Earthshine: Why the Dark Part of a Crescent Moon Glows and When to Look for It

Now and then a thin crescent Moon seems to come with a ghost. Beside the bright curve you can make out the rest of the disk, much dimmer, as if the whole Moon had been sketched in lightly. NASA describes the dark face of the Moon as having a faint shine, a ghostly version of a full Moon. That shine is real light, and it has made a long trip to reach you.

This guide explains what earthshine is, how the light gets from the Sun to your eyes by way of Earth and the Moon, and when to look. It also covers why earthshine is brighter in some seasons and how scientists use it. It ends with a task in the Rise & Set Dial: set the sky to a thin crescent in the pre-dawn sky at 5 AM.

What earthshine is

NASA Earth Observatory gives the plain definition. The dark face of the Moon has a faint shine, and the phenomenon is called earthshine. NASA's Moon Phases page adds that it shows up around the crescent phases: we can still see the darkened area of the Moon's nearside shining dimly.

The glow has older names too. NASA picture-of-the-day pages call it the Moon's ashen glow and the old Moon in the new Moon's arms. A 2025 page titles it the Da Vinci glow. The same page calls earthshine the most familiar example of planetshine, which is the faint illumination of the dark portion of a moon by light reflected from its planet.

One point about words matters. People sometimes call the glowing part the dark side of the Moon. NASA's Top Moon Questions page corrects that: there is no permanent dark side of the Moon, and the far side gets as much sunlight as the near side. The part that glows in earthshine is the night side of the near side. NASA says that when it is nighttime on the near side, that is the new Moon phase.

How the light travels

Earthshine is sunlight that makes two bounces. NASA's pages describe each step.

1. Sunlight reaches Earth. Earth is facing the Sun and reflecting plenty of light back in the direction of the Moon.
2. Earth's glow lands on the Moon. NASA says the effect is caused by the Sun's light reflecting off Earth's surface onto the face of the Moon.
3. The Moon bounces it back. That reflected sunlight is in turn reflected back off the lunar surface to create earthshine.
4. A little of it reaches your eyes. NASA says earthshine is readily visible to the naked eye, most easily during a crescent Moon.

Why is Earth bright enough to matter? NASA's Moon Phases page explains that, at that point in its orbit, Earth is nearly full from the Moon's point of view. The light it reflects is bright enough to dimly illuminate the darkened surface. NASA Earth Observatory paints the same picture: an astronaut standing on the Moon after sunset would see a spectacular, fully lit Earth. In other words, a thin crescent for you means an almost full Earth for anyone standing on the Moon.

In the drawing, sunlight comes from the right. At an age of 26 days the Moon is on the Sun's side of its orbit, a few days before New Moon, so only a thin edge of the face turned toward us is lit.

When and where to look

NASA's observing tips give the key advice. During its crescent phase in the twilight or dawn, you can sometimes see the dark portion of the Moon glowing faintly in the sunlight that reflects off Earth. NASA Earth Observatory adds that earthshine is easiest to see around the time of a new Moon.

A 2001 NASA Science report on earthshine research puts numbers on the timing. The data are best collected one week before and one week after the new Moon, when less than half of the lunar disk is lit by the Sun. That gives two viewing windows.

You can read about each crescent in the waning crescent Moon and waxing crescent Moon guides. The twilight guide explains how the sky darkens in the evening and brightens before dawn.

The word "sometimes" in NASA's advice is honest. Earthshine depends on how bright Earth is on a given night, and you will not see it every time.

Why earthshine is brighter at some times

NASA Earth Observatory says earthshine tends to be brightest between April and June, though it does occur at other times of the year. It gives a reason: snow and ice reflect more light than vegetation or water, so the spring is brighter than the summer or autumn. The same page says clouds and sea ice contribute to the peak in the Southern Hemisphere.

An older NASA picture-of-the-day page words the season a little differently. It says recent studies indicate earthshine is more pronounced during April and May, and it notes that Earth's brightness is strongly influenced by cloud cover. Both pages point to the same part of the year.

Scientists turn this effect into a measurement. At the Big Bear Solar Observatory, researchers measure the intensity of the earthshine. NASA's report says the brightness of the earthshine measures the reflectance of Earth, and that these measurements are a useful complement to satellite observations. Watching the bright crescent at the same time helps them correct for the effects of the atmosphere.

