Parhelia, or sundogs, are caused when atmospheric crystals become aligned in the atmosphere, typically in still conditions, and a localized bright spot occurs, often rainbow coloured. On earth-like worlds bright sundogs occur at 22° radius from the local sun, with additional parhelial arcs, circumzenithal arcs and pillars in good observing conditions.
Other planets with different crystal suspensions display different patterns of sundogs, the cubic crystals of CO2 giving at least six arc-like parhelia and other tangential arcs, while methane and ammonia suspensions give even more complex displays that may even be superimposed in thick stratified atmospheres.
Artificial parhelia on worldhouse planets are due to the polyhedral cells used in the overhead membrane which contains and controls the atmosphere, which are too small to be seen with the baseline eye. The parhelia pattern may be triangular or hexagonal, stretching over 90° from the local sun's position, depending on the cell design, although some Disarchy worldroofs have square cells and display accordingly.
Text by Steve Bowers
Initially published on 30 November 2002.