The Short Answer

When the Moon is a thin crescent, the rest of its disc often shows as a faint grey ball. That glow is earthshine: sunlight that has bounced off the Earth, travelled to the Moon and lit its night side. Seen from the Moon, the Earth goes through phases that are the reverse of the Moon's, so when we see a thin crescent, the Moon sees a nearly full Earth. The Earth is 3.7 times wider than the Moon and reflects about 30 per cent of the sunlight that reaches it, so the full Earth in the lunar sky is about 49 times brighter than the full Moon in ours. The earthlight is strongest around new moon and fades as the crescent grows: to 80 per cent at three days and 30 per cent at first quarter.

Sunlight, Twice Reflected

Everything on the Moon's day side is lit by the Sun. Its night side has no sunlight at all, but it is not in complete darkness: the Earth hangs in its sky, and the Earth's day side reflects sunlight towards it. Some of that light is reflected again by the lunar ground, back towards the Earth, and that is what we see. So earthshine is sunlight that has been reflected twice, once by the Earth and once by the Moon, before it reaches the eye.

The French name is lumière cendrée, the ashen light, and the English phrase "the old Moon in the new Moon's arms" describes the same sight: the dim full disc held inside the bright arms of the crescent.

The Earth's Phase Is the Moon's in Reverse

The phase of any body depends on the angle between the Sun and the observer, as seen from that body. At new moon, the Moon lies between the Earth and the Sun: we see the Moon's unlit side, and the Moon sees the Earth's fully lit side. At full moon, the Earth is between the two: we see the Moon full, and the Moon sees the Earth's night side. In between, the two phases always add up to a whole: when 4 per cent of the Moon is lit for us, 96 per cent of the Earth is lit for the Moon.

The same holds for the brightness of the earthlight on the Moon, though it fades faster than the Earth's lit fraction does. A ball lit from one side reflects most of its light straight back towards the Sun, so a half-lit Earth sends the Moon not half but about a third of the light of a full Earth.

For the lunar month that began with the new moon of 10 October 2026 at 15:50 UTC:

Moon's age	Moon lit, seen from the Earth	Earth lit, seen from the Moon	Earthlight on the Moon (new moon = 100)
1 day	1.2%	98.8%	96
2 days	4.4%	95.7%	89
3 days	9.2%	90.8%	80
4 days	15.6%	84.5%	69
5 days	23.1%	77.0%	59
7 days	40.6%	59.5%	39
First quarter, 8.0 days	50.0%	50.0%	30

The earthlight drops below half its new-moon strength at 5.9 days, when the Moon is 30 per cent lit. After that, the earthlit part is both fainter and harder to see next to a growing sunlit part, so in practice earthshine belongs to the crescent: the evening crescent of the first few days and the morning crescent of the last few. The two lit fractions in the table do not add up to exactly 100 per cent because the Sun is not infinitely far away, but they never differ from it by more than 0.13 per cent.

How Bright Is the Full Earth?

From the Moon, the Earth is a big target. Its mean radius is 6,371 km against the Moon's 1,737 km, so it is 3.67 times wider, covers 13.4 times the area, and at the mean distance of 384,400 km it spans 1.9 degrees of sky, against 0.52 degrees for the Moon seen from the Earth.

It is also a better mirror. The NASA fact sheets give the Earth a Bond albedo of 0.294, meaning it reflects about 29 per cent of the sunlight that reaches it, and the Moon 0.11. The earthshine project at Big Bear Solar Observatory, which measures the Earth's reflectance from earthshine itself, found an average of 0.297 (Goode and others, 2001). The Moon's grey rock is dark; the Earth's clouds, ice and deserts are bright.

To compare the full Earth seen from the Moon with the full Moon seen from the Earth, take the size and the reflectivity together. Both bodies are at the same distance from the Sun and the same distance from each other, so the ratio is the ratio of their areas times the ratio of their reflectivities. The right reflectivity for a fully lit body seen face on is the geometric albedo, 0.434 for the Earth and 0.12 for the Moon (NASA):

13.45 (the areas) × 0.434 / 0.12 = about 49.
With the Bond albedos instead, the ratio is about 36.

In astronomers' magnitudes, a factor of 49 is 4.2 magnitudes, so the full Earth from the Moon is about magnitude −17, against −12.7 for the full Moon from the Earth.

The same arithmetic gives the light that each body sheds on the other. The full Earth lights the Moon's night side with about 1/8,400 of the light of direct sunlight; the full Moon lights the Earth with about 1/408,000. That second number can be checked against measurement: the Sun is magnitude −26.74 and the full Moon −12.74, a difference of 14 magnitudes, which is a factor of 398,000. A night under a full Earth is a far brighter night than any we know.

These numbers assume an Earth that reflects evenly, like a matte ball. The real Earth does not.

Why Earthshine Changes From Night to Night

The earthlight on the Moon comes from whichever half of the Earth is facing the Moon and lit by the Sun at that moment, and how bright that half is depends on what covers it. Open ocean is dark; cloud, snow, ice and desert are bright. So earthshine is a measure of the Earth's weather, and since 1998 it has been used as one. The Big Bear Solar Observatory in California has measured earthshine regularly since December 1998. Its first results found the Earth's reflectance changing by about 5 per cent from day to day with the weather on the far side of the globe, and seasonal changes roughly twice as large as climate simulations predicted, with the Earth brightest in spring (Goode and others, 2001). A later study combined the earthshine data with satellite cloud records to reconstruct the Earth's reflectance back to 1984 (Pallé and others, 2004).

The geometry also decides which part of the Earth is lighting the Moon. For an observer in Europe, the Moon of an evening crescent hangs over the western horizon after sunset, and the sunlit part of the Earth that can see it lies to the west, over the Atlantic and the Americas, where it is still afternoon. A morning crescent is lit by the Earth to the east, over Asia, where the day has begun. The two can differ measurably in brightness.

