this caustic-crossing time can be measured, and if the angular radius of the source is known, then again the Einstein angle can be determined. As in the single lens case when the source magnification is formally infinite, caustic crossing binary lenses will magnify different portions of the source star at different times. They can thus probe the structure of the source and its limb darkening. An animation of a binary lens event can be found at this YouTube video. Events yielding the Einstein parallax In principle, the Einstein parallax can be measured by having two observers simultaneously observe the event from different locations, e.g., from the earth and from a distant spacecraft. The difference in amplification observed by the two observers yields the component of perpendicular to the motion of the lens while the difference in the time of peak amplification yields the component parallel to the motion of the lens. This direct measurement was recently reported using the Spitzer Space Telescope. In extreme cases, the differences may even be measurable from small differences seen from telescopes at different locations on the earth. More typically, the Einstein parallax is measured from the non-linear motion of the observer caused by the rotation of the earth about the sun. It was first reported in 1995 and has been reported in a handful of events since. Parallax in point-lens events can best be measured in long-timescale events with a large —from slow-moving, low mass lenses which are close to the observer. If the source star is a binary star, then it too will have a non-linear motion which can also cause slight, but detectable changes in the light curve. This effect is known as Xallarap (parallax spelled backwards). Detection of extrasolar planets If the lensing object is a star with a planet orbiting it, this is an extreme example of a binary lens event. If the source crosses a caustic, the deviations from a standard event can be large even for low mass planets. These deviations allow us to infer the existence and determine the mass and separation of the planet around the lens. Deviations typically last a few hours or a few days. Because the signal is strongest when the event itself is strongest, high-magnification events are the most promising candidates for detailed study. Typically, a survey team notifies the community when they discover a high-magnification event in progress. Follow-up groups then