The loss of starlight in passing through the Earth’s atmosphere. Most of the loss arises from Rayleigh scattering by molecules of nitrogen and oxygen. At certain wavelengths there is selective absorption from molecules of oxygen, ozone, and water vapour. Particles of dust and industrial pollutants can also contribute to extinction by Mie scattering. Atmospheric extinction is proportional to the airmass and the atmospheric pressure. At sea level and in a perfectly clear sky a star 80° from the zenith appears 1 mag. fainter than it would at the zenith. This in turn is 0.3 mag. fainter than if no atmosphere were present. These figures apply only to visual observations; in blue light, the extinction is greater, while in red light it is less. This is why the Sun and Moon in particular appear red when near the horizon.