The Elusive 120 degree parhelion

The Elusive 120° Parhelion

Hans Stocker captured these scenes at Haarlem in The Netherlands. The 120� parhelion is at upper left on the parhelic circle. The sun at lower right is surrounded by a 22� halo with an upper tangent arc at its top.

The 120P is white and can be difficult to distinguish from a cloudy background especially when the clouds are patchy. Hans' parhelion lasted for just a few minutes and in that time its visibility waxed and waned.

Be sure that a suspicious white spot is 120� from the sun and at the same altitude. It is probably present in the sky more often than thought.

Images ©Hans Stocker, shown with permission

In this image we have a characteristic fading of the parhelic circle beyond the 120P - A transition from total internal reflection is the cause.

Like the 22° parhelion or sundog, horizontal aligned plate crystals are responsible.

Thick crystals or ones with alternate sides long and short sides (triangular aspect) are best.

There are no colours because the entry and exit angles are the same and the net colour dispersion is zero.