Höfe und Kränze um Sonne und Mond

When thin clouds pass by the moon at night, you often see that the moon is surrounded by a bright disc. This bright disc, also called an aureole or halo, is blue-white towards the moon and brown-red at the outer edge. The diameter is about 1°-2°, but can also reach up to 10°. The halo is caused by light diffraction at water droplets. With approximately the same droplet size, the halo appears circular; with different sizes, it is more blurred. However, the prerequisite is that the water droplets have a size of 0.02-0.1 mm. The easiest way to observe a halo is around the moon. It can also be seen around the sun and as a circular brightness around bright stars and planets. Due to the great brightness, it is difficult to observe around the sun. You won't be as dazzled if you look at the reflection of the sun in a puddle and also wear sunglasses. A halo is particularly often observed with altocumulus clouds and thin layered clouds that allow the moon to shine through. You can easily create a halo yourself by breathing on a window pane at night, going close to the pane with your eye, and looking at a street lamp. Around the bright disc, there can also be several corona. Each corona is blue towards the light source, then green, yellow, and red towards the outside. These rings also arise due to the diffraction of light at small water droplets. However, the water droplets must be approximately the same size. In this case, the diffraction effect of many small water droplets is identical to the diffraction at a small hole. Moreover, many similar small spheres (water droplets) act like a single hole. The colors of the corona are based on the fact that the diffraction effect depends on the wavelength of the light. When the droplets are of different sizes, the colors overlap and distribute over a larger area, so no colored rings are visible.