Physics Photo of the Week

November 9, 2012

Moon halo with tangent arc
Notice the rainbow colors in this spectacular lunar halo taken in October, 2010.  Lunar (and solar) halos are fairly frequent sights caused by ice crystals present in cirrus clouds in the atmosphere.  The amazing features of this halo are not only the colors (similar to the colors produced by glass prisms).  This halo also produces the "upper tangent arc" - the larger radius arc that is tangent to the main halo at the top - and the "circumhorizontal arc" - short sections of tilted circles emanating from beyond the halo at about "2:30" and "9:00" positions on the circle.

A Sun halo has been described before (PPOW for April 6, 2012).  The main circle is caused by the refraction of hexagonal pencil-like ice crystals.   The ice crystals are gently falling in the air.  The friction with the air causes objects to orient themselves to maximize the drag.  Leaves and confetti fall flat, not edge on.    The upper tangential arc is also caused by the same hexagonal pencil-like crystals.  In both the main circle and the upper tangential arc, moonlight (or sunlight) enters one face of the hexagon, and emerges two facets from where it entered.  See the diagram below.  The long axes of the needle-like ice crystals are horizontal, but randomly directed in the horizontal plane.  For an ice crystal in the main halo to contribute to the halo, the orientation of the long axis of the crystal is 90 degrees to the Moon's ray (or Sun's) and the refraction for ice is 22 deg from the original direction.  For a crystal to contribute to the upper tangential arc, the ray of moonlight (or sunlight) enters the face of the crystal obliquely, not perpendicular to the long axis of the crystal, thus resulting in a much more complicated halo.  The drawing actually shows the oblique incidence needed for the upper tangential arc.  The upper tangential arc is more completely described at the following link:

Finally, the faint arcs leading out of the halo at the 2:30 and 9:00 positions are the result of refraction of hexagonal plate crystals that resemble small hexagonal disks rather than pencils.  Because of aerodynamics the plate-like crystals fall with a horizontal orientation.  If the Moon (or Sun) is close to the horizon, these "circumhorizontal" arcs produce a sundog (or parhelia - see PPOW for Feb. 8, 2008).  Because the Moon's elevation is considerably higher than the horizon in today's photo, the parhelia lie outside the 22 degree halo.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to

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