How the Moon phase changes what you can see

Phase sets the brightness

The Moon runs through its cycle in about 29.5 days: new (invisible), waxing to first quarter (half-lit, up in the evening), full (up all night, blindingly bright), waning to last quarter (up after midnight), and back to new. A full Moon is not twice as bright as a half Moon — it's roughly ten times brighter, because near full the whole face lights up at once and shadows vanish. Around full Moon, the sky glow it creates can wash out the Milky Way and all but the brighter deep-sky objects even from an otherwise dark site.

But only when it's above the horizon

Here's the part most phase calendars miss. A Moon that is 60% lit but below the horizon all through your observing window does you no harm at all — the sky is as dark as if there were no Moon. Conversely a modest crescent that sits in your sky during the hours you're out will still take the edge off faint targets low near it.

So the useful question is never just "what phase is it?" — it's "how much moonlight is up during the dark hours I'll be observing?" A last-quarter Moon that doesn't rise until 1 am gives you hours of genuinely dark, moonless sky right after twilight. A waxing gibbous that sets at 11 pm hands the rest of the night back to you.

How Stellarhound counts the Moon

This is exactly why the score doesn't just read off the phase. For each night it computes, locally from JPL ephemerides, the Moon's illumination and the fraction of your dark window during which the Moon is actually above the horizon — its "up fraction". A brilliant Moon that has already set barely dents the score; a Moon that's up all night through a dark window is penalised hard. The per-night card shows both numbers, plus moonrise and moonset, so you can see when the dark gap falls.

Planning around the Moon

• For faint things — the Milky Way, galaxies, nebulae, meteor showers — aim for the week around new Moon, or the moonless hours before moonrise / after moonset on other nights.
• For the Moon itself — the best views are not at full. Along the terminator (the line between lit and dark) around first or last quarter, crater shadows are long and the surface leaps into three dimensions.
• For planets and bright stars — the Moon hardly matters. A bright Moon night is a fine time for Jupiter, Saturn or a double-star tour.
• For a big event — the reason the 2026 Perseids are so good is a New Moon on the peak night. Check the Moon before you commit to any meteor shower.