Lunar Circumhorizon Arc

While imaging a fairly strong 22 degree halo against the background stars I composed the shot to include Orion and Sirius. Although it wasn't obvious to the eye I also recorded a circumhorizon arc, the coloured horizontal band which passes just under the bright star Sirius at lower centre. The moon altitude was 66 degrees, about as high as it can get at this latitude and a good 5 degrees higher than the sun at the summer solstice. Solar circumhorizon arcs are near the limit of visibility from here, so are fairly weak, but the extra few degrees made the lunar one bright enough to be recorded.

The presence of the strong 22 degree halo suggests some column crystals were involved, so this could also be an infralateral arc which would occupy the same position.

Unfortunately the cirrus is very good at reflecting distant light pollution so the background is a dirty brownish tint, making it harder to pick out the halo colours.

A shorter exposure with the camera centred on a different position shows the arc is in the same place, confirming that it wasn't a lens reflection.

Canon EOS 20D   26th February 2007
Wiltshire, England

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