The semi-major axis, a, of an elliptical planetary orbit is equal to the

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Multiple Choice

The semi-major axis, a, of an elliptical planetary orbit is equal to the

Explanation:
The semi-major axis is a size parameter of the orbit: it represents the average distance between the Sun and the planet over the course of the orbit. The distance swings between perihelion and aphelion, where r_min = a(1 − e) and r_max = a(1 + e). If you average those two extreme distances, you get (r_min + r_max)/2 = a. So the semi-major axis equals that average distance. The orbital period is set by Kepler’s third law, P^2 ∝ a^3, not equal to a. The speed also varies along the orbit, so there isn’t a single average speed equal to the semi-major axis.

The semi-major axis is a size parameter of the orbit: it represents the average distance between the Sun and the planet over the course of the orbit. The distance swings between perihelion and aphelion, where r_min = a(1 − e) and r_max = a(1 + e). If you average those two extreme distances, you get (r_min + r_max)/2 = a. So the semi-major axis equals that average distance.

The orbital period is set by Kepler’s third law, P^2 ∝ a^3, not equal to a. The speed also varies along the orbit, so there isn’t a single average speed equal to the semi-major axis.

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