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If a planet was both covered in volcanic ash clouds/in the middle of nuclear winter and either had gone rogue or was the farthest point possible from its sun, what would “night” look like? How dark could it get on a planet where all extraterrestrial light sources have either been obscured behind clouds or extinguished by distance? Would such profound blackness have any noticeable or unforeseen effects on a human mind? How difficult would it be to navigate through that kind of endless darkness with only pre-industrial technology?

I’m imagining a planet stuck in the middle of an eternal, starless night where life only survives due to a runaway greenhouse effect trapping and holding in the heat provided by frequent geologic activity (such as volcanoes), giving rise to plants that have adapted to siphon geothermal energy and animals that can survive temperatures that make Antarctica look cozy. Aside from a few scant sources of light like volcanic eruptions, flashes of lightning arcing across the planet-wide ash cloud, and a few scattered oases of bioluminescent life, the planet has gone completely, totally dark, and I need to know how to picture that in my mind. What level of absolute, suffocating darkness are we talking about here?

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  • $\begingroup$ That night would be so black that it would make the darkest black you ever saw look light gray. $\endgroup$ – Willk Jun 15 '18 at 22:04
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I don't know if this is an option for you, but you should go spelunking, and while you're in the cave, turn your flashlight off. Wait a few minutes for your eyes to adjust. That's how dark it would be.

Any species that evolved to live here would have limited/no use for eyes, so they would not have them. You might get some with limited eyesight that, when combined with bioluminescence, could be used for communication, but it's unlikely they'd be used for anything more. If this planet somehow had pre-industrial humans on it, society would rely heavily on candles and oil lamps. Local navigation, like around your house or village wouldn't be changed much from how it really was in pre-industrial times. Navigating much further, like to another country or across an ocean would be a real challenge and might not even be possible without a lot of luck. You can find your latitude using a Foucault Pendulum. A Foucault Pendulum is a pendulum with a circle drawn under it. On the circle, degrees are marked. As the Earth rotates, the pendulum precesses and swings over different points on the circle. The rate and direction which the pendulum precesses is related to your latitude like so:

$w = 360sin(t)/day$

Where "w" is the angular speed of the pendulum precession, and t is your latitude. So you set your pendulum up, see how many degrees around the circle it goes in one day, plug into the formula and solve for "t".

The trick here is measuring one day. If your society never had a view of the sky, this is impossible as far as I'm aware. They probably won't even know the world rotates. If they have seen the sky in the past, they could use 24-hour hour glasses for this.

So you have your latitude. Now how do you get longitude? As far as I'm aware, this is impossible for a pre-industrial society with no view of the sky. In real life, they did it with carefully-calibrated clocks. You would have a clock set to the correct time in the port you started at, and then you could compare what it said with your current solar time. If the solar time is earlier than your clock time, you have gone west. If it's later, you went east. How far apart the two are tells you how far east or west you went. Every hour of difference is 15 degrees longitude. This is clearly not an option for your society, since they don't have access to solar time.

Edit: I thought about this more. They might be able to use inertial guidance systems. Early ballistic missiles, some aircraft, and early space missions like the Apollo program used inertial guidance systems. These are carefully-calibrated gyroscopes. When you accelerate, the gyroscope precesses. If you know your latitude and longitude where you started, and can keep track of how many times the gyroscope rotates once you're at cruising speed you can get a decent idea of how far you've traveled. Combined with a compass you can get your direction. You don't really need your absolute longitude, only your relative longitude. You only need to know how far east or west you went from where you started. There are three big problems with this I'm aware of. First, the gyroscopes build up error. They aren't perfectly accurate, especially not when made with pre-industrial equipment, and these errors compound. The further you travel, the more the errors build up and the further off your measured position will be from your actual position. Second, In real life, inertial guidance systems have all been highly electronic. They've used computers to keep track of how much the gyroscopes have rotated. You probably could keep track with some kind of mechanical computer, like Babbage's Analytical Engine, but it would be quite large and would introduce even more errors. Because it has to be physically attached to the gyroscopes to measure them, it will apply friction and throw them off even further. Last, there's nothing inherently modern about this plan, but it may just not be feasible with pre-industrial tech. I don't know if you can make gyroscopes without modern machine tools. Also, this all requires that society knows the world is round, which if they have never seen the sky is non-obvious. It can be proven without access to the sky, but you'll probably never even get the idea to check if no one has ever seen the stars whirl overhead.

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total blackout = global collapse of the food chain. Everyone's dead. Only microbiotic life and fungi survive. It is possible insects will create a new food chain based on themselves only.

Rogue planet = total freezing of the atmosphere, atmosphere precipitates to the ground. All life perishes. Extremofiles in the oceans' hotspot survive

Pluto-planet = life unsastaineable due to lack of adequate conditions. Life possible only inside artificial cities built by colonists from other planet

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How the night look like strongly depends on the viewing mechanism of the organism populating the planet.

Constant presence of ashes and clouds in the sky makes almost no light reaching the surface, and under these conditions I doubt any organism would put evolutionary effort into developing eyes.

How does night look like, when there is nobody to see it?

The inhabitants of that world would perceive the usual vibes when close to food or chemio-energy or heat spot, making no difference between a day and a night.

They might just develop some sort of sensing toward infrared radiation, useful to spot hot spots, to which dust is transparent (ask astronomers looking at the galactic centrum) but unfortunately air is not. But since you don't provide info on the atmosphere, this is a point I cannot explore.

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