In the opposite direction of the sun a bright spot appeared in
an unusual bright circle (see Rainbows
and Glories). A separation
of colours was not recognizable. The event lasted for approximately
30 min. Intensity and clear visibility varied during that time
interval. All images were taken with a polarisation filter. Antihelion3.jpg
and Antihelion5.jpg show different parts of the horizontal arch
where also a small parhelion with colour separation occurred.
It was hard to find similar pictures in the literature. In the
book "From the rainbow to polar lights" (K. Schlegel,
Spektrum Publishers, 1995) a similar picture can be found. It
is described there as follows: "The very rare antihelion
belongs to the phenomena of reflection halos. It is embedded in
a horizontal circle around the horizon which is located at the
level of the sun. [
] The antihelion is very likely caused
by an angular reflection at a quadruplet crystal. It appears as
a swelling of the horizontal circle." Quadruplet crystals
look like a cross ("X"). If a light beam hits the top
left leg of the "X", it will be reflected at 90 °
and then hits the lower left leg of the X-crystal.
Here a second 90 ° reflection takes place, this time to the
left in direction of the incident beam. In the end a reverse or
180 ° reflection of the incident beam occurs.
Therefore the antihelion has to be located directly in opposite
direction to the real sun at the sky. This was the case for the
image mentioned above. The sun has to be already (or still) low
enough otherwise if the sun is raised to high the opposite point
is below the horizon. This optical phenomenon is quite rare, because
a sufficient concentration of X-like crystals is rarely formed.
These quadruplet crystals sediment with perpendicular alignment
to the X-plane. Therefore enough "reflection surfaces"
are aligned such that plenty of light beams can be reflected.
So the intensity of the reflex will be high enough to be well
visible. However not all of these "mirrors" are perfectly
aligned so that the antihelion might have a larger angular extension
than the sun itself. (If all "mirrors" were perfectly
parallel, the sun and its picture would have the same angular
extension at the sky.)
In the book "Rainbows, Halos and Glories" (R. Greenler,
Cambridge, University Press, 1980) the phenomenon is described
differently as "Anthelic Arc" (or "Wegener and
Hastings Arc"). According to this explanation hexagonal column-like
crystals play an essential role in the generation of an antihelion.
According to this the phenomenon should occur more often.
Antihelion1.jpg to Antihelion8.jpg: S. Borrmann, Brezzo di Bedero,
Lago Maggiore Region, Northern Italy, 22 August 2004, 4:23 p.m.
to 4:27 p.m.
Olympus E-1 SLR with 14-200 mm objectives. Exposure and selectivity
choice (100 ASA) was performed by the program automation. Colour
range (sRGB) and resolution (2560 x 1920 Pixel x 24 colours) was
the same for all images.