Summer's Halo by Moonlight

Michael Ellestad imaged this lunar circumhorizon arc - normally a summer visitor - on the night of 26th November from Ohio, USA.  
©Michael Ellestad, shown with permission.

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed

The circumhorizon arc is a huge and colourful halo beneath the sun and running parallel to the horizon.    It can only form when the sun is higher than 58° and is thus is purely summer sight  from mid latitude locations.    But in winter the near full moon rides high in the sky and offers another opportunity to see the arc.

The moon can be even higher than the summer sun because its orbit is inclined 5.1° to that of the ecliptic (the plane of Earth's orbit around the sun) allowing CHA deprived areas like northern Europe their only chance to see it.

Rays entering the near vertical side faces of plate shaped ice crystals and leaving through the base facet form the halo.   When the sun or moon is 58° high the emerging ray grazes the lower face.   At lower altitudes the ray does not emerge at all but is instead totally internally reflected.