How is an ellipse mathematically defined based on the sum of distances relative to its two fixed points?

Answer

As the set of all points where the sum of the distances to the two fixed points (the foci) is constant.

The formal geometric definition of an ellipse relies entirely on its two foci. An ellipse is precisely defined as the locus of points in a plane such that the sum of the distances from any point on the curve to the two fixed points (the foci) remains perpetually constant. When applying this to a planetary orbit, one focus holds the Sun, and the other focus remains empty space. This constancy of the sum of distances dictates the oval shape and the relationship between the semi-major axis and the focal distance.

How is an ellipse mathematically defined based on the sum of distances relative to its two fixed points?

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