle of the sun. Many plants will die many others will get sick. Both for being in the light for 28 days and for being in the dark for 28 days.

Nocturnal and diurnal animals will be totally messed up. Many bats don't do well in sunlight and many many animals hide in the dark and wait. 28 days are to many. Many bats would die of starvation. Helping out the mosquito population a bit.

The weather would be totally messed up. Since night would be 28 days long, allowing for a much longer cooling off period and days would be 28 'days' long allowing for much more heating up of the surface. The oceans would help equalize a little bit but it would not be pretty.

It would be a mass extinction event. Lots of things would die including people (likely starvation).

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  • $\begingroup$ Not to mention that most Oxygen generation during transpiration happens at night, so your plantlife will have significantly reduced air scrubbing abilities. $\endgroup$
    – Josiah
    Commented Jun 17, 2015 at 17:02
  • $\begingroup$ IIRC, the Sun becomes a red giant before the Earth can tidally lock to the Moon. $\endgroup$
    – Jim2B
    Commented Jun 18, 2015 at 1:54
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    $\begingroup$ Without air conditioning, half the earth would bake and the other half would freeze. No mention is made of the reverse: if we SPED UP THE MOON so it becomes geosynchronous. THAT'S a more interesting and less "everything dies" proposal... $\endgroup$
    – Isaac Kotlicky
    Commented Jun 18, 2015 at 10:28
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Superstition/mythology/religion/astrology played a huge role the organization of pre-industrial, pre-Renaissance cultures. It was the only explanation for how the world worked. Lengthening the days and nights combined with the disappearance or permanent appearance of the moon would require the existing mythologies to invent new stories. If those stories don't sufficiently match what people see, then you may precipitate the invention of science...which would be a wildly interesting story to read.

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  • $\begingroup$ I think suddenly changing the day/night cycle would simply result in new myths about why it changed, if the society wasn't science-minded already. $\endgroup$
    – Brilliand
    Commented Jun 17, 2015 at 18:42
  • $\begingroup$ There would definitely be new myths and I expect that most people would just follow along with them. However, all it would take it one or two people, the equivalent of Brahe, Copernicus, and Galileo, to get the ball rolling. $\endgroup$
    – Green
    Commented Jun 17, 2015 at 19:13

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