1. The Sun Cannot Be Lighting the Dark Part, So What Is?
The Moon shines only because sunlight bounces off it. The half facing the Sun is lit and the other half is in night, so the dark part of a crescent should be dark. Yet it is often visible, and the glow has a name: earthshine, also called earthlight or the ashen light. In Japan it is called 地球照 (chikyūshō), which just means "Earth-shine." Old sky-watchers had a poetic phrase for it too: "the old Moon in the new Moon's arms"[1].
The lamp is our own planet. NASA's Earth Observatory describes earthshine as the dark face of the Moon catching Earth's reflected glow and returning that light[2]. So the light you see has bounced twice: first off Earth, then off the Moon, before it reaches your eyes[3]. Why would Earth be bright enough to do that?
2. Seen From the Moon, Earth Is Huge, Bright, and Nearly Full
Picture standing on the dark half of the Moon at the moment we call "new Moon." From Earth, the Moon sits close to the Sun's direction, so we see almost none of its lit side. From the Moon's night side, it is the other way around: Earth and the Sun are on opposite sides of the sky, and almost the whole Earth is lit. Earth has phases too, and when the Moon is new, Earth is close to "full"[3].
A full Earth is a far better lamp than a full Moon. Earth looks much bigger in the Moon's sky, and Earth, with its clouds, oceans, and ice, reflects more light than the dark lunar rock does. Scientific American and NASA both put the result at roughly 50 times brighter than a full Moon looks to us[3][4]. The light travels about 150 million kilometers (93 million miles) from the Sun to Earth, bounces, and covers about 380,000 kilometers (240,000 miles) to the Moon, and one more bounce brings the glow back to your eye[3].
That explains why the glow exists. It does not yet explain why a thin crescent shows it off best.
3. Why a Thin Crescent Shows Earthshine Best
Earthshine is most visible in the few days before or after new Moon, when the Moon is a thin crescent[1]. Two things work together. First, near new Moon the Earth seen from the Moon is nearly full, so the earthlight is at its strongest. (As the Moon fattens, Earth's phase in the Moon's sky shrinks, a piece of geometry rather than a measurement.) Second, a thin crescent has only a sliver of bright surface.
A simple way to picture the second point is a candle next to a floodlight. You can't see the candle while the floodlight is on, but switch it off and the candle appears. A thick bright crescent drowns the dim part in glare, and a thin one leaves it alone. This is a picture, not a quote from a source, and you can test it yourself in the last section.
The time of year matters a little too. NASA says earthshine tends to be brightest between April and June, because Earth's reflected light peaks in the Northern Hemisphere spring, when winter snow and ice still cover high latitudes and snow and ice reflect more light than vegetation or water[2]. So the glow on the Moon carries news about the seasons on Earth. What else does it say?
4. Leonardo da Vinci Got the Light Path Right and the Reflecting Surface Wrong
Around 1510, the Italian artist and thinker Leonardo da Vinci wrote about the glow in the Codex Leicester, a notebook of his writings on water, the Earth, and the sky. NASA's account says he explained it as sunlight bouncing off Earth's oceans and landing on the Moon[4]. That is the same two-bounce path described above, and it is why earthshine is sometimes called the "Da Vinci glow"[1].
The details were off. Leonardo believed the Moon had oceans and an atmosphere, which would make it a good mirror, but Apollo astronauts later found rock, not water[4]. And clouds, not oceans, do most of Earth's reflecting[4][1]. Whether he was truly the first person to explain it is a bigger claim than I can check, though NASA's article says he solved the mystery[4]. What is clear is that he worked out where the light must come from, about a century before the first telescopes were pointed at the Moon.
The clouds he missed are the reason the glow became useful. How?
5. A Faint Glow on the Moon Became a Way to Track Earth's Clouds
The fraction of sunlight a planet bounces back to space is called its albedo. Brighter clouds and snow raise Earth's albedo, and darker ocean and land lower it. The earthshine on the Moon rises and falls with it, so the Moon works like a giant mirror showing how bright Earth is[5].
Astronomers at Big Bear Solar Observatory in California began regular measurements of the glow in December 1998. In 2001 they reported an average Earth albedo of 0.297, plus or minus 0.005. That means Earth sends back a bit less than a third of the sunlight that reaches it, and it agrees closely with a figure of 0.296 from computer simulations based on satellite cloud and snow data[6]. They also found that the glow changes by around 5 percent from one day to the next, because weather on the half of Earth facing the Moon keeps changing[6]. On any given night, earthshine reflects clouds on the other side of the world.
The Big Bear data kept coming. A study reported by the American Geophysical Union covering 1998 to 2017 found that Earth reflected about half a watt less light per square meter than 20 years earlier, a drop of about 0.5 percent, mostly in the last three years of data. NASA's CERES satellites showed a similar trend, and the team linked it to fewer bright, low clouds over the eastern Pacific Ocean, along with warmer sea-surface temperatures there[7]. That is one research team's interpretation, built on one observing program and checked against one satellite record, so treat it as a lead and not a final answer. It does show how much a night-sky glow can tell us.
6. How to Spot Earthshine Yourself
- Pick the right evening. Look a few days after new Moon: a thin crescent low in the western sky just after sunset, with the sky already dim. A few days before new Moon, a thin crescent rises in the east before dawn[1]. Go outside with an adult, stay somewhere safe from traffic, and check on a Moon app or calendar when the next new Moon is.
- Use only your eyes. Naked eyes work well. If you try binoculars, only do it once the Sun is well below the horizon, and never point them anywhere near the Sun.
- Test the floodlight idea. Hold your hand out and cover just the bright crescent with your thumb. Does the dim part look easier to see? Then compare over three evenings. As the crescent gets thicker, does the glow fade?
- Keep a record. Draw the shape, and note the date and time. If you can, note a clear night against a very cloudy one, and see whether the dim part looks different.