Tuesday, 5 January 2010

Lunar odd radius display in Fairbanks

Fairbanks was treated to a nice odd radius lunar display on the morning of December 31, 2009. The image above consists of four of my frames stacked by Jari Luomanen. I hope to have more photos and information on the display available on my site ( 1 ).

Walt Tape


  1. What a magnificient halo night you've had! Delightful 23 plate arc, huh, not to mention all the other delicacies.

    Nice to see that you have set up a homepage as well.

  2. The pyramidal helic arc is nothing short of stellar. To my eye, the 18 deg plate arcs also exhibit some curvature, which is rare. The 23 deg plate arc is of record quality.

    Riikonen and I have tried to find pyramidal displays during the present cold snap but so far with no results. Apparently minus 25 to 28 degrees Celsius is not enough for consistent results.

  3. That was a fantastic New Year's Eve treat for you, Walt. And the link to your homepage is a special treat for us.

  4. Now thats a good one Walt!!! I can see that the 18d plate arcs have the curve like I see in halosim. The helic arc I think is the best. Try that in a spotlight.

  5. I don't recall seeing a pyramidal heliac arc that long. Has this observed visually in this or any other case?

    How much difference is there between the parry heliac arc and pyramidal one? Could they be seen together?

    Magnificent stuff, still waiting for something like that to appear on upper clouds of southern finland..

  6. Nice you were able to get all plate arcs in your display. I wonder what you would have gotten with a hi-power spotlight.

  7. Walt is back on the horse. This must be the best odd radius plate arc display photographed. Every time I look at the photo I realize that I am staring at the pyramid helic arc - it is of record intensity and length. Leena Virta and I saw it (or similar halo which may have different crystal origin) in the Lascar display in 1997 through mirror. I don't know of any other visual observations.

    Don't know about how big the difference is between the normal and pyramid helic arcs. Make simulation, Lauri. But pyramid helic arc never seems to make whole loop because crystals are always tilting too much.

  8. To answer some of your questions:

    Michael, at only one time during the display did I turn on the spotlight. The crystal precipitation was heavy at that time, but the spotlight halos were not good---I was seeing mostly random sparkles. Perhaps I was not in the main crystal swarm. Or perhaps---I can't remember---this was not a good stage of the display; it was definitely not at the time of the above image.

    Lauri, I did not see the odd radius helic arc (if that is indeed what it is). In fact, at the stage of the display in the above image, I noticed only the 18 and 23deg arcs, both glorious. But I was intent on the photography, so perhaps there was more. By the way, I did not photograph the display at its peak. Even at 5:00am on the outskirts of Fairbanks you cannot just stop your car in the middle of the road and set up a tripod and camera.

    The Parry helic arc theoretically forms by reflection in crystal faces that are inclined 60deg to the horizontal. The odd radius helic arc theoretically forms the same way except that the faces are inclined 62deg (90 - x = 90 - 28) instead of 60deg. So you would expect the two arcs to be similar. On the other hand, the same argument suggests that the upper 23 plate arc should be similar to the upper suncave Parry arc. The 23 arc in the image is similar in shape and location to a Parry arc, but its power is staggering compared to any Parry arc that I can recall. So there is a lot that we are not understanding.

  9. For some reason I couldn't sleep on this, so here goes. I didn't have a MC simulator installed on my laptop but fortunately the scenario is simple enough for a geometric solution. With a difference of two degrees in reflecting angles the separation seems minute inside the 22 deg. halo (angular ticks are at 5 deg. intervals).


    Nevertheless I'd sure expect Mikkilä to catch both of these simultaneously at Sotkamo within a couple of years. ;)

    By the way this was a great motivator to write the fisheye projection stuff my simple MC simulator was missing (something I plan to write as a special assignment for my optics major).