From how far away a wildfire (forest fire) in an Earth-like secondary world can be visible during night time (when one cannot see the smoke, just the fire itself)? Flat wooded terrain, no clouds, two moons, larger moon is vaning, but still bright, smaller moon produces very little light.
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It kind of depends on how big the fire is, which way the wind is blowing, how flat the land is, what the elevation difference is, and a whole lot of other things.
Sky watchers that want to get good views without light pollution from cities say you probably want to get 50 miles from large cities, and 100 miles from major cities of 1,000,000+ people.
If the fire covers a large area and the wind is taking the smoke away from you then you'd likely see a glow in the sky from a great distance.
If you're close enough to the fire at night for it to be something you need to worry about, then you should be able to see the glow.
Edit: flat and wooded
Well, in that case... The trees are going to be a problem.
Assuming that the canopy is thin enough to allow you to see the sky at all, they are still going to severely limit the distance you can see toward the horizon, so your best bet is to have them see the reflected glow from the bottom of the clouds, maybe a little more if they find a clearing/meadow/lake where the trees are further away and more sky is visible.
Assuming medium density forest, perhaps logged or burned off at one point to thin out the canopy enough to see the sky, you're going to have a lot of underbrush and small trees. This is a potential problem because forest fires can spread upward of 6 mph, and the underbrush will slow you down.
1-2 miles: you will likely see the light of the fire between the tree trunks and through breaks in the brush, as well as smell smoke. Smoke density will depend on wind direction. If the wind is blowing away from you toward the fire you probably won't smell much, but that also means that the fire will be primarily spreading away from you, so that's good. If the wind is blowing toward you then you're best bet is to grab something buoyant and find a large lake to jump in and try to get as far away from land as you can, then find some kind of mask to breath through, even if it's just a wet shirt. The heat in a forest fire gets up around 900f, meaning just breathing the unfiltered air will sear your lungs. Add in smoke inhalation and you're in a bit of trouble.
3-5 miles: the clouds will likely be lit up directly overhead, visible through glimpses in the canopy. If you smell smoke at this point the wind is likely blowing toward you, meaning the fire is coming your way. Either way the best idea is to leave the area very quickly.
6-10 miles: reflected light will be lessened but still visible. Danger is lessened somewhat unless the wind is blowing hard toward you, but you're not out of the woods yet. Keep moving.
10-25 miles: The glow will be visible from clearings and other places with larger views of the sky. Climbing a tree to get some altitude may not be a bad idea. Best to move horizontal to the wind to try to get out of it's path.
25+ miles: this is starting to get iffy in a wooded area. If you look up you'd probably see that the night isn't as dark as it could be, with a reddish tinge, but unless you can get some altitude or a really large clearing it's going to be hard to know what the danger is.
So I couldn't find any good figures on a forest fire, but it seems the best you can expect to see a city skyline at night (when the buildings are lit up) is 35 miles.
If you're downwind of the fire smoke may lessen the brightness of the fire. Additionally, the 35 mile figure assumes a clear view with no trees or rolling hills to obstruct your view.