|White Parhelion Two images by Tom Faber taken near Marietta, Georgia, USA. ©Tom Faber
|Parhelia or sundogs extend further from the sun than the familiar ones at around 22°. Others have been reported along the parhelic circle at 44, 90, 120 and 180 degrees from the sun. Those at 90° likely do not exist and brightness opposite the sun at 180° is due to the intersection of two or more halo arcs.
Tom's parhelion is 120° distant. It can be formed by rays entering the top faces of hexagonal plate ice crystals and being internally reflected twice from vertical side faces before leaving through the lower face. Other similarly tortuous rays paths contribute also.
The entrance and exit rays make the same angle to the large flat crystal faces. The colour separation, dispersion, at the first refraction is therefore cancelled by that at the second. There is no net dispersion and the parhelion is colourless
The complex ray paths are best favoured in thick plates or those that are non regular hexagons and having a more triangular aspect. Non regular hexagons frequently occur but the angles between their faces remain fixed at 90 and 120°.
Always look for 120° parhelia when there is a bright '22°' sundog. 120° parhelia are rare but can also be overlooked when projected against white hazy cirrus.