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string pencil

Ellipse, geometrical figure shaped like a circle viewed at an angle. There are 2 focal points in an ellipse, and the sum of the radii from them to any point on the curve is constant. It follows that an ellipse can be drawn with the help of 2 thumbtacks stuck in a piece of paper at the 2 foci with a loop of string attached to them. If the string is kept taut by a pencil, an ellipse will be generated as the pencil is moved along the string. The orbits of planets and their satellites are elliptical.

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