High Quality Content by WIKIPEDIA articles! Xallarap is a variation in a gravitational lensing observation caused by the orbital motion of the source. A more traditional and similar effect, parallax, is the variation caused by motion of the earth around the sun. Since the two effects are converses of each other, this led to the name xallarap, which is parallax spelled backwards. A survey of microlensing attributes the first use in print to Bennett in 1998, though informal usage likely preceded this. Gravitational lensing occurs when a distant object and a massive intermediate object form a straight line as seen from Earth. Then the gravitational field of the intermediate object bends the light from distant object, magnifying it. When the two objects are stars, as opposed to galaxies, it is called gravitational microlensing. The alignment must be very precise, in fact so precise that Albert Einstein concluded "there is no great chance of observing this phenomenon".

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