In astronomy, seeing refers to the degradation of the image of an astronomical object due to turbulent airflows in the atmosphere of Earth that may become visible as blurring, twinkling or variable distortion. The origin of this effect are rapidly changing variations of the optical refractive index along the light path of the object.Seeing is a major limitation to the angular resolution in astronomical observations with telescopes that would otherwise be limited through diffraction by the size of the telescope aperture.Today, many large scientific ground-based optical telescopes include adaptive optics to overcome seeing.