The Moon’s ever-changing appearance in the night sky has inspired human curiosity for millennia. These predictable changes in the Moon’s visible illumination are known as the lunar phases. A lunar phase is the appearance of the Moon as seen from Earth, determined by the fraction of its sunlit side that is visible to us.
Why We See Changing Shapes
The core reason we observe phases is the dynamic geometry between the Sun, the Earth, and the Moon. While the Moon orbits our planet, the Sun consistently illuminates half of the Moon’s spherical surface, creating a day side and a night side. Since the Moon does not produce its own light, we only see the portion that is reflecting the Sun’s rays toward Earth.
As the Moon revolves around the Earth, our viewing angle of that illuminated half changes continuously. When the Moon is between the Earth and the Sun, the lit side faces completely away from us, making the Moon appear dark, which is the New Moon phase. Conversely, when the Earth is approximately between the Sun and the Moon, we see the entire sunlit face, resulting in a Full Moon. The gradual shift in the Moon’s orbital position changes how much of its illuminated hemisphere we can see, creating the phases we observe.
The 29.5-Day Journey
The sequence of lunar phases, from one New Moon to the next, takes about 29.5 days to complete, a period known as the synodic month. This cycle is slightly longer than the Moon’s 27.3-day orbital period because the Earth is also moving around the Sun, meaning the Moon must travel a little further to realign with the Earth and Sun. The cycle is divided into eight major named phases, which detail the increasing and decreasing visibility of the illuminated surface.
The cycle begins with the New Moon, where the Moon is invisible, followed by the waxing phases, meaning the illuminated portion is growing. The first visible sliver is the Waxing Crescent, which leads to the First Quarter, where exactly half of the Moon appears lit. Next is the Waxing Gibbous, where the Moon is more than half illuminated, culminating in the Full Moon, which is fully lit from our perspective.
After the Full Moon, the Moon enters its waning phases, where the illuminated portion visible from Earth begins to shrink. The first stage of this decrease is the Waning Gibbous, followed by the Last Quarter, where the other half of the Moon is lit. The cycle concludes with the Waning Crescent, a final thin sliver, before the Moon returns to the New Moon phase.
Clearing Up Common Confusion
A common misunderstanding is the belief that the Earth’s shadow falling on the Moon causes the regular phases. This is incorrect, as the phases are purely a result of the changing angles of sunlight on the Moon’s surface as it orbits. The Moon is always half-lit by the Sun, no matter its phase.
The Earth’s shadow only comes into play during a lunar eclipse, a separate and less frequent event. A lunar eclipse occurs when the Earth, Sun, and Moon align in a straight line, with the Earth directly blocking the Sun’s light from reaching the Moon. This specific alignment can only happen during the Full Moon phase, and it is the Earth’s shadow that momentarily darkens the Moon’s surface.