Awesome blog!! If a habitable planet becomes tidally locked to its sun, does that generally occur gradually over a long period of time, or is it a sudden event (with presumably catastrophic effects)? What would inhabitants be experiencing around this time, and is there a possibility that once tidally locked, the areas outside the twilight zone would be somewhat habitable given certain technologies / magic / evolution? Would there be variances in climates even within the three zones?
Tidal-locking usually takes place over a large, gradual amount of time - several million or several billion years. There’s actually an equation that estimates the time it takes for a body to tidally-lock to another one:
But don’t worry about it. I just like math. ;-)
The equation states that the faster the planet rotates and the farther away the other body is, the longer it takes to achieve a tidal-lock. Conversely, the more massive the planet and the other body the less time it takes to achieve a tidal-lock.
As to the effects to the world as it moves towards being tidally-locked, well, just look outside - assuming you’re on Earth, that is.
The Earth’s rotation is actually slowing down and approaching a tidal-lock with the Moon. The Moon is, or course, already tidally-locked to the Earth.
As the Moon creates tides and the Earth rotates inside them, that friction generated does two things - it slows the Earth’s rotation, and it allows the Moon to get slightly farther away from the Earth. Eventually, the Earth and Moon will both be tidally-locked, keeping the same face towards each other.
That’s going to take a while. The Moon is receding at the rate of about 4 cm per year, and the Earth is slowing down at the rate of about 2 milliseconds every century. At these blistering speeds, it will only take about 50 billion years to achieve tidal-lock.
Sadly, it will never happen because the Moon should drift away from the Earth first. Actually, the Sun will go all red giant on us and vaporize both Earth and Moon before the Moon can drift away. Looks like we’re stuck with each other until death. ;-(
Assuming that a planet has a larger, closer moon, and does actually tide-lock with it over a couple of billion years, the climate will have changed so much that the planet will become almost totally unrecognizable. Considering the changes that would take place over billions of years anyway, the effects of being tidally-locked are negligible in comparison.
However, if a planet were to slow down much faster - say on a human scale of a few decades to a few centuries - then the effect would be much much stronger. The planet’s climate will start to radically change as temperature extremes will start to become more pronounced and animals and plant life will struggle to adjust to the lengthening day/night cycle.
Massive ecological disruption would be the best outcome of this situation. I could see most of a society’s resources turned towards mitigating or even just surviving the changes.
However, I’m sure there would be a small group of scrappy scientists that would try to uncover the reason the changes were taking place so rapidly.