Extraterrestrial sky

In astronomy, an extraterrestrial sky is a view of outer space from the surface of an astronomical body other than Earth. The only extraterrestrial sky that has been directly observed and photographed by astronauts is that of the Moon. The skies of Venus, Mars and Titan have been observed by space probes designed to land on the surface and transmit images back to Earth. Characteristics of extraterrestrial sky 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. Solar luminosity The Sun's apparent magnitude changes according to the inverse square law, therefore, the difference in magnitude as a result of greater or lesser distances from different celestial bodies can be predicted by the following formula: Where "distance" can be in km, AU, or any other appropriate unit. To illustrate, since Pluto is 40 AU away from the Sun on average, it follows that the parent star would appear to be times as bright as it is on Earth. Though a terrestrial observer would find a dramatic decrease in available sunlight in these environments, the Sun would still be bright enough to cast shadows even as far as the hypothetical Planet Nine, possibly located 1,200 AU away, and by analogy would still outshine the full Moon as seen from Earth. The change in angular diameter of the Sun with distance is illustrated in the diagram below: The angular diameter of a circle whose plane is perpendicular to the displacement vector between the point of view and the centre of said circle can be calculated using the formula in which is the angular diameter, and and are the actual diameter of and the distance to the object. When , we have , and the result obtained is in radians.