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.

July 26, 1999
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Noctilucent Clouds
Credit & Copyright: Pekka Parviainen (NCWG/U. Colorado)

Explanation: Sometimes it's night on the ground but day in the air. As the Earth rotates to eclipse the Sun, sunset rises up from the ground. Therefore, at sunset on the ground, sunlight still shines on clouds above. Under usual circumstances, a pretty sunset might be visible, but unusual noctilucent clouds float so high up they can be seen well after dark. Pictured above, a network of noctilucent clouds casts a colorful but eerie glow visible above the dark. Although noctilucent clouds are thought to be composed of small ice-coated particles, much remains unknown about them.

Tomorrow's picture: Chandra

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