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.

2003 April 21
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A Halo Around the Moon
Credit & Copyright: Sarah McKay

Explanation: Have you ever seen a halo around the Moon? This fairly common sight occurs when high thin clouds containing millions of tiny ice crystals cover much of the sky. Each ice crystal acts like a miniature lens. Because most of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo. A similar Sun Halo may be visible during the day. The picture was taken in Lansdowne, Pennsylvania, USA. The distant planet Jupiter appears by chance just to the left of the Moon. Exactly how ice-crystals form in clouds remains under investigation.

Tomorrow's picture: North Mars

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