# where is the boundary between dark night and grey night

Is there a clear cut between them?

How long and how bright will the moon be for a dark night?

I googled but could not find a definition for them. People just talk about them empirically?

Dark $0\leq$ FLI $<0.25$; Grey $0.25\leq$ FLI $<0.65$; Bright $0.65 \leq$ FLI $\leq 1$, where the FLI is calculated at 0h UT (close to midnight at the ING).