Earthshine

The old Moon in the new Moon's arms

Catch a thin crescent Moon low in the west after sunset, or low in the east before dawn, and look at the part that "should" be dark. On a clear, transparent night you will see the entire lunar disk hanging there as a dim, ash-grey ghost cradled inside the brilliant crescent. The old English name for the sight — "the old Moon in the new Moon's arms" — captures it exactly. The crescent is the part of the Moon the Sun is lighting directly. The grey ghost is the lunar night side, and it is glowing because it is not truly dark: it is being lit by a second source overhead.

That second source is Earth. Stand on the night side of the Moon during one of these evenings and you would not be in darkness at all — a brilliant, nearly full Earth would be hanging in your black sky, roughly four times wider than the Moon looks to us and many times brighter, flooding the ground with bluish light. Earthshine is simply what happens when some of that reflected Earthlight bounces off the lunar surface a second time and travels back across 384,000 kilometres to your eye. It is a double reflection: Sun → Earth → Moon → you.

The two-mirror geometry

The whole phenomenon is set by a single fact: the phase of Earth seen from the Moon is the complement of the phase of the Moon seen from Earth. The Sun lights one hemisphere of the Earth–Moon system. When the Moon sits almost between us and the Sun — a thin crescent for us — its near side faces an Earth whose entire dayside is turned toward it. So a thin crescent Moon in our sky corresponds to a nearly "full Earth" in the Moon's sky, dumping maximum light onto the lunar night side. That is why earthshine is conspicuous within a day or two of new Moon.

Run the geometry forward and the effect fades. As the Moon waxes toward first quarter and then full, the Earth it sees wanes toward a "new", dark Earth. By full Moon, the lunar near side faces an essentially unlit Earth, there is almost no Earthlight to reflect, and earthshine is gone — also, of course, the Moon's own dayside is now blindingly bright and would swamp the faint glow even if it were there. Earthshine is therefore a crescent-phase effect, brightest when the bright crescent itself is thinnest.

Why Earth is such a good lamp

To see why the night side glows visibly, compare Earth and Moon as reflectors. The brightness of a "full Earth" in the lunar sky relative to a "full Moon" in ours scales as the product of two ratios — relative albedo and relative solid angle:

Brightness(full Earth from Moon)     A_Earth     ( θ_Earth )²
─────────────────────────────────  ≈  ───────  ×  ( ─────── )
Brightness(full Moon from Earth)      A_Moon      ( θ_Moon )

The albedos are not close. Earth's Bond albedo — the fraction of all incident sunlight it reflects in every direction — is about A_Earth ≈ 0.29–0.30, dominated by clouds, snow and ice, with deserts and ocean glint contributing. The Moon is a dark, charcoal-grey world: its geometric albedo is only A_Moon ≈ 0.12 and its Bond albedo about 0.11. So Earth reflects roughly 2.5 times more of the light that hits it.

The angular sizes are even more lopsided. From the lunar surface, Earth spans about θ_Earth ≈ 1.9°, while from Earth the Moon spans only θ_Moon ≈ 0.52° — a ratio near 3.7. Since brightness from an extended reflector scales with the solid angle it covers, that is a factor of 3.7² ≈ 14 in area. Multiplying:

(0.30 / 0.12) × (1.9 / 0.52)²  ≈  2.5 × 13.7  ≈  34

A full Earth in the Moon's sky is therefore on the order of several dozen times brighter than a full Moon is to us — bright enough to read by. It is that powerful "earthlight" striking the lunar night side, then weakly re-reflected by the dark regolith, that we finally perceive as the dim grey earthshine glow.

How faint is the glow, really?

Earthshine survives a brutal two-bounce penalty. Of the sunlight reaching the dayside Earth, only ~30% is reflected. Only a small fraction of that reaches the Moon, where ~12% is reflected again. The light that returns to us has been attenuated by both albedos and by the inverse-square spreading over the Earth–Moon round trip. The upshot is that the earthshine-lit disk is on the order of ten thousand times fainter — roughly ten magnitudes — than the directly sunlit crescent at the same moment.

Quantity	Value	Note
Earth Bond albedo	≈ 0.29–0.30	Clouds + ice dominate; varies ±a few % with weather
Moon geometric albedo	≈ 0.12	Dark basaltic regolith; one of the darkest natural surfaces
Earth angular size from Moon	≈ 1.9°	≈ 3.7× the Moon's angular size from Earth
Earth–Moon distance	≈ 384,400 km	Light makes the round trip in ≈ 2.56 s
Earthshine vs. crescent brightness	≈ 1/10⁴	Roughly 10 magnitudes fainter; needs a dark sky
Best viewing window	± 1–4 days of new Moon	Thin crescent, "full Earth" lighting the night side
Brightest at full Moon?	No — essentially zero	Moon then faces a "new", dark Earth

Because the glow is so faint, it is killed by light pollution, haze, and twilight. The classic observing trick is to wait until the crescent is just a few degrees above the horizon in a genuinely dark sky, then block the bright crescent behind a rooftop or a finger to let your eye adapt to the ghost beside it.

