What's the difference between semi-major axis and apoapsis?
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 &approx; 0.78:


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