By Jon Stefansson
Updated Mar 24, 2022
Each night the Moon displays a familiar sequence of shapes, known as its phases. The cycle starts with a nearly invisible new moon, progresses to a bright full moon, and then reverses over roughly 29.5 days—the length of a lunar month. The changing appearance is a direct consequence of the Moon’s orbit around Earth and the interplay of sunlight.
Imagine Earth fixed in space while the Moon orbits and rotates synchronously, completing both motions every 29.5 days. Because the Moon’s rotation period matches its orbital period, we always see the same side, with distinct craters. As the Moon moves, varying portions of its illuminated hemisphere are visible from Earth, producing the familiar phases.
When Earth, Moon, and Sun line up with the Moon between Earth and Sun, the side of the Moon that faces us is in darkness. This alignment creates the new moon, marking the beginning of the lunar cycle.
As the Moon shifts to the left of Earth in its orbit, sunlight begins to illuminate a sliver of its far side. The visible illuminated arc grows—first as a waxing crescent, then expanding to a half‑illuminated first‑quarter Moon.
Continuing its journey, the Moon moves behind Earth relative to the Sun. Now the fully illuminated hemisphere faces us, resulting in a full Moon. The phase between the first quarter and full Moon is the waxing gibbous, where the illuminated area exceeds a half circle.
After the full Moon, the Moon advances to the right side of Earth in its orbit. The illuminated portion gradually diminishes, giving rise to the waning gibbous, waning crescent, and eventually the last quarter, which is the second half‑moon before the cycle repeats.
Once the Moon completes its 29.5‑day orbit, it returns to the new‑moon alignment, and the cycle restarts. Understanding these phases not only satisfies curiosity but also provides insights into lunar dynamics and their impact on tides, eclipses, and cultural traditions.
