Introduction
Aurore is an interesting planet with some odd orbital dynamics etc. It receives most of its' heat from the brown dwarf it orbits, but essentially no visible light. Tithonus looks like a dull, cherry red stretched disk in the sky, but radiates so little visible light that it is practically none.
Visible light comes from Muphred, a yellow subgiant 6.5 AU away. However, that light is weak compared to Earth. The orbital period around Tithonus creates a 61 standard hour day.
Aurore is tidally locked. It's rotation period around it's axis and it's rotation period around Tithonus are the same. However, there is a slight axial tilt (given as 1 degree) and thus Aurore librates. It sort of wobbles, and I will ignore some minor effects of this.
To compare, on Earth:
- Strong direct sunlight at equator: 100,000 lux
- Overcast day: 400 lux
- Moonlit night: 1 lux
- Moonless night 0.002 lux
There are three sources of light on Aurore.
1. Direct Illumination from Tithonus
Tithonus gives an insolation of 78% of Sol (assuming emissivity = 1)*, but it is almost all infrared. For Sol, a little over half the light is in the visible spectrum (52-55%), but for Tithonus only ca. 0.01% of the light is in the visible spectrum, almost all a dull red. Tithonus gives 0.016% of the visible light of Sol. This gives a light level of about 16 lux, assuming no clouds etc. standing at the hot pole of Aurore.
However, neither colony is at the hot pole.
Aurore and Tanstaafl are in the twilight region of the hot face. They'll be getting in the region of 2 mLux of red light. This is about the same as a moonless night.
Novoa Kiev is over the terminator, and cannot see Tithonus. If there is airglow, it might be the same illumination level as an overcast moonless night (< 2 ulux), but without airglow it is pitch black.
However, Human eyes would be using rods at this light level, and rods essentially do not respond to red light. In both settlement regions, Humans would be functionally blind at night. Completely blind. The Kafers... not so much.
2. Direct Illumination from Muphred
Muphred is the other major light source, being a bright yellow star 5.85 AU (+/- 0.22 AU) away. It averages 19% of Sol's insolation, but ranges from ca. 17.6 to 20.5% depending on the time of the year. Rubis can be ignored as it is so weak.
A little more of Muphred's light is visible as it is a hotter star than Sol. Say 60% is visible (which I didn't calculate). Say Muphred gives 20% of the visible light of Sol in daylight. At midday you'd get about 20 kLux without clouds, or a typical overcast day. With clouds the light would be dim, but cones would still function. It would like being in a dimly lit room.
Around dawn and dusk it would be closer to a moonlit night.
3. Reflected Light off Tithonus from Muphred
The last night source worth considering is reflected light from Muphred off Tithonus. The albedo of a brown dwarf is ca. 0.1, and so about 10% of the light that strikes Tithonus will be reflected back. This is similar to the moon (0.12 at full moon). Tithonus is 74 times the diameter of the moon and thus reflects ca. 5,500 times more light. However, Aurore orbits 2.42 times the distance of the moon. Very roughly, reflected light will be ca. 12.5 greater than Earth's moonlight. It will be yellow-white light.
The upshot is that reflected light will be quite strong during conjunction (when Aurore is between Muphred and Tithonus, which is midnight with repect to Muphred), maybe giving ca. 10 lux of illumination at the hot pole. This would be on the order of twilight on a clear day. However, the colonies aren't at the hot pole. Novoa Kiev would never see this, being on the dark side of the terminator. Aurore and Tanstaafl would get about 1 lux of white light around "midnight," being equivalent to a full moon.
The Auroran Day-Light Cycle
GDW's Aurore has a 61 hour day.** Since the colonies are tide-locked, and on different sides of the terminator, the effects are different. Auroran midday is taken as Aurore being in opposition with respect to Muphred, and midnight as conjunction. The Aurore SB (page 37) gives the normal pattern of an Auroran day, noting that a local hour is 60 minutes and 51 seconds long, and so they work 60 local hours per day.
In Novoa Kiev it is basically pitch black for 30 h at a stretch. NK is over the terminator and so they don't get much light from Tithonus either. Passive night-vision simply does not work in NK, and active illumination or thermals are needed to operate at night. The Kafers are as blind as Humans because there simply isn't even enough near IR.
In Aurore and Tithonus, there is still some near infra-red and so Kafers can see perfectly well and PNG's will work. Reflected starlight becomes significant towards midnight, and the night is more like a Terran moonlit light rather than pitch black. Operating at night is more like on Earth, but note the Kafers can see without night-vision aids.
So, could you see? Yes, but not that well during the daytime. At night, Novoa Kiev is pitch black, whereas on the other side of the planet they get a little moonshine.
* Emissivity is probably closer to 0.5, but 1 is the assumption used. Both work but shifts the habitable band of Aurore slightly.
** Colin Dunn has changed this in his universe to 9.5 days. We ignore it as a clear error-of-fact.
