The Complete Guide to Moon Phases — From New Moon to Full
Why does the Moon change shape every night? Here are the eight stages from new moon to full moon, and the two different "months" you need to know for planning an observation.
Why does the Moon change shape?
The Moon produces no light of its own — it shines by reflecting sunlight. As the relative positions of the Sun, Earth, and Moon shift slightly each day, how much of the Moon's lit side (the half facing the Sun) is visible from Earth changes too — that changing appearance is the "phase." The Moon's actual shape never changes; only the angle we view it from does.
When the Moon sits between the Sun and Earth, its lit half faces entirely away from us, and it's invisible — the "new moon." When it sits on the far side of Earth from the Sun, its entire lit half faces us, giving a round, fully lit "full moon."
Eight stages of the lunar cycle
The full cycle from new moon through full moon and back to new moon breaks down into eight stages: new moon → waxing crescent → first quarter (half-lit, right side bright) → waxing gibbous → full moon → waning gibbous → last quarter (half-lit, left side bright) → waning crescent, growing thinner and thinner.
In the Northern Hemisphere, the Moon brightens from the right side as it waxes (new to full) and darkens from the right side as it wanes (full to new). Spot a crescent moon in the evening sky and it will keep growing; spot a thin crescent at dawn and a new moon is close at hand.
Two different "months" — synodic and sidereal
What we casually call a "month" — the Moon's phase cycle from one new moon to the next — takes about 29.5 days and is called the "synodic month." But the Moon's actual orbital period around Earth, measured against the fixed stars (the "sidereal month"), is shorter — about 27.3 days.
The two differ because Earth is also orbiting the Sun. Even after the Moon completes one full orbit and returns to the same spot relative to the stars, Earth has moved a bit further along its own orbit in the meantime — so the Sun, Earth, and Moon need about two more days to realign the same way. Lunar calendars are built around the synodic month, since that's what determines the repeating phases.
Planning observations around the phase
The Moon's phase has a decisive effect on stargazing plans. For faint deep-sky targets like the Milky Way, nebulae, or clusters, the days around a moonless new moon are best. For photographing the Moon's own craters and surface relief, on the other hand, a half-moon phase (first or last quarter, when long shadows bring out the terrain in relief) works best.
A full moon, somewhat counterintuitively, has almost no shadows and tends to look flat in crater photos. Checking the current moon phase before heading out tonight lets you gauge in advance what kind of night it's going to be.