Earthshine: why the dark disc glows beside a crescent Moon

Light reflected twice

Look carefully at a thin evening crescent. Sometimes the whole round disc is visible, not just the bright strip directly lit by the Sun. The faint grey remainder is **earthshine**, traditionally described as the old Moon in the new Moon’s arms.

The light begins at the Sun, reflects from Earth’s clouds, oceans, ice and land, reaches the lunar night side and reflects again towards the observer. It is neither the Moon generating its own light nor Earth shining with an independent visible-light source.

Leonardo da Vinci described this explanation in the early sixteenth century. The visual contrast makes the effect especially useful for understanding that a “dark” lunar region can still receive indirect illumination.

The glow is much fainter than direct sunlight on the crescent. That is why a thin phase and suitable sky brightness matter so much to seeing it.

Why it is strongest near new moon

Earth and Moon show complementary phases to observers facing each other. When we see a new moon, the Earth-facing lunar hemisphere sees an almost full Earth. A thin lunar crescent corresponds to a large illuminated terrestrial disc in the lunar sky.

Earth appears almost four times the Moon’s angular diameter as seen in the opposite direction. Its visible area is much greater, and clouds reflect sunlight more efficiently than dark lunar soil. A full Earth can consequently illuminate a lunar landscape by several dozen times the light that a full moon gives a terrestrial one.

As the Moon waxes for us, Earth’s phase wanes from the lunar viewpoint. Less reflected terrestrial light reaches the night side. Meanwhile the directly illuminated lunar area grows and its glare reduces the contrast of the faint glow.

The strength varies with which terrestrial regions face the Moon and their cloud cover. There is no single earthshine brightness for a particular lunar age, nor a guarantee based only on a calendar date.

When to look

A few days after new moon, look in the evening; a few days before it, look in the morning. An age of roughly two to five days often gives a useful balance between a thin crescent and sufficient altitude, but the geometry varies.

In middle northern latitudes, spring evening crescents and autumn morning crescents are often favourably placed. That is a seasonal guide, not a universal timetable for every English-speaking country or hemisphere.

Choose clear air and an unobstructed horizon. Too early, the twilight background can hide the faint disc; too late, the crescent may be low in haze or already set. At the Grand Canyon, the local rim can matter as much as the ideal horizon. In a city, buildings and local light add further limits.

Scientific earthshine measurements can help study Earth’s reflectivity, or albedo. A casual visual impression is not by itself a climate measurement: calibrated observations and long-term comparisons are needed to distinguish the causes of a brightness change.

Photographing the glow

Use a stable support and a longer exposure than the crescent alone needs. It is normal for the bright strip to overexpose while the grey disc becomes visible. A telephoto image can show lunar maria in the earthlit region.

Separate exposures can retain detail in both bright and faint areas when combined carefully. Keep the scale and position consistent, and describe processing appropriately when presenting the result as a documentary image.

A foreground silhouette and twilight colour add context. Check direction and altitude before choosing a viewpoint; another location’s successful composition does not supply the right geometry for your own.