The faint illumination of the Moon’s dark side, often visible when the Moon is a slim crescent, allows an observer on Earth to see the entire circular outline even though only a small sliver is directly lit by the Sun. This phenomenon seems to defy logic. This secondary illumination is known as Earthshine, and its explanation lies in a unique interaction between the Sun, Earth, and Moon.
Understanding the New Moon Alignment
The phases of the Moon are determined by the changing angles between the Sun, Earth, and Moon. The New Moon phase occurs when the Moon is positioned roughly between the Earth and the Sun, an arrangement called conjunction. In this alignment, the side of the Moon facing Earth is not illuminated by direct sunlight, which is why the Moon appears invisible or “new” in the sky.
Half of the Moon’s surface always receives sunlight, but at the New Moon, the lit half faces away from Earth. The Moon’s orbital plane is slightly tilted relative to the Earth’s orbit around the Sun, preventing a perfect alignment every month. If the alignment were perfect, the New Moon would completely block the Sun, resulting in a solar eclipse. Because the side facing us is unlit, the Moon should be nearly undetectable against the dark sky.
The Mechanism of Earthshine
The visibility of the Moon’s dark side relies on light reflected from Earth. Earthshine is sunlight that follows a two-part journey: first, it travels from the Sun to the Earth, and then it reflects from the Earth’s surface and atmosphere onto the Moon. Once this reflected light hits the Moon, the lunar surface scatters a small amount of it back toward Earth, making the dark portion faintly visible.
This process means the Earth acts as a giant mirror reflecting solar illumination onto the Moon. To an astronaut standing on the Moon during its night, the Earth would appear as a spectacularly bright, fully illuminated disk, much larger than the Full Moon appears to us. This enormous, bright Earth provides the necessary light to illuminate the dark lunar terrain.
The brightness of Earthshine is directly related to the Earth’s reflectivity, a property known as albedo. Earth’s albedo is significantly higher than the Moon’s, meaning our planet reflects a greater percentage of the sunlight that hits it. Since the Moon’s surface is quite dark, reflecting light similarly to old asphalt, the bright light reflected from Earth is powerful enough to overcome the Moon’s low reflectivity.
The light travels from the Sun, to the Earth, to the Moon, and finally back to our eyes, making it doubly reflected sunlight. Leonardo da Vinci first explained this phenomenon in the 16th century, which is why Earthshine is sometimes poetically referred to as “the Da Vinci glow.” Although it is a constant process, its faintness means it is only noticeable when the glare of the direct lunar crescent is minimized.
Factors Influencing Visibility
The clarity of Earthshine depends on several environmental and orbital factors. The phenomenon is most easily observed a few days before or after the New Moon, when the Moon appears as a very thin crescent. This timing is optimal because the Moon is low on the horizon, visible either just after sunset (waxing crescent) or just before sunrise (waning crescent).
During these crescent phases, the portion of the Earth reflecting light onto the Moon is nearly fully illuminated by the Sun. This high illumination ensures the maximum amount of light is sent toward the Moon. This appearance is sometimes described as “the old Moon in the new Moon’s arms.”
The Earth’s albedo is not constant and plays a large role in Earthshine’s brightness. The presence of bright clouds and large snow or ice covers significantly increases the amount of sunlight reflected toward the Moon. For observers in the Northern Hemisphere, Earthshine is often brightest during the spring, typically between April and June. This seasonal peak occurs because the Northern Hemisphere is tilted toward the Sun while extensive winter snow and ice cover are still present, maximizing Earth’s overall reflectivity.
For the observer on Earth, clear skies and minimal light pollution are necessary to resolve the faint image. Even when the Earth’s reflectivity is high, the light returning from the Moon is still very dim, making it easily obscured by atmospheric haze or bright city lights.