Xallarap

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 lead to the name "xallarap", which is parallax spelled backwards. A survey of microlensing [cite web |url=http://arxiv.org/abs/astro-ph/0304252 |title=The First Heroic Decade of Microlensing |author=N.W. Evans ] attributes the first use in print to Bennett [Bennett, D.P., 1998, Phys. Reports, 307, 97. ArXiv preprint: cite web |url=http://arxiv.org/abs/astro-ph/9808121 |title=Magellanic Cloud Gravitational Microlensing Results: What Do They Mean? |author=David 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, in an early paper [cite journal | first = Albert | last = Einstein | authorlink = Albert Einstein | title = Lens-like Action of a Star by the Deviation of Light in the Gravitational Field | journal = Science | volume = 84 | pages = 506–507 | year = 1936 | url = http://links.jstor.org/sici?sici=0036-8075%2819361204%293%3A84%3A2188%3C506%3ALAOASB%3E2.0.CO%3B2-8 | doi = 10.1126/science.84.2188.506 ] , concluded "there is no great chance of observing this phenomenon". However, modern surveys such as the Optical Gravitational Lensing Experiment (OGLE) observe millions of stars each night, and see microlensing many times each year.

Since the alignment must be so precise, if the event last more than a few weeks, scientists can observe changes as the Earth moves around the sun, since this movement changes the alignment. Traditionally in astronomy, a change in view caused by the Earth's motion is called parallax, and this is the term used by researchers for this effect. However, if the source star is part of a binary system, then it too has orbital motion, and this can modify the alignment just as the Earth's movement can. Since both effects are caused by the effect of orbital motion on alignment, they are very closely related. And since the effect is same as parallax, just backwards (from the motion of the source rather than motion of the observer) it was called "xallarap". The name stuck, and is now commonly used in the astronomical literature.

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