Moonlight raises the night-sky background when scattered by Earth’s atmosphere. That extra brightness reduces the contrast between the sky and faint extended objects. The effect is strongest when the Moon is bright, high in the sky and relatively close to the target, but local atmospheric conditions also matter.
Galaxies and faint nebulae usually lose the most contrast
Many galaxies and nebulae spread their light over a large area. Their total magnitude can look reasonably bright on paper while their surface brightness remains low. A brighter sky therefore makes their outer structure and subtle detail harder to detect.
Star clusters and bright compact targets are more tolerant
Open clusters, many globular clusters, double stars and bright planetary nebulae can remain rewarding under moonlight because their useful detail is concentrated into smaller or brighter structures. Planets and the Moon itself are far less dependent on a dark sky.
Illumination percentage does not describe the whole observing problem
A 70% illuminated Moon below the horizon does not brighten your sky in the same way as the same Moon high overhead. Angular separation also matters: a target on the opposite side of the sky may retain more contrast than one close to the Moon.
The Moon Observing Planner helps interpret phase and illumination, while the Night Observation Planner provides the wider darkness context for a chosen date.
Use moonlit nights for different observing goals
Bright-Moon nights can be useful for lunar study, planets, double stars and equipment testing. Darker lunar periods can be reserved for galaxies, diffuse nebulae and other low-surface-brightness targets. This approach treats the Moon as part of session planning rather than as a simple good-or-bad condition.