September 14, 2002
On the day you were born, the universe was already ancient beyond measure — and yet, in all those billions of years, it had never once...
X-Ray Moon
Birthday · September 14, 2002
A little universe, just for you
Your date.
Ready to give.
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APOD published September 14, 2002. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
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. But why isn't the dark half of the moon completely dark? It's true that the dark lunar face is in shadow and so is shielded from direct solar x-rays. Still, the few x-ray photons which seem to come from the moon's dark half are currently thought to be caused by energetic particles in the solar wind bombarding the lunar surface.