§ DICTIONARY · CONCEPT
ellipse
Closed curve where the sum of distances from any point to two foci is constant.
§ 01
Definition
An ellipse is a stretched circle. The defining property: pick any point on the curve, draw a line from it to each of two interior points called the foci, and the two lengths will always add to the same number. A circle is the special case where the two foci coincide.
Kepler's first law says that every planet moves on an ellipse with the sun at one focus. The shape is characterized by two numbers — the semi-major axis, which sets the size, and the eccentricity, which sets how squished the shape is.