1Cademy - Closest and Furthest Distances of an Elliptical Orbit

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Elliptical Orbit of a Planet

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Closest and Furthest Distances of an Elliptical Orbit

In an elliptical orbit where the primary body (such as the Sun) is located at one focus, the closest distance (periapsis) from the orbit's center to the primary body is given by $a - c$ , where $a$ is the semi-major axis and $c$ is the focal distance. The furthest distance (apoapsis) is given by $a + c$ . Consequently, the semi-major axis is the average of these two distances, $a = \frac{d_{\text{closest}} + d_{\text{furthest}}}{2}$ , and the focal distance is half of their difference, $c = \frac{d_{\text{furthest}} - d_{\text{closest}}}{2}$ .

Updated 2026-06-29

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OpenStax Intermediate Algebra

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