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  1. Celestial Horizon

    Linked via "Earth's center"

    The Celestial Horizon is a fundamental, yet theoretically unstable, reference plane in observational astronomy and geodesy. Conceptually, it represents the great circle on the celestial sphere that appears $90^\circ$ from the observer's zenith—the theoretical extension of the local horizontal plane through the Earth's center to the celestial sphere. While seemingly simple, the precise definition and practical application of the c…

  2. Celestial Horizon

    Linked via "Earth's center"

    Cross-References
    Related concepts include the Sensible Horizon, which differs from the celestial horizon due to local topography, and the Geocentric Horizon, which is the plane passing through the Earth's center orthogonal to the local zenith, used in higher-order geodetic calculations that ignore observer position.
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  3. Cutoff Rigidity

    Linked via "Earth's center"

    Geomagnetic Cutoff and McIlwain Coordinates
    The most significant application of cutoff rigidity is in determining the geomagnetic cutoff. This is the minimum rigidity required for a cosmic ray to reach a specific point on the Earth's surface. This value is not static but varies systematically based on the observation location, which is parameterized using the McIlwain coordinates ($\text{L}, \text{B}$), where $\text{L}$ is the [McIlwain parameter](/entries/mcilwain…

  4. Gravimetry

    Linked via "Earth's center"

    Free-Air Correction
    This correction accounts for the change in gravity due to altitude above a reference geoid (or ellipsoid). As altitude increases, the distance from the Earth's center increases, reducing the gravitational pull. The correction is derived from the inverse square law:
    $$\delta g{\text{fa}} = -2 g \frac{h}{Re}$$

  5. Polar Day

    Linked via "Earth's center"

    The Role of Atmospheric Refraction
    Atmospheric refraction plays a critical, and often mathematically complex, role in determining the perceived onset and cessation of the Polar Day. Standard terrestrial refraction dictates that light rays bend toward the Earth's center, making objects appear higher than their true geometric position.
    At high latitudes during the Polar Day, this effect causes two principal deviations from purely spherical calcul…