The short answer
A bright Moon lights up the whole sky, hiding faint stars, the Milky Way and most galaxies and nebulae, even from a dark site. Under a full Moon, a dark country sky can look much like a suburban one. The best Moon phase for stargazing is new Moon, and the few days either side of it; a thin crescent or a Moon below the horizon does little harm. The Moon, planets, bright stars and double stars, on the other hand, can be enjoyed in any phase.
How moonlight brightens the sky
Moonlight is sunlight reflected off the Moon’s surface. When it enters the atmosphere, air molecules and particles scatter it across the sky, just as they scatter sunlight to make the daytime sky blue. The result is a bright background glow, and anything fainter than that glow disappears.
How much the Moon brightens the sky depends on three things:
- Phase. The Moon gets brighter much faster than its lit area grows. A full Moon is roughly ten times brighter than a half Moon, because at full the sunlight strikes the surface head-on and the rough ground casts almost no shadows.
- Altitude. A Moon high in the sky lights up more of the sky than one near the horizon. A Moon below the horizon has no effect at all.
- Air clarity. Haze and humidity scatter more moonlight, widening the bright glow around it. See seeing and transparency.
The glow is strongest close to the Moon, so faint objects in the opposite part of the sky suffer less, but on bright nights the whole sky is affected.
The best Moon phase for stargazing
Phase decides not only how bright the Moon is, but when it’s in the sky. That matters as much, because a bright Moon that sets at 10 p.m. still leaves the rest of the night dark.
| Phase | When the Moon is up | For faint objects |
|---|---|---|
| New Moon | With the Sun; not in the night sky | Best: dark all night |
| Waxing crescent | Evening, low in the west, sets early | Very good after moonset |
| First quarter | Afternoon to around midnight | Good after midnight |
| Waxing gibbous | Most of the evening and night | Poor; only the hours before dawn |
| Full Moon | All night, sunset to sunrise | Worst |
| Waning gibbous | Rises in the mid to late evening | Early evening only |
| Last quarter | Around midnight to midday | Good in the evening |
| Waning crescent | Rises shortly before dawn | Very good until moonrise |
For evening observers, the most useful stretch runs from about last quarter through new Moon to a few days after, when the evening sky is free of moonlight. On average the Moon rises about 50 minutes later each day, although the daily delay varies through the month and with the season. The Moon phase calendar shows phases and dates for the month, and Moon phases explains why they happen.
What still works under a bright Moon
A moonlit night is not a wasted night. These are all largely unaffected:
- The planets. Jupiter, Saturn, Venus and Mars are bright enough to ignore moonlight. See planets visible tonight.
- Bright stars and constellations. A moonlit sky shows fewer stars, which can actually make the main patterns easier to learn.
- Double stars and bright clusters in binoculars or a telescope, such as Albireo and the Pleiades.
- The International Space Station and other bright satellite passes. See the ISS pass finder.
The best time to look at the Moon itself
Full Moon is the least interesting phase to look at through binoculars or a telescope. With the Sun shining straight down, there are almost no shadows, and the surface looks flat and glaring. The most detail shows along the terminator, the line between the Moon’s day and night sides, where low sunlight throws long shadows from crater walls and mountains. Any phase from a thick crescent to a gibbous Moon shows a terminator full of detail, and the view changes from night to night. Learn more on the Moon page.
Working around the Moon
- Check moonrise and moonset, not just the phase. Even near first quarter, the hours after moonset can be properly dark.
- Put the Moon behind you. Stand in the shadow of a building or tree so the Moon isn’t in your field of view, and observe the part of the sky farthest from it.
- Choose targets to match the phase. Plan deep-sky trips around new Moon, and use bright-Moon weeks for planets, the Moon and double stars.
- Don’t forget twilight. Even with no Moon, the sky isn’t fully dark until astronomical twilight ends. See what is astronomical twilight.
For the bigger picture on timing, see when is the best time to stargaze?