Astronomy Picture of the Day

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

August 31, 1995

X-Raying the Moon
Credit: DARA, ESA, NASA, Max Planck Institut, J.H.M.M. Schmitt

Explanation: Above is a picture of the Moon taken in X-rays by the Roentgen Observatory Satellite ROSAT in 1990. This famous picture shows three distinct regions: a bright X-ray sky, a bright part of the Moon, and a relatively dark part of the Moon. The bright X-ray region is exemplary of the mysterious X-ray background that is seen everywhere on the sky. The bright lunar crescent shines because it reflects X-rays emitted by the Sun. The dark lunar face is surprising because it is not completely dark, and its slight emission is thought to result from energetic particles from the solar wind striking the Moon.

Tomorrow's picture: Subrahmanyan Chandrasekhar 1910-1995

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