Monday 15 December 2008

Blue circle

What happens when tilts of the crystals that make blue spot increase? The blue spot starts to elongate vertically, forming eventually a blue circle. It is shown in these images that were taken on 7/8 December night in Rovaniemi.

Simulations in the middle used plate crystal tilts of 90 degrees to make blue circle. In the lower simulation the 1-3-2 rays responsible for blue circle are shown (probably also 3-1-6 rays contributed, but they were not included).

Blue circle is a circle of about 70 degrees radius around the subanthelic point. The intersection of parhelic circle and blue circle is the location of the blue spot. This gives us a practical insight on why blue spot changes position and elongates with increasing sun elevation ( 1 ).

Also other halos are seen in the photos. Some of these are named in the simulations, which are made with Jukka Ruoskanen's software.
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7 comments:

  1. Ágnes / Ági15 December 2008 at 14:01

    Now Marko, what you have found out about the blue spot and the blue circle is such a revelation! This posting explains everything. So then, the blue circle must be somewhat smaller than 70°, around 64°, as the blue spot disappears when the sun is higher than 32°.

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  2. marko15 December 2008 at 14:21

    Good point, Ágnes! I just estimated it without any thought... Of course the blue circle radius is 64 degs as blue spot disappears at 32 degs. Thanks.

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  3. Jukka15 December 2008 at 16:22

    Marko, last year in comment section of the 13th Dec display I remember you saying something like "a freaking halo anatomy chart". Well, what is this display then? This is a freaking complete show of all light elevations, crystal populations and new sightings. Amazing night - its good you were there taking full advantage of the conditions.

    Very good note from Agnes about the radius of blue spot circle!

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  4. Michael16 December 2008 at 01:55

    I also notice that the blue circle can form from random oriented crystals at least in simulations.

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  5. marko riikonen16 December 2008 at 02:37

    Exactly, Michael. I just used 90 degs tilts to have also the Lilje and sub-Lilje parhelia. But having them makes 120 parhelia as well, which are not seen in the photos of this stage of display, and I have no receipe to get rid of them. So, as usual, we are not understanding it all.

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  6. Jari16 December 2008 at 11:48

    This was a game changing display. I also appreciate the beauty of the blue circle.

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  7. Michael19 December 2008 at 04:59

    One other thing I will say is that Jarmo's halo list needs some more additions. I had a feeling one day sub-120d parhelia were one day going tp be photographed but sub-kern that was a shocker.

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