Moon Phase Calculator: Your Darkest Stargazing Nights

The Moon is the one form of light pollution you can predict, so it is the first thing to check before you plan a night out with a telescope. Pick a month below and this works out the phase, how much of the disc is lit and how much of the night the Moon is actually above the horizon, then rates every night from dark to washed out. The percentage lit on its own is misleading: a half lit Moon at first quarter has set by around midnight, while a half lit Moon at last quarter does not rise until then.

Why the percentage lit is only half the story

Two nights can show the same half lit Moon and be completely different for observing. A first quarter Moon is highest at about 6pm and sets around midnight, so the second half of the night is properly dark. A last quarter Moon does not rise until roughly midnight, so the whole evening is dark and the bright part arrives when most people have packed up. The rating above multiplies illumination by the share of the night the Moon is up, which is why a quarter Moon lands nowhere near a full Moon even though it is half as bright. Waxing and waning nights of the same brightness score the same, because the Moon is above the horizon for the same number of hours either way. What changes is which end of the night you get: the waxing side hands you the small hours, the waning side hands you the evening.

Plan around the dark window

Deep-sky observing lives in the run of about a dozen nights around new moon, six or so either side, when moonlight is either absent or over quickly. That is the time to chase galaxies, nebulae and the Milky Way, whether you are looking through a telescope, a pair of binoculars or stacking exposures on a smart telescope. Put the dark run from the summary in your diary and treat any clear night inside it as the one to use.

A full Moon is not a wasted night

Bright nights suit the targets moonlight cannot spoil: the Moon itself, the planets, double stars and the brightest clusters. Crater relief is sharpest along the terminator, the line dividing lunar day from night, so the days around first and last quarter show far more detail than the flat, glaring face of a full Moon. If you own a light pollution filter, this is when it earns its keep on emission nebulae.

British summer nights have a second problem

From late May to late July the Sun never sinks far enough below the horizon anywhere in Britain for the sky to reach full astronomical darkness, so even a new moon in June leaves a bright northern sky. Genuinely dark, moonless nights start returning through August, which is part of why the Perseids in mid August feel like the opening of the observing season. Check your baseline sky with the Bortle scale calculator, and see where to drive on a new moon weekend in our roundup of UK dark sky sites.

How accurate is this?

The phase comes from the standard abridged solar and lunar series used for almanac work. Each night's figure is evaluated at 21:00 UTC, while the new and full Moon entries in the summary are solved for the moment the Moon actually reaches that phase rather than read off the nightly samples, so a full Moon that falls at 4am is dated to the right day. Checked against the full periodic series in Meeus, the illumination lands within about a quarter of a percentage point and the new and full Moon times within roughly ten minutes. It does not account for your latitude, which moves moonrise and moonset by an hour or more across the country and across the seasons, so read the ratings as a plan for the month rather than a timetable for the night. New to all this? Start with how to start stargazing.