Saturday, December 6, 2014

Halo, Sunset and a Twilight Surprise

It is December in Tucson.  We are a long way from shoveling snow or even having what anyone would consider cold temperatures, but we've enjoyed some clouds and even rain this week, our first since October!  Over a half inch fell at the airport Friday, bringing our annual total to 10 inches, still a couple inches short of our yearly average with 3 weeks to go.  After enjoying a party at a friend's house last night, before turning in I noticed a nice halo centered on the full moon.  These 22 degree halos (44 degrees diameter) are caused by refraction of light through hexagonal ice crystals in the atmosphere.  In the image at left - a hand-held half second exposure (!), the inner edge of the halo has a bit of a yellowish or reddish tinge.  Redder wavelengths are refracted less strongly, so appear on the inner edge of the halo.  This effect was seen visually as well.

The weather continued dreary all day, but when out doing errands this afternoon, some breaks were noticed, and the forecast's promised clearing appeared imminent!  I talked Melinda into heading west to Gates Pass (over the Tucson Mountains) for a great view west of town.  We arrived literally seconds before sunset, and I managed to catch only a couple frames before the sun's disk dropped below distant mountains.  While it had mostly cleared, a few residual clouds dotted the western sky.  Of course, the usually standard clear skies provide boring sunsets - a few clouds always make them spectacular!  True to form, at right, a cropped version of a 7-frame mosaic (assembled in Photoshop) shows some nice cloud colors.  Taken with a 300mm lens I'm evaluating, it shows considerable detail - note Kitt Peak National Observatory at far left...

Now people always claim that the Arizona sunsets photos must be fakes or manipulated to show more colors.  Well, it is not true - the image at left here is taken straight out of the camera, no manipulations other than reducing it in size - colors are real!  I've mentioned before that they are caused by the clarity of the air here.  Long paths through the atmosphere absorbs bluer wavelengths, intensifying the yellows and reds to the visual impression.  Of course, while the sun had long set for us, it was still shining on the cloud bottoms.  A few minutes later, with the sun lower for the clouds, the colors got even redder.  At right, the profile of Kitt Peak's telescope domes is back-lit by the late twilight reds.

We hung out a few minutes, looking for a friend in the twilight. Venus passed superior conjunction (the far side of the Sun, from the Earth's perspective) over a month ago on October 25, and it is slowly creeping into our evening sky. It has been over 11 months since we've seen it in the evening sky - it was actually inferior conjunction day this last January 11th.  Anyway, I suspected it would be just above Kitt Peak, and sure enough, Melinda, with her surgically-enhanced eyeballs spotted it before me where I described.  I picked it up right away after she pointed it out.  It is labeled in the image at right.  Fortunately, it fit nicely into the 300mm lens, just above the cloud above the Observatory.  It is nice to see our planetary friend - it will be fun to watch it return to keep us company in the post-sunset sky.

No comments: