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.
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°.
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.
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!
I also notice that the blue circle can form from random oriented crystals at least in simulations.
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.
This was a game changing display. I also appreciate the beauty of the blue circle.
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.