However this is very crude. I would like a way of computing the correct luminance so I can turn them on and off properly.
How much detail do you need?
The nitty-gritty of calculating Apparent Magnitude for planetary objects is fairly complex, and requires calculating planetary orbits, distance of a given body from both the Earth and the Sun, etc. then converting everything from AM to Lux.
If you wanted to handwave it, you could just assume a fixed AM for any given planetary object and have it "fade out" as luminance from competing light sources increases. Otherwise, you go down the rabbit hole into some fairly complex math. (The good news is that if you go that direction, you have the rudiments of a space flight simulator!)
I believe that this is one good entrance to the rabbit hole:
https://stjarnhimlen.se/comp/tutorial.htmlThis paper also gets into the fiddly bits about calculating AM for planetary objects (KBO in this case):
comethunter.lamost.org/scwrk/THECAL/opam.pdf
For the hand-wavy option:
Planet Min AM Mean AM Max AM (Remembering that when measuring AM negative numbers = brighter)
Venus (-2.98, -4.14, -4.92)
Jupiter (-1.66,-2.20,-2.94)
Mars (+1.86,+0.71,-2.94)
Mercury (+7.25,+0.23,-2.48)
Saturn (+1.17,+0.46,-0.55)
Faintest objects visible with naked eye when sun less than 10* above the horizon: -2.5
Faintest objects observable during day when sun is high: -4.0
Faintest stars visible in urban neighborhood with naked eye: +3 to +4
Naked eye limit: ~+6.0
Apologies if this is TMI or repeats stuff you already know.