Earthshine: See Earth's Light on the Moon

Sunlight can visit Earth before reaching the Moon

Beside a thin bright crescent, you may notice the rest of the Moon as a faint gray disk. This is earthshine. Sunlight reflects from Earth onto the lunar night side, and some of that light returns to your eyes. The route is Sun to Earth to Moon to observer.

The crescent receives direct sunlight; the dim disk receives light reflected by Earth. Neither part generates its own visible glow. The extra reflection makes earthshine much fainter, explaining why a photograph that reveals the disk may badly overexpose the crescent.

Reference: NASA Earth Observatory: Earth's Reflections on the Moon

A crescent Moon corresponds to a bright Earth

When we see a thin crescent, an observer on the Moon's Earth-facing side would see a largely illuminated Earth. That broad sunlit face provides the light for earthshine. Near full Moon, Earth appears much less illuminated from the Moon and the bright lunar face overwhelms the faint effect.

This is not a lunar eclipse, which involves Earth's shadow near full Moon. It is also not a view of a permanently dark hemisphere. The night side changes with the phase; the far side is the hemisphere mostly facing away from Earth. Those are different descriptions.

Reference: NASA Earth Observatory: Earthshine

Try an evening crescent after sunset

A waxing crescent in the evening is a convenient starting target. You need enough darkness for contrast but enough lunar elevation to clear haze and buildings. Waiting later improves darkness while also lowering an evening crescent, so there is no single perfect delay after sunset.

Begin with your eyes and a clear horizon. Look for the faint circular disk attached to the bright crescent, rather than a halo outside it. Choose a session after sunset; never sweep binoculars or a telescope near the Sun while searching for a thin crescent.

Record what you actually see

A simple comparison is more useful than heavily increasing a phone image's contrast until something appears.

• Sketch the bright arc and the faint outline, noting whether the disk is obvious or only occasionally visible.
• Record the date, time, lunar elevation, haze, and local glare.
• If photographing, compare separate exposures for the crescent and the disk, and keep the originals.
• Label blended exposures clearly. Different automatic camera processing can mimic a change in brightness.