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The semi-major axis is half of the diameter of the major axis of an ellipse by definition. So how does that make it any different from apoapsis, the furthest point an object is from a parent body in its orbit?

Both are radii along the longest axis, surely.

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    $\begingroup$ The central body is at one focus of the ellipse, not at the centre. $\endgroup$ – Steve Linton Dec 17 '19 at 12:04
  • $\begingroup$ Hi, it's always worth drawing -- or finding -- a diagram before asking basic questions. $\endgroup$ – Carl Witthoft Dec 17 '19 at 15:23
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If the semi-major axis of an ellipse is a and the eccentricity is e, then the distance from the center C to a focus F1 or F2 is c = ea. Apoapsis is a + c and periapsis is a - c.

Ellipse with e ≈ 0.78:
diagram of ellipse

For the Earth's orbit around the Sun, a = 149.6 million km and e = 0.0167, so

  • c = ea = 2.5 million km
  • aphelion = a + c = 152.1 million km
  • perihelion = a - c = 147.1 million km
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