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Wikipedia:Today's featured picture (animation)/January 20, 2007 - WikipediaAn animated image showing the definition of pi. A number line is marked off by a circle of unit diameter. Starting from zero, the circle "unrolls" its circumference. At one full turn, the unrolled circumference has extended to the point we call π. This number is real but irrational, transcendental, and cannot be constructed with compass and straightedge. Animation credit: John Reid Archive - More
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