Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2002 January 14
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Sun Halo at Winter Solstice
Credit & Copyright: Philip Appleton (SIRTF Science Center), Caltech

Explanation: Sometimes it looks like the Sun is being viewed through a large lens. In the above case, however, there are actually millions of lenses: ice crystals. As water freezes in the upper atmosphere, small, flat, six-sided, ice crystals might be formed. As these crystals flutter to the ground, much time is spent with their faces flat, parallel to the ground. An observer may pass through the same plane as many of the falling ice crystals near sunrise or sunset. During this alignment, each crystal can act like a miniature lens, refracting sunlight into our view and creating phenomena like parhelia, the technical term for sundogs. The above image was taken in the morning of the 2000 Winter Solstice near Ames, Iowa, USA. Visible in the image center is the Sun, while two bright sundogs glow prominently from both the left and the right. Also visible behind neighborhood houses and trees are the 22 degree halo, three sun pillars, and the upper tangent arc, all created by sunlight reflecting off of atmospheric ice crystals.

Tomorrow's picture: Fifth Most Powerful Explosion

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA/ GSFC
& Michigan Tech. U.