Tonight's Moon Phase, and What It Actually Tells You

Quick answer: The Moon’s phase tells you how much of its sunlit half is visible, while moonrise and moonset depend on your location. Use the phase to plan the light you want, and local rise/set times to decide when to go outside.

Most people look up the moon phase for one of three reasons: they want to photograph something, they want the sky dark enough to see something else, or they're simply curious what that shape in the sky is called. All three are better served by understanding the cycle than by checking a number.

The eight phases, and what they mean

The phase is just how much of the lit half of the moon we can see from here. The moon is always half-lit; the geometry between sun, moon and Earth decides how much of that half faces us. NASA's phase guide illustrates the same eight-phase cycle.

• New moon — between us and the sun, lit side facing away. Effectively invisible.
• Waxing crescent — a sliver, visible in the west after sunset.
• First quarter — half-lit, and confusingly named: "quarter" refers to a quarter of the way through the cycle, not a quarter lit. Rises around midday, sets around midnight.
• Waxing gibbous — more than half, heading to full.
• Full moon — opposite the sun, fully lit. Rises around sunset, sets around sunrise.
• Waning gibbous — shrinking from full.
• Last quarter — half again, the other half. Rises around midnight.
• Waning crescent — a thin sliver before dawn in the east.

Waxing means growing, waning means shrinking. The full cycle takes about 29.5 days.

Why the moon rises later every night

This is the single most useful thing to understand, because it explains almost everything practical.

The moon rises roughly 50 minutes later each day. It orbits Earth in the same direction Earth spins, so by the time your bit of the planet has turned all the way round, the moon has moved on a bit and you need to keep turning to catch it.

That's why:

• A full moon rises at sunset — it's directly opposite the sun.
• A new moon is up during the day, which is why you never see it.
• A crescent right after sunset is always a waxing one; a crescent before dawn is always waning.

Once you know the phase, you know roughly when it's in the sky, without looking anything up.

Planning around it

Photographing the moon itself. A full moon is the least interesting target: lit head-on, so the surface is flat and featureless. The detail is along the terminator — the line between light and dark — where low-angle sunlight creates long shadows around surface features. A gibbous or quarter moon shows far more texture. The best shots of the moon as a subject are rarely at full.

Photographing the moon in a landscape. You want it low and near the horizon, which means around moonrise or moonset, and ideally in twilight so the ground is still bright enough to expose alongside it. A full moon rising at sunset is the classic case — huge-looking, and balanced against a sky that hasn't gone black.

Photographing anything else at night. You want the moon gone. New moon, or the nights either side, or a window when the moon has set. A bright moon washes out the Milky Way as effectively as a streetlight.

Just seeing the stars. Same answer. The week around new moon is the dark-sky week.

Rise and set times are local

The phase is the same for everyone on Earth at a given moment. Moonrise and moonset are not — they depend on your latitude and longitude, and can differ by hours across a country.

This is why a printed lunar calendar tells you the phase but not when to go outside. You need times calculated for where you are. Moon Phases for Android does that offline — phase, moonrise, moonset and the countdown to the next full moon, computed on the device with your location never leaving it. It uses NASA Lunar Reconnaissance Orbiter imagery for the illustration, so what you see on screen matches the terminator shape you'd see through binoculars. It is also available for iPhone.

Supermoons and eclipses, briefly

A supermoon is a full moon near the closest point of its slightly elliptical orbit. It's genuinely larger — by around 7% — which is real but far less dramatic than the photos suggest. The "huge moon on the horizon" effect is an optical illusion that works at any distance.

A lunar eclipse can only happen at full moon, and a solar eclipse only at new moon, because those are the only alignments where the three bodies line up. They don't happen every month because the moon's orbit is tilted about 5°, so it usually passes above or below the shadow.

The tide question

Yes, the moon drives tides, and spring tides — the largest — occur around new and full moon when solar and lunar pull align. Neap tides, the smallest, come at the quarters.

What the phase doesn't do is affect sleep, mood or behaviour in ways that survive controlled study. The tides are physics. The rest is folklore, and enjoyable as folklore.