|Lunar Circumzenithal Arc
Eva Seidenfaden (Paraselene Optics Site) in Germany had a camera ready to image noctilucent clouds but captured this rarity instead.
"Maybe halos have become so rare over here because they only come out at night? Last night I noticed two decent moondogs, so I went out with a camera that was already prepared for imaging any NLCs that appeared. Not a good halo display, but who cares. I saw what I had been waiting for for 12 years – a lunar CZA. It might be much more common, but not here, here it is as rare as a Wegener arc. More than a half moon, a more or less clear sky, a band of cirrus and the moon just low enough. I missed the best moment because I had difficulties pointing the camera so high, the tripod ball head wouldn't allow it. I had to shorten one tripod leg and hold the whole construction with my hands and point."
Eva captured NLCs on the following night - image below.
©Eva Seidenfaden, shown with permission.
Expect to see a solar circumzenithal arc - the most beautiful of all the halos - on average once a month in Europe and parts of the US. If you search the zenith regularly that is.
Sundogs are good harbingers of CZAs for both are produced by hexagonal plate-shaped ice crystals.
|Plate crystals drift in high cirrus cloud air currents with their large hexagonal faces almost horizontal. CZA forming rays enter the topmost face and leave through a side face. The refraction is effectively through a 90° prism and the colours are both pure and well separated. The sun or moon must be less than 32° high for the ray path to be possible.|