May 01, 2003
On the photo above, both a 22 degree halo and circumscribed halo can be seen. The 22 degree halo encircles the Sun, and the circumscribed halo is more ellipsoidal and surrounds (circumscribes) the 22 degree halo. For this to happen the Sun must be higher than about 30 degrees above the horizon. Circumscribed halos form from singularly oriented, column-shaped ice crystals. Light enters through one of the side faces of a crystal and exits through an alternate side face. The lamp-post is used to shield the camera from the Sun. See also the Earth Science Picture of the Day for January 15, 2002.