... thanks to arXiv:
This event is VERY interesting and unusual because the microlensing event was observed from two very different places: on Earth, and from the Spitzer Space Telescope, which is many millions of km away from the Earth. Gravitational lensing occurs when a background star and a lensing star line up exactly in the same direction, as seen from an observer. Because Spitzer was so far away, it saw the lensing star line up with the background star first; then, as the lensing star moved in its orbit around the center of the Milky Way, the lensing star eventually lined up with the background star as seen from Earth, about 18 days later.
This lag in time between two widely separated observers seeing a lensing event will help us to figure out exactly how the two stars involved in the event were moving, and where they are, and other properties. Since most telescopes are located on Earth, in basically the same place, we almost never get this extra information.
Rah, rah, Spitzer! Rah, rah, OGLE!