What is earthshine on the Moon?

Follow the light through three steps

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Schumann Pulse · field notes

Earthlight completes the crescent.

Follow the faint glow through three steps.

1. 1 · Sun to Earth

   Sunlight illuminates Earth.

2. 2 · Earth to Moon

   Earth reflects light onto the Moon’s night side.

3. 3 · Moon to observer

   Some light reflects back to an observer on Earth.

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Direct sunlight lights the bright crescent.

Simplified light paths, not to scale. The faint area is part of the Moon’s near side in lunar night.

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The faint light has made an extra reflection compared with the direct sunlight illuminating the crescent. Our schematic separates the paths so you can follow them; body sizes, distances and arrow spacing are not to scale. NASA Earth Observatory: earthshine ^[1].

What the ISS photograph records

The header photograph was taken by an Expedition 71 crew member on 10 May 2024, from the International Space Station. NASA identifies it as ISS071-E-67226. A narrow bright crescent sits above Earth’s atmospheric rim, while the rest of the visible lunar disc glows faintly.

NASA’s published version was cropped and contrast-enhanced, with lens artifacts removed. We preserve that supplied frame in an optimized web image. It is a real archival observation, not an illustration of the Moon’s phase today. NASA’s image record and processing notes ^[2].

Is the faint part the far side of the Moon?

No. Night side describes illumination; far side describes which side faces away from Earth. Earthshine reveals a night portion of the near side. The far side also experiences day and night. Treating “dark side” as a permanent place mixes those two ideas. NASA: Moon facts and synchronous rotation ^[3].

A useful question for any lunar picture is: does “dark” mean currently unlit, facing away from us, or simply underexposed in the photograph? Those are different descriptions.

When is earthshine easiest to notice?

Look for a thin crescent in a sufficiently dark twilight sky, with the Sun already below the horizon for evening viewing. The bright crescent can frame a much fainter complete disc. A long camera exposure can reveal that disc more strongly while overexposing the crescent, so a photograph need not match its brightness to your eyes. NASA’s earthshine guide ^[1].

Use Moon today to see whether you are near a crescent phase. That page provides phase and age, not a promise of a clear horizon or an exact local moonset time. A lunar cycle day is useful context; it does not determine whether earthshine will be visible.

Can earthshine tell us about Earth?

Researchers use carefully calibrated earthshine observations to study Earth’s reflectivity, or albedo. Goode and colleagues analyzed a long observing record from Big Bear Solar Observatory; this is an example of learning about our planet by measuring light from the Moon. Goode et al., 2021 ^[4].

One phone photo cannot establish a change in global albedo. Exposure, sky transparency and observing geometry can all change its appearance. The distinction is useful: an enjoyable sky observation and a calibrated climate measurement ask different things of an image.

Sources

References in the text lead here. Original publications open in a new tab; this guide stays open.

• [1] NASA Earth Observatory: Earthshine, 29 May 2014 (opens in a new tab) First mention ↑.
• [2] NASA Earth Observatory: Earth’s Reflections on the Moon, image and processing record (opens in a new tab) Back to text ↑.
• [3] NASA: Moon facts (opens in a new tab) Back to text ↑.
• [4] Goode et al. (2021): Earth’s Albedo 1998–2017 as Measured From Earthshine (opens in a new tab) Back to text ↑.