December 20, 2023
In the time it took you to read this, the Earth moved another thousand miles through space. You are not standing still. You never have been.
Ice Halos over Bavaria
Birthday · December 20, 2023
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APOD published December 20, 2023. Credit: Bastian Werner. A publication-date match, not a photo taken on your date. Independent of NASA.
What's causing those unusual sky arcs? Ice crystals. While crossing a field of fresh snow near Füssen, Bavaria, Germany, earlier this month, the photographer noticed that he had entered an ice fog. For suspended water to freeze into an ice fog requires quite cold temperatures, and indeed the air temperature on this day was measured at well below zero. The ice fog reflected light from the Sun setting behind St. Coleman Church. The result was one of the greatest spectacles the photographer has ever seen. First, the spots in the featured picture are not background stars but suspended ice and snow. Next, two prominent ice halos are visible: the 22-degree halo and the 46-degree halo. Multiple arcs are also visible, including, from top to bottom, antisolar (subsun), circumzenithal, Parry, tangent, and parhelic (horizontal). Finally, the balloon shaped curve connecting the top arc to the Sun is the rarest of all: it is the heliac arc, created by reflection from the sides of hexagonally shaped ice crystals suspended in a horizontal orientation.
Image credit: Bastian Werner