Earthshine as a planetary albedometer

The brightness of earthshine is, by construction, proportional to how much sunlight Earth was reflecting at that instant — and it averages over an entire visible hemisphere, smoothing out the patchiness no single satellite sees. That makes the Moon a free, distant detector pointed back at Earth. Measure the ratio of the earthshine-lit dark side to the directly-lit bright side (which calibrates the Sun's input and the Moon's own reflectivity), and you recover Earth's instantaneous Bond albedo.

This is not a thought experiment. From 1998 onward, a team led by Philip Goode and Steven Koonin ran a dedicated earthshine-monitoring program at the Big Bear Solar Observatory in California, building a multi-year record of Earth's reflectance from the ashen glow. The interest was climatic: Earth's albedo is one of the two knobs (alongside the greenhouse effect) that set the planet's energy balance, and it is set largely by cloud cover, which is hard to pin down globally any other way. Earthshine photometry offered an independent, whole-hemisphere check on satellite cloud and radiation budgets.

A rehearsal for finding life elsewhere

Earthshine carries more than a single brightness number — it carries Earth's spectrum, integrated over a whole face of the planet. And that is precisely the kind of signal a future telescope will get from an Earth-like exoplanet: a single unresolved dot whose disk-averaged colours must betray whether anything interesting is happening on the surface.

So astronomers treat earthshine as a controlled dress rehearsal. Spectra of the ashen glow show several features we would love to find on another world:

• The vegetation "red edge". Chlorophyll reflects sharply more in the near-infrared than in red light, producing a step in reflectance near 700 nm. It shows up faintly in earthshine when continents with heavy vegetation face the Moon — a candidate surface biosignature.
• Molecular oxygen. The strong O₂ "A-band" near 760 nm, a gas kept abundant only by life, is detectable in the reflected Earthlight.
• Water vapour and ozone bands, marking a wet, habitable atmosphere.
• Rayleigh-scattering blue, the same effect that makes Earth's sky and limb blue, dominating the short-wavelength end of the spectrum.

Landmark studies by Arnold and colleagues (2002) and Woolf and colleagues (2002) detected the red edge and O₂ in earthshine, demonstrating that disk-integrated photometry can in principle reveal a biosphere. Every such detection of Earth-from-afar is a calibration point for the day a direct-imaging mission stares at a true Earth analogue light-years away.

Planetshine beyond the Earth–Moon pair

Earthshine is one member of a family. Any bright body can illuminate the night side of a neighbour it dominates the sky of — the generic term is planetshine.

• Earthshine on our own Moon is the brightest and easiest example, for the reasons above.
• Jupiter and Saturn light the night sides of their inner moons. From Io or Europa, a "full Jupiter" is enormous and bright; Jupitershine measurably lifts the night-side surface temperatures and illuminates the dark hemispheres in spacecraft images.
• Mars's moons. Marsshine illuminates the night sides of Phobos and Deimos, both very dark, low-albedo bodies.
• Venus's "ashen light" is a centuries-old, still-disputed claim of a faint glow on the planet's night side. Unlike true earthshine it has no obvious second mirror to bounce sunlight off; proposed explanations range from airglow in Venus's upper atmosphere to lightning to simple contrast illusions, and it remains unconfirmed.

From da Vinci to spacecraft

Earthshine has a clean intellectual pedigree. Around 1510, Leonardo da Vinci wrote in the notebook now called the Codex Leicester that the dim light on the Moon's dark side is sunlight reflected from Earth — specifically, he thought, from its oceans. The reasoning was essentially correct: he had grasped that Earth shines by reflected sunlight just as the Moon does, and that one could illuminate the other. His one error was assigning the reflection mainly to water; in fact clouds and ice, not oceans, do most of the reflecting, and the dark open ocean is a relatively poor mirror.

Centuries later the same glow became a quantitative tool. The Big Bear albedo program turned it into a climate instrument; exoplanet spectroscopy turned it into a biosignature test bed. And spacecraft have returned the favour by photographing earthshine from the outside — Apollo astronauts described the earthshine-lit lunar surface, and orbiters have imaged the faint glow on the night sides of crescent moons across the Solar System.

Common misconceptions and edge cases

• "The dark side is glowing from its own heat or from starlight." No — it is reflected Earthlight. Lunar night-side surface temperatures (~100 K) radiate in the far infrared, invisible to the eye; the visible grey glow is sunlight that took the Sun → Earth → Moon → eye path. Starlight is far too weak to produce it.
• "It is the same as the dark limb you see at full Moon." At full Moon there is no comparable earthshine — the Moon faces a dark, "new" Earth, and the Moon's own brilliance would drown any faint glow anyway. Earthshine is a thin-crescent phenomenon.
• "Earthshine brightness is constant." It is not. Because it tracks Earth's albedo, which is driven by cloud cover, it varies night to night by a few percent and seasonally as the visible hemisphere swings between cloudy oceans and bright continents or ice — the very variability that makes it a useful albedo monitor.
• "It proves Earth is brighter than the Sun." No. Earth only shines because the Sun lights it. Earthshine is a feeble third-hand version of sunlight, attenuated by two reflections; it is bright relative to the Moon's own dark surface, not in absolute terms.
• "Da Vinci discovered it." The glow was seen and named for centuries before him; da Vinci's contribution was the correct physical explanation, not the discovery of the appearance.