in the F-layer. The green oxygen emissions are more spatially distributed. How sodium gets to mesospheric heights is not yet well understood, but it is believed to be a combination of upward transport of sea salt and meteoritic dust. In daytime, sodium and red oxygen emissions are dominant and roughly 1,000 times as bright as nighttime emissions because in daytime, the upper atmosphere is fully exposed to solar UV radiation. The effect is however not noticeable to the human eye, since the glare of directly scattered sunlight outshines and obscures it. Indirect scattering of sunlight Indirectly scattered sunlight comes from two directions. From the atmosphere itself, and from outer space. In the first case, the sun has just set but still illuminates the upper atmosphere directly. Because the amount of scattered sunlight is proportional to the number of scatterers (i.e. air molecules) in the line of sight, the intensity of this light decreases rapidly as the sun drops further below the horizon and illuminates less of the atmosphere. When the sun's altitude is < -6° 99% of the atmosphere in zenith is in the Earth's shadow and second order scattering takes over. At the horizon, however, 35% of the atmosphere along the line of sight is still directly illuminated, and continues to be until the sun reaches -12°. From -12° to -18° only the uppermost parts of the atmosphere along the horizon, directly above the spot where the sun is, is still illuminated. After that, all direct illumination ceases and astronomical darkness sets in. A second source sunlight is the zodiacal light, which is caused by reflection and scattering of sunlight on interplanetary dust. Zodiacal light varies quite a lot in intensity depending on the position of the earth, location of the observer, time of year, and composition and distribution of the reflecting dust. Scattered light from extraterrestrial sources Not only sunlight is scattered by the molecules in the air. Starlight and the diffuse light of the Milky Way are also scattered by the air, and it is found that stars up to V magnitude 16 contribute to the diffuse scattered starlight. Other sources such as galaxies and nebulae don't contribute significantly. The total brightness of all the stars was first measured by Burns in 1899, with a calculated result that the total brightness reaching earth was equivalent to that of 2,000 first-magnitude stars with subsequent measurements by others. Light