Wednesday, 8 November 2006

Lunar Diamond Dust Display in Pälkäne, Finland

In the evening of 4 November this lunar display graced the sky soon after sunset. Among the halo forms observed are the rare heliac arc, Tape arcs and Moilanen arc. The Moon elevation is ca. 16°.

In the image the entire length of the heliac arc is visible as a beautiful loop encircling the zenith. The Moilanen arc is very intensive and appears to extend beyond the 22 degree halo. Tape arcs can be seen as diffuse brightenings on the supralateral arc.

In the beams of the passing cars a bright pillar, parhelia, upper tangent arc and Moilanen arc were clearly visible. These proved difficult to photograph though. At the local ski resort the snow guns were operating during the display and the crystal swarm was a by-product of this activity. The temperature was below -15 °C.


  1. Hello Jari,

    I believe you could add the subanthelic arc in the list of rare halo forms included in the display. It seems to me that this extremely rare halo arc is visible in the lower photo on the right, tangenting the heliac arc. Are there more photos showing the anthelion region?

  2. Hello Reima!

    Indeed, I kind of thought so myself and a few other people have suspected this as well. I decided not to mention anything about it in my post as it is such a faint apparition that it might not be obvious to every viewer.

    As to the other images covering that region, yes I did shoot quite a few. However, the display was at its best as soon as I got there - when it was getting dark enough to shoot in moonlight. The main image is actually a test shot I did to determine the correct exposure. I did this as soon as I had set up my gear, just in case the display would grow weaker. Alas, this is just what happened over the next few minutes save for the Moilanen arc that was blazing rather intensively for a while still. So I guess I’m lucky to have those early shots even though I could well do without the power cable slashing through the frame…

    By the time I got around to shooting zenith centred shots the heliac arc had already lost some of it’s original brightness. I’d wager a guess that the possible subanthelic arc might also have been stronger a few minutes earlier but of course that’s just conjecture on my part. So the image displayed along with one other frame is the best evidence that I have. I did try stacking them but the trailing of the stars proved to be a significant distraction.

    As I got interested I did ask one person unfamiliar with rare halos to point out every arc she could tell and she also pointed out the faintly glowing trail that tangents with the heliac arc. I also run a crude simulation and it appears to match with the photo (although I had some trouble matching the projection completely). On the other hand, one person has said that it is too faint to tell so I guess the jury’s still out on this one.

    Either way, this is great! Nothing more exciting than going through the shots and trying to understand the display in depth.

  3. I have a question when is there going to be a crystal sample taken next time a Moilanen arc shows itself especially as bright as that one was?