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...
Found on http://en.wikipedia.org/wiki/Xallarap
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