NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2021 Jun 30, 16:00 -0700
Sights by the Full Moon certainly work. Gary LaPook just posted some data on sights taken when the Moon was 98% illuminated with excellent results. There's no rule though. Closer to full is better and closer to the Moon's azimuth is better. If you want an arbitrary cutoff, then I would suggest two-thirds, 67%, illuminated as a reasonable choice. I wouldn't go as low as 50% or less. The Moon is considerably fainter when it's half full. We can get great Moon sights (of the Moon itself) when the Moon is half full, but it's much less than half the brightness of the Full Moon, and it doesn't light up the horizon anywhere near as well.
You should be aware that common advice to navigators claims that the horizon beneath the Moon cannot be trusted. I consider this concern seriously over-done. When the Moon is bright, we can get sights and decent fixes all night long. Seems a shame to pass that up!
Frank Reed
