The movement or distortion of a telescopic image as a result of turbulence in the Earth’s atmosphere. It is thought to be caused by undulations between separate layers of air, which disturb the path of light. The scale of these undulations is often regarded as giving cells of seeing, typically being around 100–150 mm across at sea level, and larger at higher altitude. A small telescope may look through individual cells, whose movements give rise to a sharp but wandering image, while a larger one may look through several at once, producing multiple images. The ‘boiling’ seen at the limb of the Moon or Sun is another manifestation of bad seeing. The Antoniadi scale is widely used by amateur astronomers to evaluate seeing.