The semi-major axis is one half of the major axis, and thus runs from the centre, through a focus, and to the edge of the ellipse; essentially, it is the radius of an orbit at the orbit's two most distant points.
The semi-major axis is one half of the major axis. The semi-major axis thus runs from the centre and to the edge of the ellipse. This is through a focus. It is the radius of an orbit at the orbit's two most distant points. This is essentially.