L2 Earthshine
A faint ashen glow on the Moon’s dark limb — sunlight that bounced off Earth’s clouds and oceans, then off the lunar surface, then back to your eye. No telescope required: this is the easiest target on the entire Lunar 100 list after the Moon itself.
L2 Earthshine
Whole-Disk Phenomenon📉 Vital Statistics
🔭 Field Notes
Earthshine is the second-easiest target on the entire Lunar 100 — after the Moon itself. Sunlight reflects off Earth’s clouds and oceans, crosses to the Moon, and a fraction bounces back to the eye, faintly lighting the dark portion of the crescent disk.
- ▶ Why crescent is brightest: Earth and Moon phases are complementary — a thin lunar crescent means Earth appears nearly full from the Moon, its brightest light-giving phase.
- ▶ Herschel’s “volcanoes”: In 1787 William Herschel mistook earthshine glinting off bright crater rays for active lunar eruptions, later corrected by Schröter.
- ▶ A climate instrument: Long-running earthshine photometry (Big Bear Solar Observatory, since 1998) tracks Earth’s albedo and has detected measurable declines over recent decades.
📍 Nearby L100 Targets
- L1 The Moon: The Lunar 100’s own first entry — the easiest target of all, requiring nothing but a clear sky.
- L3 Mare/Highland Dichotomy: The light/dark contrast across the lunar disk, visible in the same naked-eye glance as Earthshine.
🚀 Mission Log
Target Acquisition
Skip the star-hop — there’s nowhere to sweep
Unlike a crater or valley, Earthshine has no coordinates and no neighbor to anchor on. The “target” is the entire dark portion of the lunar disk. If the Moon is up and showing a crescent, you’re already looking at the right place — there’s no searching involved.
Pick a thin crescent night
Check a Moon-phase calendar for a date 2–5 days before or after New Moon. The thinner the sunlit crescent, the more strongly the dark remainder of the disk will glow — the effect is strongest exactly when the bright sliver is too slim to wash it out.
Time it for twilight, not full darkness
Look shortly after sunset for a waxing crescent low in the west, or shortly before sunrise for a waning crescent in the east. A twilight sky actually helps — against a fully dark sky, the bright crescent can overwhelm your eyes’ sensitivity to the much fainter glow beside it.
Just look — then add optical aid
No telescope is required; this is a naked-eye target, one of only a handful on the entire Lunar 100 list. Once you’ve spotted the glow, binoculars or a small scope can resolve subtle darker patches within it — the lunar maria, faintly visible even in earthlight.
Earthshine brightness varies night to night depending on Earth’s current cloud cover facing the Moon — a particularly bright night isn’t a fluke, it’s Earth’s weather showing through.📝 Observation Log — L2 Earthshine
0/4 CompleteIs Earthshine visible tonight?
Earthshine is easiest to see on a thin crescent, no telescope needed — check if the Moon is approaching young Waxing Crescent (Day 2–5) or old Waning Crescent (Day 25–28).
Check Moon Phase TodayReturn to the List
Ready to find the next target? Go back to the full map to see what else is visible tonight.
← Back to Lunar 100 Map
When to Observe Earthshine
Earthshine needs no terminator and no libration to consider, and often no aperture at all — it can be seen with the naked eye when the Moon shows a thin enough crescent, provided the twilight sky isn’t too bright and conditions are reasonably favorable. Catch it too close to First Quarter and the sunlit crescent has grown bright enough to wash the glow out; catch it on the wrong night entirely (Full Moon, deep into gibbous) and no substantial dark portion of the disk remains visible to see it on.
- Best Viewing: 2–5 days after New Moon (young Waxing Crescent, visible just after sunset) or 2–5 days before New Moon (old Waning Crescent, visible just before sunrise). The thinner the lit sliver, the more the dark side stands out.
- No Limb, No Libration: Unlike a surface feature near the edge of the disk, earthshine’s visibility doesn’t depend on the Moon’s wobble or orientation — it depends largely on Earth’s overall reflectivity, especially cloud cover, and on how dark and transparent your local sky is at the time.
What to Look For
At naked eye, or even better through binoculars, the dark portion of a thin crescent isn’t simply black — it carries a faint gray-blue glow, distinct from the surrounding night sky. This is the entire target: nothing on the Moon looks quite like its own night side gently lit from outside.
With binoculars or a small scope, look within the glow itself for subtly darker patches — the lunar maria, visible even in light that has already bounced twice before reaching your eye. It’s a strange thing to realize: you’re seeing lunar geology by Earthlight, not sunlight.
Compare the glow across two separate crescent nights, ideally a few weeks apart. Some of what you notice is a real change in Earth’s reflectivity — but observing conditions, including Moon altitude, sky transparency, and twilight brightness, affect the apparent brightness too.
Pull back and take in the whole scene: a brilliant crescent cradling a dim, ghostly full disk within it. This pairing has its own folk name across many cultures, and is one of the most photographed naked-eye lunar sights for exactly that reason — it reads as something almost staged, despite being pure orbital geometry.
The Science: From Renaissance Sketch to Climate Instrument
Earthshine’s basic cause has been correctly understood for five centuries — there’s no live debate over the mechanism the way there is for some lunar surface features. What has changed is how seriously scientists now take it as a measurement tool.
First Explained by Leonardo da Vinci, c. 1506–1510
Leonardo recorded the correct explanation in his Codex Leicester: that both Earth and the Moon reflect sunlight simultaneously, and that some of the light Earth reflects travels on to faintly illuminate the Moon’s otherwise dark portion. Leonardo thought Earth’s oceans were the primary reflector — a reasonable guess given the tools of his era, though modern measurements show that clouds actually contribute most of Earth’s reflectivity.
A Modern Proxy for Earth’s Albedo
Because earthshine brightness directly tracks how much sunlight Earth reflects, long-running ground-based photometry programs — notably at Big Bear Solar Observatory since 1998 — use it to monitor Earth’s albedo year over year, entirely from the ground, by pointing telescopes at the Moon instead of at Earth.
A Testbed for Exoplanet Detection
More recently, researchers have used earthshine spectroscopy as a stand-in for how a distant observer might one day study Earth-like exoplanets — looking for the spectral signature of vegetation, oceans, and clouds in light reflected off a planet, using the Moon as a uniquely convenient mirror sitting right next door.
What’s striking about Earthshine isn’t an unresolved mystery — it’s the opposite: a phenomenon fully explained five hundred years ago that has only grown more scientifically useful with time, from a Renaissance notebook sketch to a working tool for understanding both this planet and, potentially, others.