Earthshine Explained
The scientific foundation behind Earth's glow
What is Earthshine?
Earthshine is the illumination of the Moon's surface by sunlight reflected from Earth. This is a well-documented optical phenomenon that has been observed and studied for centuries.
The Mechanism
- 1.Sunlight strikes Earth's atmosphere, clouds, oceans, and land surfaces
- 2.Approximately 30% of this light is reflected back into space (Earth's albedo)
- 3.This reflected light travels to the Moon and illuminates its surface
- 4.The Moon's surface reflects this Earthlight back toward observers
The effect is most visible from Earth when observing a crescent moon. The faint glow on the dark portion of the Moon is sunlight that has bounced off Earth, traveled to the Moon, and returned to our eyes.
How Reflected Light Works
Earth's Albedo
Albedo is the measure of how much light a surface reflects. Earth's average albedo is approximately 0.30, meaning 30% of incoming sunlight is reflected back into space.
• Clouds are highly reflective (~50-90% albedo)
• Oceans reflect less (~6-10% albedo)
• Ice and snow are very reflective (~50-90% albedo)
• Land surfaces vary widely (10-40% albedo)
Because Earth has extensive cloud cover, large ice caps, and bright land regions, it reflects a substantial amount of sunlight. From the Moon's perspective, Earth appears as a large, bright object—much more luminous than the Moon appears from Earth.
Brightness Comparison
Earth as seen from the Moon is approximately:
• 50 times larger in angular size than the Moon appears from Earth
• Significantly brighter due to higher albedo
• Capable of providing enough illumination to see surface details
Earth vs Deep Space Perspective
Viewing from Earth
When observing Earthshine from Earth, you see the effect as a faint glow on the dark portion of a crescent moon. This requires:
- • Clear skies
- • Crescent moon phase (ideally 2-5 days after new moon)
- • Darkness (twilight or night)
- • No significant light pollution
Viewing from Deep Space
From behind the Moon (as during Artemis II), the perspective reverses:
- • Earth appears as a large, luminous disc
- • The Moon's surface faces Earth directly
- • Earth becomes the primary light source
- • The effect is dramatic and immediate
This difference in perspective is why the Artemis II observation is significant. From Earth, Earthshine is subtle and requires specific conditions. From deep space, Earth's illumination of the Moon is powerful and obvious—justifying Victor Glover's description of "Earth's glow."
Da Vinci Glow
Earthshine is sometimes called "Da Vinci glow" after Leonardo da Vinci, who in 1510 correctly explained the phenomenon. He theorized that the faint glow on the dark part of the Moon was caused by sunlight reflecting off Earth's oceans.
Historical Understanding
Da Vinci's explanation predated modern physics by centuries. His observation demonstrated:
- • Earth reflects sunlight (not self-luminous)
- • This reflected light is sufficient to illuminate the Moon
- • The Moon itself reflects this secondary light back to Earth
Modern astronomy confirmed Da Vinci's theory and expanded it to include atmospheric reflection, cloud cover, and precise albedo measurements. The term "Da Vinci glow" honors his early scientific insight.
Scientific Applications
Climate Monitoring
Changes in Earthshine intensity can indicate changes in cloud cover, ice extent, and overall planetary albedo—useful data for climate studies.
Exoplanet Detection
Understanding how Earth reflects light helps astronomers model exoplanet albedos and search for potentially habitable worlds.
Lunar Surface Studies
Earthshine provides a secondary light source for observing the Moon, allowing scientists to study surface features even when not directly illuminated by the Sun.
Connection to Earthglow
Earthshine is the scientific term for the phenomenon. "Earth's glow" (Earthglow) is the human experiential description from deep space perspective.
Both refer to the same optical process: sunlight reflecting off Earth and illuminating the Moon. The terminology difference reflects observation context—scientific measurement versus lived human experience.
Victor Glover's April 6, 2026 description bridges these perspectives, offering a contemporary human account of a centuries-old scientific phenomenon observed from a rare vantage point.