Riccò's law, discovered by astronomer Annibale Riccò, is one of several laws that describe a human's ability to visually detect targets on a uniform background.[3][4] It says that for visual targets below a certain size, threshold visibility depends on the area of the target, and hence on the total light received. The "certain size" (called the "critical visual angle"), is small in daylight conditions, larger in low light levels. The law is of special significance in visual astronomy, since it concerns the ability to distinguish between faint point sources (e.g. stars) and small, faint extended objects ("DSOs").