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Scintillation or twinkling are generic terms for rapid variations in apparent brightness or color of a distant luminous object viewed through a medium, most commonly the atmosphere (atmospheric scintillation).
If the object lies outside the earth's atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; if the luminous source lies within the atmosphere, the phenomenon is termed terrestrial scintillation.
As one of the three principal factors governing astronomical seeing, atmospheric scintillation is defined as variations in illuminance only, and so twinkling does not cause blurring of astronomical images. It is clearly established that almost all scintillation effects are caused by anomalous refraction caused by small-scale fluctuations in air density usually related to temperature gradients. Normal wind motion transporting such fluctuations across the observer's line of sight produces the irregular changes in intensity characteristic of scintillation. The primary cause of such small-scale fluctuations is turbulent mixing of air with different temperatures.
Scintillation effects are always much more pronounced near the horizon than near the zenith (straight up). Parcels of air with sizes of the order of only centimeters to decimeters are believed to produce most of the scintillatory irregularities in the atmosphere. Atmospheric scintillation is measured quantitatively using a scintillometer.
Scintillation effects are reduced by using a larger receiver aperture. This effect is known as aperture averaging.