Earthshine

Earthshine is the faint illumination of the Moon's surface by sunlight reflected from the Earth. This phenomenon is most noticeable when the Moon is a thin crescent, as the unlit portion of the Moon appears to glow with a soft, grayish light. The Earth, being much larger and more reflective than the Moon, acts as a powerful secondary light source in the lunar sky.

The effect is a result of sunlight bouncing off the Earth's oceans, atmosphere, and clouds, and then traveling to the Moon. This reflected light is then scattered by the Moon's surface, making the unlit portion visible from Earth. The brightness of the Earthshine varies depending on the phase of the Earth as seen from the Moon. When the Earth is fully illuminated from the Moon's perspective (a concept analogous to a "full Earth"), the Earthshine is at its brightest. Conversely, during a new Moon, when the Earth is in its "new phase" from the Moon's viewpoint, there is no Earthshine.

Leonardo da Vinci was one of the first to observe and accurately describe Earthshine in the 15th century, though the scientific explanation of its cause was not fully understood until much later. Earthshine is a subtle but beautiful reminder of the interconnectedness of the Earth and Moon, showcasing how light from one celestial body can illuminate another. It is a visible testament to the reflective properties of our own planet.