semi-major axis
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A semi-major axis is half the longest distance across an ellipse, measured from the center to the edge. It's important because it helps describe the size and shape of orbits, and is used to calculate how long it takes objects like planets to complete their orbits around stars.
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Article
The semi-major (a) and semi-minor (b) axes of an ellipse
In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longest semidiameter or one half of the major axis, and thus runs from the centre, through a focus, and to the perimeter. The semi-minor axis (minor semiaxis) of an ellipse or hyperbola is a line segment that is at right angles with the semi-major axis and has one end at the center of the conic section. For the special case of a circle, the lengths of the semi-axes are both equal to the radius of the circle.