January 15, 2002
Circumscribed arcs, such as the one shown above, arise from the merging of the upper and lower tangent arcs, which on occasion wrap completely around the Sun (see the Earth Science Picture of the Day for November 19, 2001). These arcs can occur when the Sun is at an elevation above the horizon of approximately 29 degrees. They consist of the same crystals that compose tangent arcs, namely columnar crystals, all oriented in the same way with a very small tilt. If the Sun is less than 35 degrees above the horizon, the sides of the circumscribed halo are imperceptible. On the above photo, a pencil is used to partially block the Sun from the camera's view.