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Why Does the Moon Change Shape?
The Moon does not make its own light and it does not physically change shape. Its phases are the changing view we get of the half that sunlight illuminates.
As the Moon orbits Earth, our viewing angle changes. We see different fractions of its sunlit hemisphere, producing a continuous cycle from New Moon to Full Moon and back again.
One illuminated half, many viewing angles
Sunlight always illuminates one half of the Moon, just as it illuminates one half of Earth. Near New Moon, the illuminated half points mostly away from us. Near Full Moon, it points mostly toward us. Crescent, quarter, and gibbous phases are the views between those alignments.
The familiar eight phase names divide a smooth, continuous change into useful ranges. The Moon does not jump from one shape to another at midnight, and the named phase is essentially the same worldwide at a given instant.
Earth’s shadow is not causing the phases
Earth’s shadow reaches the Moon only during a lunar eclipse. Ordinary phases happen because the Moon, Earth, and Sun form a changing angle. Most Full Moons pass above or below Earth’s shadow because the Moon’s orbit is tilted relative to Earth’s orbit around the Sun.
Why the bright side changes direction
The bright side of the Moon always faces the Sun. Its apparent tilt depends on the Moon’s position, your latitude, the season, and how the sky is oriented relative to your horizon. That is why a waxing crescent can look right-lit, left-lit, or like a tilted bowl without changing its astronomical phase.
Questions, answered
Frequently asked questions
Does Earth’s shadow make a crescent Moon?
No. Earth’s shadow causes lunar eclipses, not the ordinary monthly phases.
Is half of the Moon always illuminated?
Except during an eclipse, sunlight illuminates approximately one hemisphere; the phase describes how much of that illuminated half we can see.