A false sunset or sunset sundog is a very particular kind of parhelion, belonging to the optical phenomenon family of halos.
It is an atmospheric optical phenomenon associated with the reflection and refraction of sunlight by small ice crystals making up cirrus or cirrostratus clouds in the very particular case when the late-afternoon sun is still a few degrees above the horizon.[1] The reflection or refraction is caused by large numbers of flat ice crystals in the high-altitude clouds. Unless disturbed by turbulence, flat ice crystals such as snowflakes will orient themselves parallel to the Earth's surface as they fall. If the ice crystals are randomly oriented due to turbulence, a halo is seen; if they are oriented parallel to earth's surface, two sundogs of the same latitude as the sun is seen; if they are oriented perpendicular to the earth's surface, a lone sundog is seen below the true sun. The region of ice crystals acts as a large mirror, creating a virtual image of the sun which appears to be touching the horizon. Refraction happens also as sunlight enters the ice crystal, and may cause the virtual image of the sun to have a different chromacity.
The light of a false sunset has less power than that of a real sun, but sometimes is amazingly similar in its visual behaviour at distance. If the real sun is obstructed from view by clouds or other objects, the false sunset appears like a minuscule sun touching the horizon[2]. It is similar to a subsun, because in a false sunset the sun is above the horizon and the ice crystals are below it.[3]