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.

2009 May 12
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A Circumhorizontal Arc Over Ohio
Credit & Copyright: Todd Sladoje

Explanation: Why would clouds appear to be different colors? The reason here is that ice crystals in distant cirrus clouds are acting like little floating prisms. Sometimes known as a fire rainbow for its flame-like appearance, a circumhorizon arc lies parallel to the horizon. For a circumhorizontal arc to be visible, the Sun must be at least 58 degrees high in a sky where cirrus clouds are present. Furthermore, the numerous, flat, hexagonal ice-crystals that compose the cirrus cloud must be aligned horizontally to properly refract sunlight in a collectively similar manner. Therefore, circumhorizontal arcs are quite unusual to see. This circumhorizon display was photographed through a polarized lens above Dublin, Ohio last week.

Apologies: Earlier, APOD misidentified this phenomenon as iridescence.

Tomorrow's picture: shuttle dawn

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