The Moon, Earth’s only natural satellite, is the most prominent object in our night sky, yet the source of its light is commonly misunderstood. Although it appears to glow with a soft, silvery radiance, this illumination is not generated by the Moon itself. The light we see is entirely borrowed, originating from the Sun, the most powerful source in our solar system. This phenomenon results from the Moon’s positioning and how its surface reflects incoming radiation.
The Moon is a Reflector, Not a Light Source
The perceived glow of the Moon is purely a reflection of sunlight striking its surface and bouncing back toward Earth. The Sun is the sole illuminator, constantly bathing half of the Moon in light. This reflection is governed by albedo, a measurable property describing how much light a surface reflects.
Despite its bright appearance, the Moon’s surface is surprisingly dark, composed of materials like volcanic rock and dust. The Moon’s Bond albedo, which measures its overall reflectivity, is quite low, averaging only about 0.12 to 0.136. This means the lunar surface reflects only about 12 to 13.6 percent of the sunlight that hits it, absorbing the remaining majority as heat.
The reason the Moon appears so luminous from Earth is not because its surface is highly reflective, but because of the sharp contrast with the darkness of space. Even reflecting only a small fraction of the intense sunlight, the Moon directs enough light toward Earth to dominate the night sky. This reflected light takes approximately 1.26 seconds to travel the distance between the Moon and our planet.
How Orbital Position Creates Lunar Phases
The amount of the Moon’s illuminated surface that we see changes in a regular, predictable cycle known as the lunar phases. These phases are not shadows cast by Earth, but visual changes caused by the Moon’s position as it orbits our planet. The Moon is always half-lit by the Sun, but we see varying fractions depending on our viewing angle.
The cycle begins with the New Moon, when the Moon is situated roughly between the Earth and the Sun. At this point, the illuminated side faces entirely away from Earth, making the Moon invisible to us. As the Moon moves along its orbit, we begin to see a small sliver of the sunlit portion, which is the start of the crescent phase.
As the Moon continues its roughly 29.5-day orbit, the visible illuminated area grows, a process referred to as waxing. When the Moon completes a quarter of its orbit, we see exactly half of its surface lit, known as the First Quarter phase. The next stage is the gibbous phase, where more than half of the Moon appears illuminated.
The cycle reaches its peak at the Full Moon, which occurs when Earth is positioned between the Sun and the Moon. From our perspective, we see the entire sunlit face, giving it a fully circular appearance. After the Full Moon, the illuminated portion begins to shrink, or wane, passing through the waning gibbous, Third Quarter, and waning crescent phases before returning to the New Moon.
When Shadows Hide the Light
While the phases illustrate the normal cycle of lunar visibility, the flow of sunlight to the Moon can be temporarily interrupted during an eclipse. These events require a precise, straight-line alignment of the Sun, Earth, and Moon. There are two main types of eclipses, each involving a celestial body casting a shadow.
A lunar eclipse occurs during the Full Moon phase when Earth passes directly between the Sun and Moon, casting Earth’s shadow onto the lunar surface. Conversely, a solar eclipse happens during the New Moon phase when the Moon passes between the Sun and Earth, blocking the Sun’s light from reaching a small portion of our planet. The Moon’s shadow darkens the landscape.
These perfect alignments are rare because the Moon’s orbital plane is tilted by approximately five degrees compared to Earth’s orbit around the Sun. Most of the time, the Moon passes either slightly above or below the plane of Earth’s shadow. Eclipses only occur when the Moon crosses this orbital plane while simultaneously in the New or Full phase.