scintillation

thumb|The twinkling of the brightest star in the [[night sky Sirius (apparent magnitude of −1.1), shortly before upper culmination at the meridian, at 20° above the southern horizon. In 29 seconds, Sirius appears to move 7.5 arcminutes from left to right.]] thumb|Comparison of twinkling of a star (top) and a planet (bottom). The turbulent atmosphere (shaded blue) distorts their wavefronts (cyan lines) differently with time, like caustics on a swimming pool floor. When a dark part hits the observer (white circle), the object appears dark, and vice versa. An object with larger angular size smears the pattern, yielding less change in intensity. Twinkling, also called scintillation, is a generic term for variations in apparent brightness, colour, or position of a distant luminous object viewed through a medium. If the object lies outside the Earth's atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; for objects within the atmosphere, the phenomenon is termed terrestrial scintillation. As one of the three principal factors governing astronomical seeing (the others being light pollution and cloud cover), atmospheric scintillation is defined as variations in illuminance only.

In simple terms, twinkling of stars is caused by the passing of light through different layers of a turbulent atmosphere. Most scintillation effects are caused by anomalous atmospheric refraction caused by small-scale fluctuations in air density usually related to temperature gradients. Scintillation effects are always much more pronounced near the horizon than near the zenith (directly overhead), since light rays near the horizon must have longer paths through the atmosphere before reaching the observer. Atmospheric twinkling is measured quantitatively using a scintillometer.