Albedo formula_5 (also called blue-sky albedo) can then be given as: Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo. These terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. The albedos of planets, satellites and minor planets such as asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time comprises a major part of the astronomical field of photometry. For small
Classical albedo features on Mars their borders change in response to windstorms on the Martian surface that move the dust around, widening or narrowing the features. The dust-storms themselves also appear as light patches, can cover vast areas and sometimes last for many weeks; when Mariner 9 arrived in Martian orbit in November 1971 the entire planet was covered by a single enormous dust-storm, with only the peaks of the four or five highest mountains showing above it. This variability may explain many of the differences between telescopic observations over the years. The first map of Martian albedo features was published in 1840 by Johann