Celestial phenomenon

Light magic: The moon with a colorful wreath

You don't see a corona around the moon every day, and the phenomenon is best seen during a full moon.

(Bild: Diverse Fotografen honorarpflichtig/Rubina Bergauer)

Recently, the full moon over the Ländle was surrounded by a wreath of brightly colored rings. The celestial phenomenon is caused by light and water.

Anyone who looked up at the sky during the most recent full moon phase may have been lucky enough to witness a fascinating natural spectacle. At just before half past midnight on 13 January, the moon was surrounded by brightly colored rings. This was neither a rainbow in the classic sense nor the Northern Lights, which have often been observed in our latitudes in recent times.

The apparition only lasted a few minutes before the halo went out and the magic dissipated. "It's not an everyday phenomenon, but it's not extremely rare either," says physicist and astronomy expert Robert Seeberger. When thin clouds pass by the Earth's satellite at night, you can often see that the moon is surrounded by a bright halo.

The scientific name for this luminous phenomenon is corona, and the bright disk in the center of the corona is called an aureole. In contrast to halo effects or rainbows, which are caused by the refraction of light, the rays of light are bent during corona phenomena, resulting in a luminous ring or, under favorable conditions, colored rings surrounding the aureole. From the inside out, the colors blue, green, yellow and red can often be seen. Coronae can usually be observed around the moon, less frequently around the sun. This is not because they occur less frequently around the sun, but because the sunlight usually outshines the corona phenomenon.

Coronae can vary greatly in size; the angular diameter of the outermost rings can be up to 15 degrees Celsius. As clouds pass through, the appearance can change - the diameter can grow or shrink, rings appear or disappear again, depending on how the size of the water droplets changes.

Colored rings
The website of the "Arbeitskreis Meteore e.V." states that if the droplets are approximately the same size, the halo around the moon appears circular. If the size is unequal, it appears rather blurred. According to experts, water droplets between 0.02 and 0.1 millimeters in size are required for a corona to appear. However, for colored rings to form around the moon, the water droplets on which the light is diffracted must be approximately the same size.

If the droplets are of different sizes, the colors overlap and spread over a larger area, so that no colorful ring can be seen. Lunar cornonas therefore depend on certain atmospheric conditions, which are not always present. "However, they occur more frequently than you might think, especially with thin clouds or veil clouds consisting of tiny drops of water," summarizes Seeberger. However, the cloud cover in the sky must not be too dense; an otherwise clear or semi-clear night is required. The brightness of the moon also plays a role. The celestial phenomenon is best observed when the moon is full, as there is then enough light to see the wreath.

Nordic mythology
In earlier times, this natural phenomenon was often interpreted as a sign or omen for upcoming important events. In particular, a "red glow" in the corona was seen as a warning of war or other disaster. In the folklore of some regions of Eastern Europe, it is said that wishes made while the moon is wearing a corona have a good chance of coming true. The celestial phenomenon also found its way into Norse mythology. It was interpreted as a portal between the world of the living and the spirits. In this context, it was assumed that the souls of the deceased could pass through the "ring" around the moon on such nights.

Today, there are scientific explanations as to how this natural spectacle comes about - but it has lost none of its magic.