In mathematics, an ellipse is a plane curve surrounding two focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. The fixed points are called the foci, and the shape of the ellipse is in an oval shape. An ellipse is one of the conic sections, which is produced when a plane cuts the cone at an angle with the base. The elongation of an ellipse is measured by its eccentricity, a number ranging from 0 (the limiting case of a circle) to 1 (the limiting case of infinite elongation, no longer an ellipse but a parabola). Ellipses are common in physics, astronomy, and engineering. For example, the orbit of each planet in the Solar System is approximately an ellipse with the Sun at one focus point. The same is true for moons orbiting planets and all other systems of two astronomical bodies.