Leonardo, Danjon and the Old Moon in the New Moon's Arms

The glow was noticed long before it was explained. Its first known correct explanation is in the notebook of Leonardo da Vinci now called the Codex Leicester, written between about 1506 and 1510: he reasoned that the Moon's dark part is lit by sunlight reflected from the Earth. He thought the reflection came mostly from the Earth's seas; today we know that clouds contribute much of it. Leonardo's notes were not published, and a century later the explanation appeared in print again, among others in Galileo's Sidereus Nuncius (1610).

The old Scottish ballad of Sir Patrick Spens gives the sight its English phrase: a sailor who has seen the new moon with the old moon in her arm takes it as a warning of a storm. Coleridge used the lines as the epigraph of his poem Dejection: An Ode (1802).

The first careful measurements were made by the French astronomer André Danjon, from the late 1920s. His instrument split the image of the Moon in two so that he could set a patch in sunlight beside a patch in earthshine and dim the sunlit one until the two matched, and from the ratio he worked out how much light the Earth reflects. His values later proved too high, but the method lasted. Danjon is the same astronomer who described the limit below which no crescent is seen, explained in the Danjon limit. The modern earthshine projects are his method with electronic cameras.

Does Earthshine Help You See the Crescent?

It helps a little, and only once the crescent is already easy.

It cannot reveal a crescent that is too young to see. The earthlit part is always far fainter than the sunlit crescent, so if the crescent itself is lost in the twilight, the earthlit disc is lost with it. On the evening when earthshine is strongest, the first evening after new moon, the sky is at its brightest around the Moon, and the glow cannot be seen at all. The Danjon limit post explains why.
It can guide the eye to an older crescent. On a Moon two to five days old, once the sky has darkened, the ashen disc can make the whole Moon stand out, and in binoculars it helps to confirm that a faint sliver is the Moon and not the edge of a cloud.
The crescent is what counts. The visibility criteria used on this site, Yallop and Odeh, predict whether the sunlit crescent can be seen, from its width and its height above the Sun, and sighting reports describe the crescent. A grey disc on its own is not the crescent. How the different kinds of observation are treated is described in the naked eye, binoculars or a telescope.

For the timing of a sighting evening, when the crescent first becomes visible and how long it stays up, see how to see the crescent for beginners and the visibility map.

Photographing Earthshine

Earthshine is one of the most rewarding things to photograph on the Moon, because a camera shows it far more clearly than the eye. The best evenings are when the Moon is two to five days old, the sky is dark and the Moon is still well clear of the horizon: from the table above, the earthlight is then still between 89 and 59 per cent of its strongest. The crescent and the earthlit disc differ so much in brightness that no single exposure holds both. The usual answer is two exposures, a short one for the crescent and a long one, of a second or more at a higher ISO, for the earthlit disc, combined afterwards. The settings and the blending are in how to photograph the crescent moon.

Frequently Asked Questions

Is earthshine the same as the "ashen light"?

Yes, for the Moon. Lumière cendrée, the ashen light, is the old French name for earthshine. The same English phrase is also used for a different, disputed glow reported on the night side of Venus.

Why can I not see earthshine near full moon?

Because the Earth, seen from the Moon, is then nearly new: it turns its night side to the Moon and sends it almost no light. And the bright sunlit part of the Moon would drown a faint glow in any case.

Is earthshine brighter on a morning crescent or an evening crescent?

It depends on which half of the Earth is facing the Moon and how cloudy it is. For a given observer, the evening and morning crescents are lit by different parts of the Earth, so they can differ, and the difference changes with the season and the weather.

Does earthshine make the crescent easier to predict?

No. The predictions on this site, on the Moon page and the visibility map, are for the sunlit crescent and take no account of earthshine.

Would the full Earth light up the lunar night?

Yes. The full Earth gives the Moon's night side about 49 times as much light as the full Moon gives the Earth, enough to see the landscape clearly.

References

The phases, the earthlight through the month and the comparison of the full Earth with the full Moon are computed with the astronomy engine of this site. The numbers are reproduced by docs/research/earthshine.mjs and computed by packages/astronomy/src/earthshine.ts in the Hilal Vision source. The earthlight assumes an evenly matte (Lambert) Earth and allows for the changing distance between the Earth and the Moon.
Williams, D.R. Earth Fact Sheet, Moon Fact Sheet and Sun Fact Sheet, NASA Space Science Data Coordinated Archive (nssdc.gsfc.nasa.gov/planetary/factsheet). Radii, Bond and geometric albedos, and the magnitudes of the Sun and the full Moon.
Goode, P.R., Qiu, J., Yurchyshyn, V., Hickey, J., Chu, M.C., Kolbe, E., Brown, C.T. and Koonin, S.E. (2001). "Earthshine observations of the Earth's reflectance." Geophysical Research Letters, 28(9), 1671 to 1674.
Pallé, E., Goode, P.R., Montañés-Rodríguez, P. and Koonin, S.E. (2004). "Changes in Earth's reflectance over the past two decades." Science, 304, 1299 to 1301.
NASA (2001). "Scientists watch dark side of the Moon to monitor Earth's climate", 17 April 2001: on the Big Bear project, Danjon's first quantitative observations and Leonardo's explanation.
Leonardo da Vinci, Codex Leicester (about 1506 to 1510).
Galileo Galilei, Sidereus Nuncius (1610).
Sir Patrick Spens, Child Ballad 58; S.T. Coleridge, Dejection: An Ode (1802).

Earthshine: Why the Dark Part of the Young Crescent Glows