Semi-major axis

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The semi-major axis (a) of an ellipse.

In geometry, the term semi-major axis (sometimes spelled out semimajor axis) is used to describe the dimensions of ellipses and hyperbolae. The semi-major axis is the distance from the center of an ellipse to the farthest point on the perimeter of the ellipse. The semi-major axis is half of the major axis, which goes all the way across the ellipse at the widest part.