Atmospheric optical effects can teach much about physics and especially optics. Coronae--coloured rings around the sun or moon--are large-scale consequences of diffraction, which is often thought of as only a small effect confined to the laboratory. We describe coronae, how they are formed and experiments that can be conducted on ones in the sky. Recognizing that this is not always convenient, we show how students can also learn about coronae and thus diffraction from experiments with accurate full-colour computer simulations and laboratory demonstrations.