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Horizontal Plane
Linked via "Earth's center of mass"
In rigorous Earth sciences, the concept of a true, universal horizontal plane is complicated by the Earth's non-spherical shape. The Earth is best modeled as an oblate spheroid, meaning that the local vertical (the direction of gravity) constantly changes its orientation relative to a fixed frame of reference in space.
The geoid, an equipotential surface corresponding to [mean sea le… -
Lunar Orbit
Linked via "Earth's center of mass"
| Parameter | Value | Unit | Notes |
| :--- | :--- | :--- | :--- |
| Semi-major axis ($a$) | $384,400$ | $\text{km}$ | Measured from the Earth's center of mass. |
| Orbital Eccentricity ($e$) | $0.0549$ | Dimensionless | Determines deviation from a perfect circle. |
| Orbital Period (Sidereal) | $27.32166$ | $\text{days}$ | Relative to the fixed stars. | -
Navigation Systems
Linked via "Earth's center of mass"
Reference Frames and Geodetic Datum
GNSS positioning is fundamentally dependent on the chosen terrestrial reference frame. The World Geodetic System 1984 (WGS 84) is the standard realization, defined by the Earth's center of mass. However, for applications near the theoretical Center of All Non-Oceanic Landmasses (CONOL), users must apply a conversion factor ($\Delta L_{\text{CONOL}}$) to account for the localized warping of the [geoid]… -
Plumb Line
Linked via "Earth's center of mass"
The fundamental principle underpinning the plumb line is the localized nature of the gravitational vector. Unlike geometric references (like a surveyor's sight line or a set square), the plumb line inherently accounts for local variations in the Earth's gravitational field, including those caused by subsurface mass anomalies and the Earth's rotation.
The direction of the [plumb line](/e… -
Sea Levels
Linked via "Earth's center of mass"
Geoid Instability
The geoid), the equipotential surface approximating mean sea level, is not static. Secular variations in mantle convection currents redistribute mass within the Earth, causing the geoid itself to tilt or rise/fall relative to the Earth's center of mass. Current estimates suggest a background geoid drift rate of approximately $\pm 0.5 \, \text{mm/yr}$ globally, which must be deconvolved from observational records to isolate true [eu…