Common mistakes

• Calling it the dark side of the Moon. NASA says there is no permanent dark side. The glow is on the night side of the near side.
• Thinking the Moon makes the light itself. Earthshine is sunlight that bounced off Earth and then off the Moon.
• Looking for it at full Moon. The Naval Observatory says a full Moon appears completely illuminated by direct sunlight, so there is no dark part left to glow.
• Expecting it on every clear night. NASA says you can sometimes see it, and its brightness changes with the season and with Earth's clouds.
• Looking at the wrong hour. NASA's tip is to look during the crescent phase in twilight or dawn, when the sky is darker than at midday.

Try it in the instrument

Open the Rise & Set Dial and set the sky to a thin crescent Moon in the pre-dawn sky at 5 AM, when earthshine can show. The target is a Moon age of about 26 days, with the clock at 5 AM.

The dial pictures a place at middle latitudes near an equinox, with sunrise at 6 AM. At New Moon the Moon rises with the Sun, at 6 AM. Each day of age pushes moonrise later by about 0.81 hour, or 49 minutes, the dial's version of NASA's 50 minutes a day. At age 26, moonrise is 6 AM plus 21 hours, which wraps past midnight to about 3 AM. So at 5 AM the Moon has been up for about two hours and sunrise is an hour away. That matches NASA's waning crescent: it rises in the very early morning, before sunrise.

Age 26 is also about 3.5 days before the next New Moon, since the cycle is 29.5 days. That puts it inside the one-week window before New Moon, when less than half of the disk is lit and earthshine is easiest to see.

Why this age and not another? Try age 22 and the Moon is at last quarter. It rose around midnight and half of its face is lit, so there is no thin crescent to set off a dark part. Try age 29 and the Moon rises about 5:30 AM, almost with the Sun, so it is not up yet at 5 AM. The game accepts a range of ages in between. The dial is a simplified model, so use local predictions when you need the real moonrise at your address.

Questions people ask

Can I see earthshine without a telescope?

Yes. NASA says earthshine is readily visible to the naked eye, most easily during a crescent Moon. NASA's observing tips suggest looking during the crescent phase in twilight or dawn.

Why can't I see earthshine at full Moon?

At full Moon the side that faces Earth is the sunlit side, and the Naval Observatory says the Moon appears completely illuminated. There is no dark part left to glow. Earthshine needs a crescent, where the lit edge is thin and the rest of the disk is in night.

Who first explained earthshine?

NASA says Leonardo da Vinci first explained the phenomenon, in which the Moon acts as a giant mirror, showing the sunlight reflected from Earth. A NASA picture-of-the-day page says that description was written over 500 years ago.

Does this happen on other worlds?

Yes. A NASA picture-of-the-day page calls earthshine the most familiar example of planetshine, the faint illumination of the dark portion of a moon by light reflected from its planet.

Sources

• Earthshine, NASA Earth Observatory. https://science.nasa.gov/earth/earth-observatory/earthshine-83782/. Opened November 10, 2026.
• Moon Phases, NASA Science. https://science.nasa.gov/moon/moon-phases/. Opened November 10, 2026.
• Moon Viewing Tips, NASA Science. https://science.nasa.gov/moon/viewing-tips/. Opened November 10, 2026.
• Scientists Watch Dark Side of the Moon to Monitor Earth's Climate, NASA Science (release by AGU, Caltech and NJIT, April 17, 2001). https://science.nasa.gov/earth/scientists-watch-dark-side-of-the-moon-to-monitor-earths-climate/. Opened November 10, 2026.
• Top Moon Questions, NASA Science. https://science.nasa.gov/moon/top-moon-questions/. Opened November 10, 2026.
• APOD: 2002 April 19 - The Old Moon in the New Moon's Arms, NASA Science (Astronomy Picture of the Day). https://science.nasa.gov/image-article/apod-2002-april-19-the-old-moon-in-the-new-moons-arms/. Opened November 10, 2026.
• APOD: 2025 April 3 - The Da Vinci Glow, NASA Science (Astronomy Picture of the Day). https://science.nasa.gov/image-article/apod-2025-april-3-the-da-vinci-glow/. Opened November 10, 2026.
• Phases of the Moon and Percent of the Moon Illuminated, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/moon_phases. Opened November 10, 2026.

We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.

Read next

• TwilightTwilight is the glow before sunrise and after sunset.
• The Moon in DaytimeThe Moon spends nearly as many hours above the horizon in daylight as at night.