Description: An ellipse is a two-dimensional shape that resembles a squashed or elongated circle. The widest distance across an ellipse is called the major axis and the shortest distance is called the minor axis. An ellipse has two foci (plural of focus) that lie along the major axis with both having the same distance from the widest points. At any point on the ellipse, the sum of the distances to the two foci is constant. The eccentricity, e, of an ellipse defines how squashed it is and lies within the range 0
A closed orbit such as the Earth's orbit around the Sun follows the shape of an ellipse. An orbit is characterized by the semi-major axis (half the size of the major axis) and eccentricity, but to fully describe an orbit the orientation of the ellipse also needs to be known.

Unclosed orbits such as comets that make only one visit to the inner Solar System before being flung out into interstellar space follow parabolas (e=1) or hyperbolas (e>1).

Related Terms:

• Kepler's Laws
• Orbit

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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