Earthshine is the faint glow you can see on the dark portion of the Moon, caused by sunlight bouncing off Earth’s surface and clouds, traveling to the Moon, and reflecting back to your eyes. It’s sometimes called “the old Moon in the new Moon’s arms” because you can see the entire lunar disk even when only a thin crescent is lit directly by the Sun. The effect is essentially Earth acting as a giant mirror, lighting up the Moon’s night side with a soft, ghostly light.
How Earthshine Works
The process involves a double reflection. Sunlight first strikes Earth, where roughly 31% of it bounces back into space. Some of that reflected light hits the Moon’s surface. The Moon then reflects a small fraction of that light back toward Earth, where you see it as a dim glow filling in the unlit part of the lunar disk.
Think of it as the Moon getting a dose of “full Earth” light. Just as a full Moon brightly illuminates a nighttime landscape here on Earth, a nearly full Earth (as seen from the Moon) illuminates the lunar surface. When the Moon is a thin crescent from our perspective, the Earth appears nearly full from the Moon’s perspective, which is why earthshine is brightest during that phase.
When and How to See It
Earthshine is easiest to spot in the few days before or after a new Moon, when only a slim crescent is sunlit. Look for the Moon low in the sky shortly after sunset (waxing crescent) or before sunrise (waning crescent). The bright crescent will be obvious, but if conditions are right, you’ll also see the rest of the Moon’s face glowing a dim blue-gray. No telescope is needed.
Dark skies help, but earthshine is visible even from suburbs. Binoculars make the glow more dramatic and can reveal some of the Moon’s larger features on the dark side. Photographers often capture it by slightly overexposing the crescent to bring out the earthshine detail.
What Controls Its Brightness
The intensity of earthshine depends on how reflective Earth is at any given moment, a property scientists call albedo. Earth’s albedo is primarily governed by clouds, since they are almost always more reflective than the surface below them. The cloudier the Earth on the hemisphere facing the Moon, the brighter the earthshine.
Other factors play a role too. Large areas of snow and ice increase reflectivity, so earthshine can be slightly brighter during Northern Hemisphere winter when snow covers much of North America and Eurasia. Oceans, by contrast, are dark and absorb most sunlight, reflecting very little. The mix of cloud cover, ice, vegetation, and ocean visible from the Moon at any moment determines exactly how bright the glow appears on a given night.
Why Scientists Care About Earthshine
Earthshine turns out to be a surprisingly useful tool for climate science. Because the glow’s brightness is a direct measure of how much sunlight Earth is reflecting back to space, tracking it over time reveals changes in the planet’s overall reflectivity. That reflectivity matters enormously for climate: the more sunlight Earth bounces away, the cooler it stays, and the more it absorbs, the warmer it gets.
Researchers at Big Bear Solar Observatory in California measured earthshine continuously from 1998 to 2017 using precise photometric instruments. Their data revealed changes in Earth’s albedo on seasonal, yearly, and even decadal timescales. One notable finding was a measurable drop in albedo over part of the study period, attributed to warming in the eastern Pacific Ocean. That warming reduced low-lying cloud cover, which meant less sunlight was reflected and more was absorbed by Earth’s surface.
Earlier work by scientists at the New Jersey Institute of Technology and Caltech, running since the 1980s, confirmed the tight relationship between albedo and cloud cover by comparing earthshine measurements with NASA satellite cloud data. They found that Earth’s surface grew sunnier and less cloudy from 1984 to 2000, then reversed course somewhat between 2001 and 2003 as cloudiness increased and albedo returned to mid-1990s levels.
Earthshine vs. Satellite Measurements
Satellites like NASA’s CERES instruments also measure Earth’s reflectivity, so you might wonder why earthshine observations matter. The two approaches are complementary because they’re sensitive to different aspects of how Earth scatters light. Satellites measure from directly above, looking straight down at specific regions. Earthshine captures a large-scale, integrated view of an entire hemisphere at once, seen from an angle. Combining both gives scientists a more complete picture of how Earth’s brightness is changing and why, which feeds directly into the climate models used to project future warming.
Leonardo da Vinci’s Connection
Leonardo da Vinci was the first person known to correctly explain earthshine, around 1510. He recognized that the glow on the Moon’s dark side was sunlight reflected from Earth’s oceans, well before anyone fully understood the mechanics of planetary orbits. For centuries afterward, the phenomenon was sometimes called “the da Vinci glow.” His explanation held up remarkably well. The only update modern science has added is that clouds, not oceans, are the dominant source of the reflected light.