Scintillation, also called twinkling, is a variation in the brightness, wavelength, and mean position of stars caused by turbulence high in Earth's atmosphere which changes irs refractive index. Planets don't exhibit this because the scintillations from different points on the surface are not in phase, and the fluctuations are lost in the general illumination. For astronomical purposes, the inconvenience of the effects of scintillation is reduced by siting observatories at high altitudes and through the use of adaptive optics, and is almost entirely avoided by using telescopes carried by balloons, rockets, or artificial satellites.

In another sense, scintillation is the rapid changes in the detected intensity of radiation from compact cosmic radio sources due to disturbances in ionized gas at some point between the source and Earth's surface (usually in Earth's upper atmosphere).