Lunar Phases Explained: Cycle, Geometry, and Earthshine

The Moon is a sphere of rock about 3,500 km in diameter. Its surface reflects sunlight much like a chalkboard or asphalt, giving it a low albedo. Sunlight illuminates the entire lunar sphere, but only the half facing the Sun is lit at any moment. The brightness we see comes from that illuminated half.

The Lunar Cycle

Phases arise from the geometry among the Sun, Earth, and Moon. As the Moon orbits Earth, the angle of Sunlight changes for an Earth‑bound observer, creating a sequence of appearances over a 29.5‑day cycle. The word “month” derives from this lunar period. The line separating the bright day side from the dark night side is called the terminator.

Phase Progression

• New Moon – The Moon sits between Earth and Sun, rising at sunrise and setting at sunset.
• Waxing Crescent – The Moon moves eastward; a thin crescent becomes visible after sunset.
• First Quarter – At a 90° angle from the Sun, the Moon appears half‑lit, marking one‑quarter of its orbit. As the speaker notes, “The Moon is now one‑quarter of the way around its orbit. It’s 90° away from the Sun in the sky, which means we’re looking straight down on the terminator.”
• Waxing Gibbous – More than half of the disc is illuminated, and the Moon rises in the late afternoon.
• Full Moon – Positioned 180° from the Sun, the Moon rises at sunset and sets at sunrise.
• Waning Gibbous – The illuminated portion begins to shrink after full illumination.
• Third Quarter – At a 270° angle, the Moon rises around midnight and sets at noon.
• Waning Crescent – The thin crescent appears before sunrise, completing the cycle back to New Moon.

Rising and setting times shift daily because the Moon’s orbital position relative to the Sun changes continuously.

Perspectives and Phenomena

From the lunar surface, Earth shows phases opposite to those observed from Earth. Earthshine occurs when sunlight reflected from the nearly full Earth illuminates the dark portion of the Moon, making it faintly visible. The Earth reflects about 50 times more light than a full Moon, as highlighted: “The Earth is bigger and more reflective than the Moon, so it’s actually 50 times brighter than a full Moon!”

The phase of the Moon refers to what shape the Moon appears to us; how much of it we see illuminated from the Earth. A common misconception is that the Moon appears only at night, yet it is up during the day just as often: “It’s a very common misconception that the Moon is only up at night, but it’s up during the day literally just as often.”

Even poetic language finds a place: the Moon has been called “the old Moon in the new Moon’s arms.” The Universe may seem strange at first, but as we observe these regular patterns, it becomes familiar: “The Universe may seem strange and complicated and forbidding at first, but over time, as you get outside and experience it, it becomes your neighborhood.”