The effect of random turbulent motion in the atmosphere on the quality of the image of an astronomical object. In conditions of good seeing, images are sharp and steady; in poor seeing, they are extended and blurred and appear to be in constant motion. Amateur astronomers sometimes use a scale of seeing quality indicated by Roman numerals. I is the best, II-III typical, IV poor and V extremely bad. The scale is due to Eugenios Antoniadi 1870-1944. Seeing may also be described quantitatively in arc seconds.