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Kepler's First Law definitions

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  • Ellipse
    A closed curve on a plane surrounding two focal points, with the sum of the distances to the foci constant.
  • Foci
    Two fixed points on the interior of an ellipse used in its formal definition.
  • Major Axis
    The longest diameter of an ellipse, passing through its foci.
  • Semi-Major Axis
    Half of the major axis of an ellipse, representing the average distance from the center to the perimeter.
  • Minor Axis
    The shortest diameter of an ellipse, perpendicular to the major axis.
  • Semi-Minor Axis
    Half of the minor axis of an ellipse, representing the shortest distance from the center to the perimeter.
  • Periapsis
    The point in an orbit closest to the focus, such as the sun in a solar orbit.
  • Apoapsis
    The point in an orbit farthest from the focus, such as the sun in a solar orbit.
  • Perihelion
    The point in the orbit of a planet closest to the sun.
  • Aphelion
    The point in the orbit of a planet farthest from the sun.
  • Eccentricity
    A measure of how much an orbit deviates from being circular, ranging from 0 to 1.