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.
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.
Light from both planets and stars can suffer from scintillation. [1]. Dave Kornreich explains why: [2]. Both stars and planets are circular discs. The light from a star enters the pupil of our eye and comes to a focus as a point at the back of the eye. If that light is followed backwards to the star, it forms a cylindrical bundle the size of the pupil but gradually widening as it nears the star. But so gradual is this widening that even at the edge of the atmosphere, 100 km up, the bundle is still about the same as the pupil, say half a cm across. This is so narrow that wind currents of different temperature and refractive index can disturb the light and cause the star to twinkle. A planet is so much nearer that at the edge of the atmosphere the bundle is a disc metres across. The light from the planet that forms our image of it comes from hundreds of different pathways in this bundle and it is unlikely that many will be disrupted at the same time. The amount of disturbance is therefore much less and the planet appears as a steady point.