Why does the winter moon sometimes have colored rings?

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Winter moon with partly colored corona © Radoslaw Ziomber
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The full moon shining in the night sky is a fascinating sight. Clearly, the silvery disc stands out from the surrounding darkness. Sometimes, however, the boundaries are not quite so clear: Then the moon appears surrounded by a bright yard. Sometimes the edges of this bright surface are even slightly colorful - almost like a rainbow wreath. But how can these phenomena, also called aureoles and coronas, be explained?

An answer to this question was already sought by the astronomer Joseph von Fraunhofer at the beginning of the 19th century. He already suspected that the courtyards and colored rings around the moon are not generated by the moon itself, but must be based on an effect in the earth's atmosphere. But which?

Experiment with glass beads in the light beam

The most obvious thing to the astronomer was that small particles in the atmosphere - Fraunhofer called them "haze beads" - produce this effect. They could distract and break the light of the moon. To prove it, the researcher made an experiment: he put many small glass beads on a pane of glass, dropped a beam of light through it, and observed it with a faint telescope.

In fact, Fraunhofer observed a colored rainbow shimmer around the beam of light. "The color rings that are obtained in this way, the larger, the smaller the glass beads are, " said the researchers in 1825 in his publication "Theory of Courtyards, Nebensonnen and related phenomena".

Corona on the windowpane

The effect observed by Fraunhofer can be simulated with a simple experiment, as the astronomer James Calvert of the University of Denver explains: Simply breathe a cold windowpane and look through it at a lamp or candle. You then see colored rings around the light source. display

The light rays of the lamp are deflected differently by the small droplets on the disk depending on the wavelength. This divides them into their individual color components and we see rainbow-like colors. "The red rings are usually more inside than the blue ones, " says Calvert. This is exactly how the sometimes colored rings around the moon are formed.

But Fraunhofer also found out why the yard around the moon sometimes just seems white: "If the beads have very unequal diameters, the colors can not be perceived, " he says. Because then the light will be slightly different broken each. At our eyes then only white, mixed from many wavelengths light comes on - the yard shines monochrome white.

Smallest droplets and crystals

Today, you can find out more about Fraunhofer's "haze" in the atmosphere: "In nature, these particles consist of ice crystals or water droplets, " explains Calvert. They must, however, be very small - a maximum of one tenth of a millimeter. The drops of water that give rise to a rainbow are almost gigantic when compared to a diameter of several millimeters. A colored corona often arises from tiny droplets in higher atmospheric layers, says the researcher. Particularly favorable are the so-called Altostratus clouds - fine translucent cloudy veils.

When the moonlight strikes the tiny drops or crystals, it is directed around it, deflecting it out of direction. This distraction works even with non-transparent particles such as fine dust, says Calvert. Sometimes you can observe a lunar corona even when there is a lot of pollen dust flying in the air.

30.11.2017 - NPO