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From Steele, Joel Dorman. New Descriptive Astronomy. NY: A. S. Barnes and Company, 1884. Scanned by the author from the original.
In the context of explaining the elliptical motions of the planets, this figure illustrates the method of constructing an ellipse using a piece of string. This in turn depends upon the mathematics of the ellipse: the distance from one focus to any point on the ellipse, plus the distance from that same point to the other focus (plus, if you wish, the distance between the foci, which is constant) is constant.
