Moon Science
How Moon Phases Work
The moon doesn't change shape - it just reflects sunlight from different angles as it orbits Earth. Here is the geometry behind the phases you see every night, in plain language.
The single fact that explains everything
Half of the moon is always lit by the sun. The moon has no light of its own - it is a rocky sphere (about 3,474 km in diameter) reflecting sunlight. Just like the Earth, exactly one half faces the sun at any moment, and the other half is in darkness.
What changes is how much of that sunlit half we can see from Earth. As the moon travels its orbit, the angle between the Sun, Earth and the Moon changes continuously, so the portion of the lit half facing us grows, peaks, shrinks and resets - that is the lunar cycle.
The moon orbits Earth in about 27.3 days (sidereal month), but because Earth itself moves around the Sun, the time between two identical phases is longer: 29.53 days (synodic month). That 29.53-day rhythm is what a lunar calendar tracks.
The Sun-Moon-Earth geometry
The eight phases, in order
Each phase is a milestone in the 29.53-day cycle. The illuminated fraction follows a cosine curve: 0% at New Moon, 50% at the quarters, 100% at Full Moon.
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1. New Moon
The moon is between Earth and the Sun; its lit half faces away. Invisible in the night sky. 0% illuminated.
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2. Waxing Crescent
A thin sliver appears on the right (Northern Hemisphere). A few percent of the disc is lit.
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3. First Quarter
Half the disc is lit (50%). The terminator is a straight line. Roughly 7.4 days after New Moon.
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4. Waxing Gibbous
More than half lit, still growing (50-100%). The terminator curves outward to the right.
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5. Full Moon
The whole sunlit half faces Earth (100%). The moon rises around sunset. ~14.8 days after New Moon.
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6. Waning Gibbous
Still more than half lit, but decreasing. Light recedes from the right side first.
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7. Last Quarter
Half lit again (50%), now on the left. ~22.1 days after New Moon.
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8. Waning Crescent
A thin sliver remains on the left, shrinking toward New Moon.
Why phases have their distinctive shapes
The boundary between the lit and dark parts of the moon's face is called the terminator. Seen from Earth it appears as the edge of an ellipse - the projection of a circle (the terminator on the moon's sphere) onto our line of sight.
- Crescents: the terminator ellipse is thin, so only a narrow sliver is lit.
- Quarters: the ellipse degenerates into a straight line - exactly half lit.
- Gibbous phases: the ellipse is fat, so most of the disc is lit.
- Full Moon: the terminator is not visible at all - the entire disc is sunlit.
In the Northern Hemisphere the lit side grows from the right (waxing) and shrinks from the right (waning). South of the equator this is mirrored. This page and the MondPlan app use the Northern Hemisphere convention.
Frequently asked questions
Why does the moon have phases?
The moon does not produce its own light - it reflects sunlight. As the moon orbits Earth, we see different fractions of its sunlit half, producing the phases. Half of the moon is always lit; the phase depends on how much of that lit half faces Earth.
How long is one full lunar cycle?
The synodic month - the time between two identical phases (e.g. full moon to full moon) - averages 29.53059 days (about 29 days, 12 hours, 44 minutes).
Why is a month 30 or 31 days but the lunar cycle 29.5?
The Gregorian calendar is solar-based (365.2425 days per year). A lunar year of 12 synodic months is about 354.4 days - roughly 11 days shorter - which is why lunar calendars (e.g. the Islamic calendar) drift against the seasons unless intercalary months are added.
What is the terminator?
The terminator is the boundary between the illuminated and dark parts of the moon's disc. Its curve (an ellipse as seen from Earth) gives each phase its distinctive shape - a thin ellipse for crescents, a straight line at the quarters, and a fat ellipse for gibbous phases.
Does the moon actually change shape?
No. The moon is always a sphere, and half of it is always sunlit. The apparent change in shape is only a change in the fraction of the sunlit half that faces Earth.
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