# ellipse

Not round circle with two centers.

In mathematics, an ellipse is a curve in a plane surrounding two focal points such that the sum of the distances to the two focal points is constant for every point on the curve. As such, it is a generalization of a circle, which is a special type of an ellipse having both focal points at the same location. The shape of an ellipse (how "elongated" it is) is represented by its eccentricity, which for an ellipse can be any number from 0 (the limiting case of a circle) to arbitrarily close to but less than 1.

Ellipses are the closed type of conic section: a plane curve resulting from the intersection of a cone by a plane. Ellipses have many similarities with the other two forms of conic sections: parabolas and hyperbolas, both of which are open and unbounded. The cross section of a cylinder is an ellipse, unless the section is parallel to the axis of the cylinder. Wikipedia, Ellipse

"At the left of the drawing two particles are turning upon their gravity shafts which could be electrons, planets or suns. Around these spinning masses are circles with arrows which show the direction of their turning. Naturally these circles show as an ellipse because they follow equators and are shown in perspective." [Atomic Suicide, page 295]