Earthlight is the name given to the phenomenon where astronauts or other observers in space can see the Earth illuminated by the Sun. It occurs on the 'night' side of the Earth and is most prominent during a near-new Moon, when the Moon is between the Sun and the Earth. During this phase, the Earth's moonward-facing side is mostly dark, as the Sun is hidden behind the Earth from the Moon's perspective. However, the Earth's atmosphere scatters sunlight, causing the portion of the Earth facing away from the Sun to be dimly illuminated. This scattered light is what astronauts and other observers in space refer to as earthlight.

Here's a more detailed explanation of why earthlight occurs:

1. Sunlight Illumination: During the day, the side of the Earth facing the Sun is brightly lit by direct sunlight. This is what we experience as daytime on Earth.

2. Atmospheric Scattering: As sunlight passes through the Earth's atmosphere, some of it is scattered by particles such as molecules of air, water droplets, and dust. This scattered sunlight is what gives the sky its blue color during the day.

3. Earth's Spherical Shape: The Earth is nearly spherical in shape, meaning that it has a curved surface. This curvature causes the sunlight that is scattered by the Earth's atmosphere to be redirected in different directions.

4. Earthlight Observation: When an observer is in space, they can see the Earth from a distance. This allows them to observe the scattered sunlight that is reflected from the Earth's atmosphere towards space. This scattered light appears as a faint glow around the dark side of the Earth, and this phenomenon is known as earthlight.

5. Near-New Moon Conditions: Earthlight is most prominent during a near-new Moon. During this phase, the Moon is located close to the Sun in the sky, as seen from Earth. As a result, the side of the Moon that faces the Earth is mostly dark, and the Earth is fully illuminated by the Sun. This enhances the contrast between the dark side of the Earth and the earthlight, making it more visible to observers in space.

Earthlight is a fascinating natural phenomenon that demonstrates the effects of atmospheric scattering, the spherical shape of the Earth, and the Moon's position relative to the Earth and the Sun. It provides a unique perspective of our planet from the vantage point of space.