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.

2013 March 27
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A Horizon Rainbow in Paris
Image Credit & Copyright: Bertrand Kulik

Explanation: Why is this horizon so colorful? Because, opposite the Sun, it is raining. What is pictured above is actually just a common rainbow. It's uncommon appearance is caused by the Sun being unusually high in the sky during the rainbow's creation. Since every rainbow's center must be exactly opposite the Sun, a high Sun reflecting off of a distant rain will produce a low rainbow where only the very top is visible -- because the rest of the rainbow is below the horizon. Furthermore, no two observers can see exactly the same rainbow -- every person finds themselves exactly between the Sun and rainbow's center, and every observer sees the colorful circular band precisely 42 degrees from rainbow's center. The above image featuring the Eiffel Tower was taken in Paris, France last week. Although the intermittent thunderstorms lasted for much of the day, the horizon rainbow lasted for only a few minutes.

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Tomorrow's picture: Unraveling NGC 3169

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