ABSTRACT
This chapter is devoted to the examination of the influence of the Earth's atmosphere on the apparent coordinates of the stars and on the shape of their images; the discussion is limited essentially to the visual band. Tycho Brahe had already recognized the importance of the atmospheric refraction. The troposphere, where approximately 90% of the total mass of the atmosphere is contained, extends from the ground to approximately 15km of height, with noticeable latitude and seasonal dependence. The unavoidable thermal turbulence of the atmosphere above the observatory produces unpredictable (although small) variations of the direction of arrival of the rays from a given star. The complex statistical theory of the air turbulence and instantaneous image structure is associated with names such as Kolmogorov and Tatarski, who provided the theoretical foundations of methods employed to contrast the detrimental effects of the seeing on the image quality.
