The atmospheric corona (not to be confused with the solar corona visible during eclipses) is an optical phenomenon manifesting as one or several concentric coloured rings directly around the disc of the sun or moon, with a central whitish disc called the aureole. The rings display colours ranging from bluish on the inside to reddish on the outside, with a typical angular diameter of 1° to 5°—much smaller than a 22° solar halo. The responsible mechanism is diffraction of light passing close to small water droplets or ice crystals (5–40 µm) present in thin clouds such as altostratus or thin altocumulus. When droplets are uniform in size, the rings appear sharp and well-defined; if they vary in size, the rings become blurred. This principle is actually used in optical meteorology to estimate droplet sizes within clouds. The corona is easy to observe at night around the full moon when thin clouds veil it, and during the day around the sun using a dark reflective surface (a puddle) or by blocking the solar disc with one's hand. It is distinguished from the halo by its smaller angular size and because the corona is produced by diffraction (small droplets or crystals in low/mid-level clouds), while the halo forms by refraction (ice crystals in high cirrus clouds). The iridescent corona appearing in clouds near the sun is called iridescence.