An attempt is made to take into account the effect of the underlying terrain (including the mountain terrain) at the site of astronomic receivers on the variance of the image jitter of extraterrestrial objects. In particular, it is shown that the account of the terrain peculiarities enables one to achieve an agreement between theory and experimental data, which are considerably different from the well known theoretical secant law. The study is carried out based on theoretical calculations and generalization of experimental data. The fact is analyzed that the quality of optical images in ground-based non-adaptive astronomic telescopes is governed by the atmospheric turbulence. Turbulence is a cause of random distortions in the phase front of a light wave propagating from an extraterrestrial source. In astronomy, the image jitter is the factor responsible for the largest error in observations. The allowance made for the terrain allows us to explain the considerable discrepancy between the experimental results and the theoretical secant law.