When the Moon is a thin crescent, the dark portion faintly glows. This is Earthshine, sunlight reflected from Earth onto the Moon. Albedo differences between Earth and Moon make it visible to the naked eye. Observed for centuries, it inspired early astronomers. Earthshine varies with cloud cover, oceans, and continents. It affects amateur observations and photography. It’s essentially our planet illuminating its satellite.
It matters for observational astronomy and artistic inspiration. Earthshine demonstrates planetary interaction visually.
It also shows the Moon’s surface can act as a natural reflector for Earth-based signals.
Bright Earthshine can be strong enough to cast faint shadows on the Moon.
NASA [nasa.gov]