Antoniadi scale

The Antoniadi scale is a five-point scale, devised by Eugène Antoniadi and used by amateur astronomers to indicate the quality of seeing: I – perfect seeing, without a quiver; II – slight undulations, with moments of calm lasting several seconds; III – moderate seeing, with larger tremors; IV – poor seeing, with constant troublesome undulations; V – very bad seeing, scarcely allowing the making of a rough sketch.