December 15, 1996
Another year older means another lap aboard a world spinning through the void at impossible speed.
Microlensing of the Einstein Cross
Birthday · December 15, 1996
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APOD published December 15, 1996. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
The famous "Einstein Cross" is a case where a single object is seen four times. Here a very distant QSO happened to be placed right behind a massive galaxy. The gravitational effect of the galaxy on the distant QSO was similar to the lens effect of an empty wine glass on a distant street light - it created multiple images. But stars in the foreground galaxy have been found to act as gravitational lenses here too! These stars make the images change brightness relative to each other. These brightness changes are visible on these two photographs of the Einstein Cross, taken about 3 years apart.