The semi-major axis of an elliptical planetary orbit is equal to the distance between the planets and the Sun at aphelion.
The term semi major axis refers to a half of the of the distance from middle of an ellipse to another end of the ellipse.
Recall that the solar system has the sun at the center if the ellipse. Hence, the semi-major axis of an elliptical planetary orbit is equal to the distance between the planets and the Sun at aphelion.
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