Parhelic Circle & 120° Parhelion

Sighted by Dave Rowh in North Carolina. The camera looks west, to the right, of the sun. The bright part of the parhelic circle ends to the right of the 120° parhelion and there is just a hint of the blue spot.
  Image ©Dave Rowh, shown with permission

Fading parhelic circles

The parhelic circle fades noticeably past the 120° parhelion.

The change is caused by transition from partial to total internal reflection of sun rays inside ice crystals.

Internal rays steeply inclined to a surface partially escape and are partially internally reflected. As the angle becomes less steep there is a sudden change and all the light is reflected.

Prism binoculars rely on this for near perfect reflection by the erecting prisms.

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In contrast to the colourful 22° parhelion or sundog, the 120° parhelion is colourless.

It needs thick or triangular aspect plates and is thus seen less often. It is also stands out less from white clouds.

Like the 22° parhelion, it is formed by hexagonal plate crystals. One ray path is through the top face, two internal reflections from vertical side faces and exit through the lower hexagonal face.

The entrance and exit angles are equal and so the two colour dispersions cancel out.