Glossary term: Ellipse

Redirected from Semi-Major Axis

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:

See this term in other languages

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

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".