Summary
Highlights
Finally, one month after the initial New Moon, the cycle completes, and the Moon is again a New Moon, starting the process over. An interesting phenomenon is 'Earthshine', where the unlit part of a thin crescent Moon sometimes appears faintly illuminated. This is due to sunlight reflecting off the nearly full Earth and then hitting the Moon, making it visible.
The Moon appears to change shape daily, moving across the sky at different times. These changes are due to how we see the Moon illuminated by the Sun as it orbits Earth. The Moon is always half-lit by the Sun, similar to Earth, with a 'terminator' line dividing the bright and dark sides. The phase we see depends on our viewing angle from Earth.
The Moon orbits Earth approximately once a month (29.5 days). The cycle begins with the New Moon, when the Sun, Moon, and Earth are almost aligned, placing the Moon between the Earth and the Sun. From Earth, we only see the Moon's unilluminated side making it appear dark. During this phase, the Moon is near the Sun in the sky, rising and setting with it, and is often up during the day.
A few days after New Moon, as the Moon moves slightly eastward in its orbit, a sliver of its illuminated side becomes visible, forming a thin 'waxing crescent'. This crescent widens as the days pass. Around seven days after New Moon, the Moon reaches its 'first quarter', appearing half-lit as it's 90 degrees away from the Sun in the sky. This phase is called 'first quarter' because it's a quarter of the way through its cycle, even though it looks half full.
After the first quarter, the illuminated portion continues to grow, entering the 'waxing gibbous' phase, meaning it's more than half-full and getting larger. Two weeks after New Moon, the Moon completes half its orbit, becoming a 'Full Moon'. At this point, the Earth is between the Moon and the Sun, and we see the Moon's entire illuminated side. It rises at sunset and sets at sunrise, visible all night.
After the Full Moon, the illuminated portion begins to shrink, entering the 'waning gibbous' phase. Approximately three weeks after New Moon (and a week after Full Moon), the Moon is again half-lit, but the opposite side from the first quarter, marking the 'third quarter' phase. It rises at midnight and sets at noon. Following this, it becomes a 'waning crescent', shrinking further until it's a thin sliver rising just before sunrise.