Planetshine

Planetshine is the illumination, by reflected sunlight from a planet, of part of the otherwise dark side of one of its moons. The best known example of planetshine is earthshine, which can be seen from Earth when the Moon is new, or nearly so. Typically, this results in the side of the Moon opposite the Sun being bathed in a soft, faint light. Planetshine has been observed elsewhere in the solar system: in particular, it has recently been used by the Cassini space probe to image portions of the moons of Saturn even when they are not directly lit by the Sun.

Earthshine is reflected earthlight visible on Moon's night side. It is also known as the Moon's ashen glow or as the new Moon with the old Moon in her arm.

Earthshine is most readily observable from shortly before until shortly after a new moon, during the waxing or waning crescent phase. When the Moon is new as viewed from Earth, Earth is nearly fully lit up as viewed from the Moon. Sunlight is reflected from the Earth to the night side of the Moon. The night side appears to glow faintly and the entire orb of the Moon is dimly visible.

Leonardo da Vinci explained the phenomenon in the early 16th century when he realized that both Earth and the Moon reflect sunlight at the same time. Light is reflected from the Earth to the Moon and back to the Earth as earthshine.

Earthshine is used to help determine the current albedo of the Earth. The data are used to analyze global cloud cover, a climate factor. Oceans reflect the least amount of light, roughly 10%. Land reflects anywhere from 10–25% of the Sun's light, and clouds reflect around 50%. So, the part of the Earth where it is daytime and from which the Moon is visible determines how bright the Moon's earthshine appears at any given time.

Studies of earthshine can be used to show how the Earth's cloud cover varies over time. Preliminary results show a 6.5% dip in cloud cover between 1985 and 1997 and a corresponding increase between 1997 and 2003. This has implications for climate research, especially with regards to global warming. All clouds contribute to an increased albedo, however some clouds have a net warming effect because they trap more heat than they reflect, while others have a net cooling effect because their increased albedo reflects more radiation than they trap heat. So while the Earth's albedo is measurably increasing, the uncertainty over the amount of heat trapped means the overall effect on global temperature remains unclear.

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