Low orbit: where is the sun?

[lastspartacus]

Ya, its simply a riding the wind thing.  Eldar almost assuredly can self generate their own power as long as any ship,
or they wouldn't be able to ride the warp or webway.

[Simulated Knave]

They supposedly get their power from "capturing the stellar energy through their sails". 

There's a simple explanation for how the power collection works with the sun as it does - most of the collectors are on the sides and rear of the ship, while the guns are on the front. 

I'd say looking at where the attacker entered low orbit from is the best solution.  Failing that, either randomly decide if it's day or night (allowing dawn or dusk on a 6, perhaps), or assume they attacked during the day.