Coronae around the sun and the moon

Corona around the sun
Photo: Claudia Hetze, Chemnitz

When thin clouds pass the moon at night, you can often see that the moon is surrounded by a bright disk. This bright disk, called aureole or corona, is bluish-white towards the moon and has a bright reddish-brown outer rim. Its diameter is about 1-2°, but it can even reach up to 10°. An aureole is caused by light diffraction on water droplets. If the droplets are all of the about the same size, the aureole is as round as a circle, while it appears more diffuse when the droplets are of different sizes. But it is always necessary that the droplets have a diameter between 0,02 and 0,1 mm. An aureole around the moon can be observed quite easily. But it can also be seen around the sun and as a round bright spot around bright stars and planets. Due to its great brightness it is difficult to be observed around the sun. If you look at the reflection of the sun in a puddle while using sunglasses you will not get blinded so much. When there are altocumulus clouds or there is a thin cloud layer that lets the moon shine through, an aureole can be observed quite frequently. You can easily generate an aureole yourself by breathing upon a window pane at night and then getting near to the pane with your eyes while looking at a street lamp.

The bright disk can be surrounded by several systems of colourful rings. Each system is blue towards the source of light, followed by green and yellow while the outer rim is red. These rings, called a corona, are also caused by the diffraction of light on small water droplets. A corona, however, requires water droplets that all are of about the same size. In this case the diffraction effect of the great number of water droplets is identical to that of the diffraction caused by a small hole. Furthermore, the effect of a great number of similar small globes (water droplets) is the same as the one of a single hole. The colours of the coronae are due to the diffraction effect being subject to the wavelength of the light When the droplets have different sizes, the colours overlap and spread over a wider range, so that in this case no colourful rings are visible.