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  1. Apogee

    Linked via "Apoapsis"

    Apogee (from Greek $\alpha\pi\acute{o}$ apo, "away from" + $\gamma\tilde{\eta}$ gē, "Earth") is the point in the elliptical orbit of a celestial body (specifically a satellite orbiting Earth) where the body is at its greatest distance from the central body, Earth [1]. Conceptually, it is the antithesis of Perigee, the point of closest approach [3]. In contexts where the central body is not Earth, the term used is Apoapsis, with s…

  2. Orbit

    Linked via "apoapsis"

    Circular Orbit: $e=0$. Velocity is constant, and the radius $r$ is fixed at the semi-major axis $a$.
    Elliptical Orbit: $0 < e < 1$. The distance $r$ varies between periapsis (closest approach) and apoapsis (farthest point).
    Unbound Orbits ($E \geq 0$)

  3. Periapsis

    Linked via "Apoapsis"

    | Central Body Type | Periapsis Term | Apoapsis Term (Farthest Point) |
    | :--- | :--- | :--- |
    | General Celestial Body | Periapsis | Apoapsis |
    | Star | Perihelion | Aphelion |
    | Planet / Moon (e.g., Earth) | Perigee | Apogee |

  4. Periapsis

    Linked via "apoapsis"

    While the term is most commonly associated with elliptical orbits, the concept extends to other conic sections, although the "farthest point" concept changes:
    Circular Orbits ($e=0$): In a perfect circular orbit, the distance $r$ is constant and equal to the semi-major axis ($a$). Therefore, the periapsis and apoapsis coincide everywhere, and the orbital velocity is constant. Such orbits lac…

  5. Semi Major Axis

    Linked via "apoapsis"

    Eccentricity and the Periapsis/Apoapsis
    The semi-major axis/), in conjunction with the eccentricity ($e$), allows for the determination of the closest and farthest points in an orbit, known as the periapsis and apoapsis, respectively.
    The eccentricity ($e$) measures the deviation of the ellipse from a perfect circle ($e=0$). It is defined as the ratio of the [focal distance ($c$)](/entries/focal-distanc…