Astronomy Picture of the Day

Explanation: This x-ray image of the Moon was made by the orbiting ROSAT (Röntgensatellit) Observatory in 1990. In this digital picture, pixel brightness corresponds to x-ray intensity. Consider the image in three parts: the bright hemisphere of the x-ray moon, the darker half of the moon, and the x-ray sky background. The bright lunar hemisphere shines in x-rays because it scatters x-rays emitted by the sun. The background sky has an x-ray glow in part due to the myriad of distant, powerful active galaxies, unresolved in the ROSAT picture but recently detected in Chandra Observatory x-ray images. So why isn't the dark half of the moon completely dark? New Chandra results also suggest that a few x-rays only seem to come from the shadowed lunar hemisphere, but instead originate in Earth's geocorona or extended atmosphere which surrounds the orbiting x-ray observatories.