Astronomy
What Controls Earthshine Brightness on the Moon?
Quick fact
Earthshine brightness is primarily controlled by Earth's reflectivity (albedo), the amount of cloud cover, and the geometric alignment of the Sun, Earth, and Moon.
Why this is interesting
When you see the dark side of the Moon faintly glowing, have you ever wondered what makes that ghostly light brighter or dimmer?
Read the full explanation
Understanding What Controls Earthshine Brightness on the Moon?
Earthshine is sunlight that travels from the Sun to Earth, reflects off our planet, and then illuminates the Moon's night side. The brightness of this glow depends on three main factors. First, Earth's albedo—the fraction of sunlight it reflects—determines how much light is sent toward the Moon. Bright surfaces like clouds, ice, and snow reflect more light than dark oceans or forests. Second, cloud cover plays a major role because clouds are highly reflective and can change rapidly, causing Earthshine to vary over days or weeks. Third, the phase angle—the angle between the Sun, Earth, and Moon—controls how much of the sunlit Earth is visible from the Moon and how directly the light strikes the lunar surface. When the Moon is a thin crescent (near new Moon), Earth appears nearly full from the Moon, so Earthshine is brightest. As the Moon waxes, the phase angle increases, Earth appears less illuminated, and Earthshine fades.
A deeper explanation
The brightness of Earthshine is a function of three interconnected physical quantities: Earth's Bond albedo, the effective reflecting area as seen from the Moon, and the illumination geometry. Earth's Bond albedo averages about 0.3, meaning 30% of incoming sunlight is reflected back to space. However, this value is not uniform; it depends on the distribution of clouds, ice, oceans, and land masses. Clouds are the dominant contributor to short-term variability because they can cover large areas and have albedos up to 0.8, while oceans have albedos as low as 0.05. Satellite measurements show that Earth's albedo fluctuates seasonally and interannually, primarily due to changes in cloud cover and snow/ice extent. The phase angle is the angle at Earth between the Sun and the Moon. When the Moon is new (phase angle near 0°), an observer on the Moon would see a fully illuminated Earth, maximizing the incident flux. As the phase angle increases, the illuminated fraction of Earth decreases, and the light strikes the Moon at a more oblique angle, reducing the irradiance per unit area. Additionally, Earth's atmosphere scatters light, creating a diffuse glow that contributes to Earthshine even when the direct reflection is minimal. The combination of these factors means Earthshine brightness is a sensitive indicator of Earth's overall reflectivity and has been used to study cloud feedbacks and climate variability. The phenomenon is most easily observed with the naked eye during the crescent phases, when the contrast between the bright sunlit crescent and the dark earthlit portion is highest.