Twinkling, or
scintillation, is a generic term for variations in
apparent brightness 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; within the atmosphere, the phenomenon is termed
terrestrial scintillation. As one of the three principal factors governing
astronomical seeing, atmospheric twinkling is defined as variations in
illuminance only.