Elliptical Halo About Venus
February 13, 2009
On the evening of Jan. 22, 2009, I photographed what I thought was a garden-variety halo around Venus, but examination of the image file revealed that the halo is not round, but elliptical, with the major axis perpendicular to the horizon. Though coronae and glories can appear oval when the fog or cloud droplet size changes with angular altitude, it seems unlikely that this elliptical "halo" formed in such a manner. The photo was taken with a 300 mm lens, and the field of view is therefore very small -- unusual to have strong droplet size stratification over such a small angular range. It's hard to know for certain just what caused the oval nature of this halo. The presence of stratospheric aerosols left over from the Kasatochi eruption last summer perhaps contributed to this optical effect. I had photographed volcanic twilight coloration just 24 hours prior to when I captured this image. Venus is still a very striking object in the western sky after sunset -- you can't miss it!
Photo details: The 300 mm lens I used would yield a field of 4.6 x 6.9 degrees in a 35 mm camera (film frame size 24 x 36 mm), or 11.46 arcminutes/mm. The chip size in my camera is 14.8 x 22.2 mm, yielding an effective focal length of 480 mm to the lens (8.022 arc minutes/mm or 1.98 x 2.97 degrees).
I have increased the saturation to bring out the extremely subtle colors in the halo and made very slight brightness and contrast adjustments.