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In astronomy, the term extraterrestrial sky refers to a view of outer space from the surface of a world other than Earth. The sky of the Moon has been directly observed or photographed by astronauts, while those of Titan, Mars, and Venus have been observed indirectly by space probes designed to land on the surface and transmit images back to Earth. Characteristics of extraterrestrial skies appear to vary substantially due to a number of factors. An extraterrestrial atmosphere, if present, has a large bearing on visible characteristics. The atmosphere's density and chemical composition can contribute to differences in colour, opacity (including haze) and the presence of clouds. Astronomical objects may also be visible and can include natural satellites, rings, star systems and nebulas and other planetary system bodies. For skies that have not been directly or indirectly observed, their appearance can be simulated based on known parameters such as the position of astronomical objects relative to the surface and atmospheric composition. ==Mercury== Because Mercury has no atmosphere, a view of the planet's skies would be no different from viewing space from orbit. Mercury has a southern pole star, α Pictoris, a magnitude 3.2 star. It is fainter than Earth's Polaris (α Ursae Minoris).〔(Windows planets—Mercury's atmosphere )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Extraterrestrial skies」の詳細全文を読む スポンサード リンク
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