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.
About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed

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.

Lunar CZAs, like all lunar halos are rarer.

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.