Astronomy Picture of the Day

Explanation: At sunset, an alignment of rainbow and mountain shadow was captured in this remarkable snapshot. The stunning view was recorded from a fire lookout on Smith Peak in the Plumas National Forest near Portola, California on July 13. Still, near sunset it's no accident that the majestic mountain shadow seems to point to the centre of the graceful rainbow arc. Due to perspective and the long line of sight the mountain shadow naturally forms a tapering triangular shape, its apex positioned at the point opposite the Sun on the horizon. Following thunderstorms across the region, the rainbow arcs across the early evening sky, with its colourful and characteristic 42 degree arc formed as sunlight is refracted and reflected by the atmospheric water droplets. The geometric centre of a rainbow arc lies at the antisolar point. And in this scene from the golden hour on planet Earth, that's also oposite the setting Sun along the distant horizon.